Airflow (CFM) Calculator

INTRODUCTION

You installed the bathroom exhaust fan.

You felt capable. You felt thrifty. You felt like a 50 CFM fan from the big box store was "plenty" for a 5×8 bathroom because the box said "handles up to 70 sq ft."

The fan ran. It made noise. You saw the grille flutter. For a month.

Then the paint above the shower started bubbling. The caulk turned black at the tub edge. The mirror fogged for 20 minutes after a shower.

You blamed the paint. "Cheap latex."

You scraped. You primed with Kilz. You repainted with "bathroom grade" semi-gloss.

Month three: Same bubbling. Worse mold. The drywall tape lifted in a strip above the shower head.

You blamed the contractor who built the house. "No vapor barrier."

You tore out the drywall. You found the vapor barrier intact. The insulation was dry above the rest of the room. But directly above the shower, the OSB subfloor was stained dark. The rafter bays held a musty, mushroom smell.

You blamed the fan again. "Defective motor."

You bought an 80 CFM fan. Louder. More flutter. Same mold.

But the real problem was the number.

You never calculated the required airflow. It did not know your 5×8 bathroom with 8-foot ceilings is 320 cubic feet, not 70 square feet. It did not know the ASHRAE standard for bathrooms is 50 CFM continuous or 20 CFM per 50 sq ft intermittent — meaning your 40 sq ft bathroom needs 20 CFM continuous or 80 CFM intermittent on a timer. It did not know that a 50 CFM fan, after duct resistance (15 feet of 4-inch flex with two elbows), delivers only 32 CFM effective. It did not know that 4-inch duct at 50 CFM creates 573 FPM velocity — above the 500 FPM maximum for quiet residential operation, which is why it roared and rattled.

Your fan was moving air. But not enough. Not fast enough. Not quietly enough. The moisture stayed. The mold grew. The paint failed.

This is what happens when you ventilate without an Airflow (CFM) Calculator.

Air is invisible. You cannot see 50 CFM. You cannot feel the difference between adequate and inadequate ventilation without instruments. But the consequences are visible: mold, rot, odors, CO₂ headaches, grease buildup, and in commercial kitchens, fire code violations.

Airflow governs comfort. It governs health. It governs energy efficiency. It governs whether your range hood actually captures cooking smoke or just makes noise while grease coats your cabinets.

A 200 CFM fan in a 1,200 sq ft house with a natural draft water heater can backdraft CO into the living space. A 50 CFM fan in a commercial kitchen with a wok station is like using a desk fan to put out a campfire.

Get CFM wrong, and you suffocate slowly or poison quickly. You freeze in one room while another roasts. You pay to heat air that escapes through an oversized exhaust. You breathe PM2.5 because the filter was sized for the wrong flow rate.

Get CFM right, and the system disappears. The bathroom dries in 10 minutes. The kitchen smells like nothing. The bedroom CO₂ stays below 800 ppm. The energy bill drops because the heat recovery ventilator matches the exhaust exactly.

An Airflow (CFM) Calculator finds that number. It converts room volume to required CFM based on air changes per hour. It sizes ducts so velocity stays quiet. It checks makeup air requirements so you do not backdraft combustion appliances. It tells you if your fan is fighting a duct that is too small, too long, or too bent.

In 2026, with airtight construction mandated by energy codes, indoor air quality standards tightening, and heat pump systems requiring precise airflow for rated efficiency, knowing your CFM is not optional.

It is essential for every HVAC technician, builder, homeowner, and anyone who breathes indoor air.

---

WHAT IS AN AIRFLOW (CFM) CALCULATOR?

An Airflow (CFM) Calculator is a tool that calculates the volume of air that must be moved to achieve ventilation, comfort, or process requirements in a given space or duct system.

It uses fluid dynamics, thermodynamics, and building code standards:

• ASHRAE 62.1 — Ventilation for Acceptable Indoor Air Quality (commercial)

• ASHRAE 62.2 — Ventilation and Acceptable Indoor Air Quality in Residential Buildings

• ACCA Manual D — Residential duct system design procedures

• SMACNA standards — Duct construction and velocity limits

• NFPA 96 — Commercial kitchen ventilation and grease exhaust

Standard inputs:

• Room dimensions (length, width, ceiling height)

• Room type / occupancy (bedroom, kitchen, bathroom, office, classroom, gym)

• Ventilation standard (ASHRAE 62.1, 62.2, or custom ACH)

• Duct size and type (round, rectangular, flex, rigid)

• Duct length and fittings (elbows, transitions, dampers)

• Fan curve data (static pressure, RPM, horsepower)

• Temperature differential (for heat load calculations)

Outputs you get:

• Required CFM for the space and application

• Air Changes Per Hour (ACH) achieved

• Duct size recommendation for target velocity

• Velocity in Feet Per Minute (FPM) — critical for noise and capture

• Static pressure loss in the duct run

• Equivalent duct length (accounting for fittings)

• System effect factors (inlet/outlet conditions)

• Makeup air requirement (to prevent depressurization)

• Energy cost of continuous ventilation

It answers the questions every building professional asks:

"Why does my bathroom still smell after running the fan for an hour?"

"Will this range hood actually capture wok smoke?"

"How big should the fresh air duct be for this classroom?"

"Why is my heat pump freezing up?"

---

HOW TO USE THE NUMOVIX AIRFLOW (CFM) CALCULATOR

Our calculator gives you instant, accurate ventilation engineering in under 30 seconds.

Step 1:

Select your calculation mode.

Example: Room Ventilation (ASHRAE 62.2)

---

Step 2:

Enter your room dimensions and type.

Example: Bathroom

• Length: 8 feet

• Width: 5 feet

• Ceiling height: 8 feet

---

Step 3:

Select your ventilation standard or target ACH.

Example: ASHRAE 62.2 — Intermittent Exhaust

---

Step 4:

Enter your existing or proposed duct information.

Example: 4-inch flex duct, 15 feet, 2 elbows

---

Step 5:

Enter your fan rated CFM (from manufacturer).

Example: 50 CFM rated

---

Step 6:

Click "Calculate Airflow."

You will instantly see:

Example: 5×8 Bathroom, 8' Ceiling, 4" Flex, 15 ft, 2 Elbows, 50 CFM Fan

---

Ventilation Results:

| Parameter | Value |

| Room Volume | 320 cubic feet |

| ASHRAE 62.2 Required (Intermittent) | 80 CFM |

| ASHRAE 62.2 Required (Continuous) | 20 CFM |

| Your Fan Rated CFM | 50 CFM |

| Duct Velocity at Rated CFM | 573 FPM |

| Duct Velocity Limit (Residential) | 500 FPM max |

| Static Pressure Loss | 0.42" w.c. |

| Effective Delivered CFM | 32 CFM |

| ACH at Effective CFM | 6.0 ACH |

| Code Compliance | FAIL (32 < 80 required) |

| Noise Level Estimate | 3.5 sones (loud for bathroom) |

---

Recommendation:

| 4" flex | 917 FPM | 1.08" w.c. | 48 CFM | FAIL — too loud, too small |

| 6" flex | 407 FPM | 0.18" w.c. | 76 CFM | Marginal |

| 6" rigid | 407 FPM | 0.11" w.c. | 79 CFM | PASS |

| 8" rigid | 229 FPM | 0.04" w.c. | 80 CFM | Excellent, quiet |

---

Key Numbers:

• Required: 80 CFM intermittent for a 40 sq ft bathroom per ASHRAE 62.2

• Your 50 CFM fan delivers only 32 CFM through 4" flex with elbows

• 4" duct is too small — velocity exceeds 500 FPM, creating noise and resistance

• 6" rigid metal duct is the minimum for quiet, effective 80 CFM delivery

• Continuous ventilation option: 20 CFM on low speed, 6" duct, whisper quiet

---

Example: Commercial Kitchen Hood — 6-Foot Type I Hood, Wok Cooking

| Parameter | Value |

| Hood length | 6 feet |

| Cooking type | Heavy-duty (wok, charbroiler) |

| NFPA 96 / IMC Requirement | 150 CFM per linear foot minimum |

| Required CFM | 900 CFM |

| Capture velocity at hood face | 50–75 FPM |

| Duct size (round, 2,000 FPM max) | 10-inch minimum |

| Grease filter area needed | 6 sq ft (at 150 FPM through filters) |

| Makeup air requirement | 80–90% of exhaust (720–810 CFM) |

---

THE MATH BEHIND AIRFLOW CALCULATION

Understanding the formulas helps you verify results and avoid ventilation failures.

---

CFM from Air Changes Per Hour (ACH):

CFM = (Room Volume × ACH) / 60

Where:

• Room Volume = L × W × H (cubic feet)

• ACH = Air changes per hour (standard varies by room type)

Example (320 cu ft bathroom, 8 ACH):

CFM = (320 × 8) / 60 = 42.7 CFM

---

ASHRAE 62.2 Residential Formula:

Q_fan = 0.03 × A_floor + 7.5 × (N_br + 1)

Where:

• A_floor = Floor area (sq ft)

• N_br = Number of bedrooms

Example (1,800 sq ft, 3 bedrooms):

Q = 0.03 × 1,800 + 7.5 × 4 = 54 + 30 = 84 CFM whole-house

Bathroom local exhaust: 50 CFM intermittent or 20 CFM continuous

Kitchen local exhaust: 100 CFM intermittent or 5 ACH

---

Duct Velocity:

V = CFM / A

Where A = Cross-sectional area of duct (sq ft)

Example (80 CFM in 6" round duct):

A = π × (0.25)² = 0.196 sq ft

V = 80 / 0.196 = 408 FPM

---

Duct Sizing for Target Velocity:

A = CFM / V_target

Then convert area to diameter or rectangle dimensions.

Example (target 400 FPM for quiet residential):

A = 80 / 400 = 0.20 sq ft

Diameter = √(4A/π) = √(4 × 0.20 / 3.1416) = √0.255 = 0.505 ft = 6.06 inches → 6" duct

---

Static Pressure Loss (Darcy-Weisbach):

ΔP = f × (L/D) × (ρ × V² / 2)

Where:

• f = Friction factor

• L = Duct length

• D = Hydraulic diameter

• ρ = Air density

• V = Velocity

For practical HVAC, use equivalent length tables:

• 90° elbow = 10–30 feet of straight duct equivalent

• Flex duct = 1.5×–2× the pressure loss of rigid duct

---

System Effect Factors:

Fans perform worse with poor inlet/outlet conditions:

• Inlet within one diameter of wall: −10% CFM

• Inlet elbow at fan: −15% CFM

• Discharge elbow too close: −10% CFM

• Flex duct compressed: −20% to −50% CFM

The calculator applies these derating factors.

---

Complete Real Example:

Priya's Restaurant Kitchen Hood Failure:

Starting Point:

• Restaurant: Indian cuisine, heavy tandoor and kadai cooking

• Hood: 8-foot Type I commercial exhaust hood

• Original installer: "A 1,200 CFM fan should handle it."

• Duct: 10-inch round, 25 feet to roof, 3 elbows

• Makeup air: None. "The dining room has gaps."

---

Month 1: The Guess Approach

The 1,200 CFM fan roars. The hood seems to work. Smoke is visible but eventually clears.

Month two: Grease drips from the ceiling tiles near the hood. The health inspector notes "grease accumulation on ceiling and light fixtures."

Month four: The fire inspector tags the system. "Inadequate capture velocity. Grease buildup in duct. No makeup air."

Month six: A grease fire in the kadai flashes into the hood. The suppression system activates. The kitchen is shut down for 3 days.

The insurance claim is denied: "Inadequate ventilation per NFPA 96. Owner negligence."

Cost: $18,000 in fire damage, code violations, lost revenue, and system redesign.

---

Month 7: Discovers the Calculator

Priya uses the Numovix Airflow Calculator.

Hood Requirements (NFPA 96 / IMC):

• Hood length: 8 feet

• Heavy-duty cooking: 150 CFM per linear foot minimum

• Required CFM: 8 × 150 = 1,200 CFM (the original guess was coincidentally correct)

But the duct analysis reveals the failure:

| Parameter | Value |

| Duct size | 10-inch round |

| Cross-sectional area | 0.545 sq ft |

| Velocity at 1,200 CFM | 2,200 FPM |

| NFPA 96 maximum duct velocity | 2,500 FPM |

| Static pressure (25 ft, 3 elbows, rigid) | 1.85" w.c. |

| Fan curve at 1.85" w.c. | Actual output: 890 CFM |

| Capture velocity at hood face | 37 FPM (target: 50–75 FPM) |

| Effective CFM at hood | 890 CFM |

| Required CFM | 1,200 CFM |

| Shortfall | 25.8% |

She realizes:

• The 1,200 CFM fan was correct in theory but the duct system strangled it.

• 10-inch duct at 1,200 CFM creates 2,200 FPM velocity. High velocity increases pressure loss exponentially (velocity squared term).

• Three elbows added 60–90 feet of equivalent length. The fan sees 25 feet of pipe as 115 feet of resistance.

• No makeup air meant the kitchen was depressurized by −0.15" w.c. This reduced fan output further and pulled grease-laden air through every crack instead of through the hood.

• The capture velocity at the hood face was 37 FPM. Smoke and grease particles need 50–75 FPM to be drawn into the hood. Below that, they escape into the room.

---

New Approach:

Priya redesigns with the calculator.

Hood and Fan:

• Required: 1,200 CFM at hood face

• Add 20% for filter loading: 1,440 CFM rated

• Select fan: 1,500 CFM at 1.5" w.c.

Duct System:

• Target velocity: 1,800 FPM (quiet, efficient, below 2,500 FPM grease limit)

• Required area: 1,500 / 1,800 = 0.833 sq ft

• Diameter: √(4 × 0.833 / π) = 13 inches → 14-inch round duct

• Straight path to roof: 25 feet

• Two 45° elbows instead of three 90° elbows: Equivalent length = 30 feet

• Total equivalent length: 55 feet

• Static pressure: 0.72" w.c.

Makeup Air:

• Required: 80% of exhaust = 1,200 CFM

• Source: Dedicated makeup air unit with heating/cooling

• Discharge: 4 diffusers around hood perimeter, not blowing into cooking area

Results after installation:

• Capture velocity: 65 FPM at hood face

• Zero grease escape visible

• Duct pressure: 0.72" w.c. — fan delivers 1,480 CFM

• Kitchen pressure: −0.02" w.c. (slightly negative, safe)

• Health inspector: Pass

• Fire inspector: Pass

• Insurance: Validated

She spent $4,200 on duct upgrades and makeup air and saved $18,000 in fire damage and code violations.

Why? Because she calculated the airflow before she installed.

---

CFM REQUIREMENTS BY SPACE AND STANDARD

ASHRAE 62.2 Residential:

| Room | Minimum CFM | Notes |

|---|---|---|

| Whole house | 0.03 × sq ft + 7.5 × (br + 1) | Continuous mechanical ventilation |

| Bathroom | 50 CFM intermittent / 20 CFM continuous | Must exhaust outdoors |

| Kitchen | 100 CFM intermittent / 5 ACH | Range hood or equivalent |

| Utility room with gas dryer | 50 CFM intermittent | Combustion safety |

| Basement (finished) | Add to whole-house calculation | Must be included in total |

ASHRAE 62.1 Commercial (per person + area):

| Office | 5 | 0.06 | Open plan or private |

| Classroom | 10 | 0.12 | Higher density, CO₂ critical |

| Retail | 7.5 | 0.06 | Varies by occupancy |

| Restaurant dining | 7.5 | 0.18 | Kitchen excluded |

| Gym/health club | 20 | 0.06 | High metabolic rate |

| Hospital patient room | 25 | 0.06 | Infection control critical |

| Laboratory | — | — | Fume hood demand system |

Commercial Kitchen (NFPA 96 / IMC):

| Cooking Type | CFM per Linear Foot | Capture Velocity |

| Light (steam table, oven) | 50–75 | 50 FPM |

| Medium (fryer, griddle) | 100–125 | 50–60 FPM |

| Heavy (wok, charbroiler) | 150–200 | 60–75 FPM |

| Extra-heavy (solid fuel) | 200+ | 75+ FPM |

---

WHY EVERY BUILDER NEEDS AN AIRFLOW CALCULATOR

1. Know Your Real Ventilation

A fan box says "80 CFM." The duct, elbows, and grille reduce it to 45 CFM.

The calculator shows the effective delivered airflow, not the fantasy number on the box.

---

2. Stop Mold Before It Starts

Bathrooms need 8+ ACH to dry surfaces before mold germinates (typically 24–48 hours on wet drywall).

A 50 CFM fan in a 320 cu ft bathroom gives 9.4 ACH. A 30 CFM effective fan gives only 5.6 ACH.

The calculator ensures you cross the mold threshold.

---

3. Prevent Backdrafting and CO Poisoning

A 400 CFM kitchen hood in a tight house depressurizes the space.

The water heater draft hood reverses. CO enters the living space.

The calculator checks makeup air requirements against exhaust flow.

---

4. Size Ducts for Quiet Operation

4-inch flex at 80 CFM = 917 FPM = roaring, rattling, whistling.

6-inch rigid at 80 CFM = 408 FPM = whisper quiet.

The calculator finds the duct size that balances flow and noise.

---

5. Pass Code and Inspection

Mechanical inspectors measure airflow with a balometer. They check duct size with a tape.

If your 4-inch duct is supposed to deliver 100 CFM, you fail. The calculator prevents this.

---

6. Optimize Heat Pump Performance

Mini-splits and central heat pumps require precise indoor airflow (typically 350–450 CFM per ton).

Too little: Coil freezes. Efficiency drops 40%. Compressor fails.

Too much: Drafty, noisy, poor dehumidification.

The calculator sizes ducts for rated airflow.

---

7. Understand Why Your Friend's House Feels Fresh

Your friend: 6" duct to bathroom, 80 CFM on timer, continuous 20 CFM whole-house ERV.

You: 4" duct, 50 CFM fan that actually delivers 32, no timer, no whole-house ventilation.

Same house size. Different math. Different air.

The calculator explains the difference.

---

KEY FACTORS THAT AFFECT AIRFLOW

Duct Resistance:

The #1 killer of delivered CFM. Every foot of duct, every elbow, every transition adds resistance.

Flex duct has 1.5×–2× the resistance of rigid metal. A 90° elbow equals 10–30 feet of straight duct.

The calculator converts fittings to equivalent length and computes total system resistance.

---

Fan Curve:

Fans do not produce constant CFM. They produce variable CFM based on static pressure.

A fan rated at 1,200 CFM at 0.1" w.c. may deliver only 800 CFM at 0.8" w.c.

The calculator plots your system resistance against the fan curve to find actual operating point.

---

Duct Velocity and Noise:

| Velocity | Application | Noise Level |

| < 250 FPM | Residential supply | Whisper quiet |

| 250–500 FPM | Residential return/exhaust | Quiet |

| 500–900 FPM | Commercial supply | Noticeable |

| 900–1,500 FPM | Commercial exhaust | Loud |

| 1,500–2,500 FPM | Kitchen grease exhaust | Very loud, code-limited |

| > 2,500 FPM | Industrial only | Prohibited for grease |

---

Makeup Air Balance:

Every CFM exhausted must be replaced. Options:

• Natural infiltration: Uncontrolled, energy-wasting, unsafe in tight houses

• Passive vent: Barometric damper, simple but cold in winter

• Fan-powered makeup air: Heated/cooled, balanced, energy-efficient

• ERV/HRV: Heat recovery, balanced, code-compliant for tight houses

The calculator computes the makeup air requirement and warns if exhaust exceeds safe infiltration rates.

---

Filter Loading:

A clean filter adds 0.05" w.c. A dirty filter adds 0.3" w.c.

That extra resistance can drop delivered CFM by 20%.

The calculator includes filter loading scenarios.

---

Temperature and Altitude:

Air density changes with temperature and elevation.

At 5,000 feet, air is 17% less dense. A fan moves the same volume but 17% less mass.

For heat transfer calculations, mass flow rate matters. The calculator adjusts for altitude.

---

COMMON MISTAKES PEOPLE MAKE

Mistake 1: Trusting the Box CFM Rating

The box says "80 CFM." That is at zero static pressure — a fan in free air, no duct, no grille, no resistance.

Install it with 20 feet of flex and two elbows, and you get 45 CFM.

Always calculate effective CFM at operating static pressure.

---

Mistake 2: Using 4-Inch Duct for Everything

4-inch bathroom fans, 4-inch dryer vents, 4-inch range hood ducts.

A 4-inch duct at 100 CFM creates 1,146 FPM. It screams. It resists. It fails.

Bathrooms need 6-inch minimum. Kitchens need 8-inch or larger. Dryers need 4-inch rigid only, as short as possible.

Size duct to CFM, not to the fan collar.

---

Mistake 3: No Makeup Air in Tight Houses

You install a 1,200 CFM range hood in an airtight home.

The house goes to −0.3" w.c. The fireplace backdrafts. The water heater spills CO. The garage fumes enter through the wall.

Every exhaust system needs calculated makeup air.

---

Mistake 4: Ignoring Equivalent Length of Fittings

You measure 15 feet of duct. You ignore three elbows.

Each elbow = 20 feet equivalent. Your fan sees 75 feet, not 15.

Static pressure triples. CFM drops 40%.

The calculator converts every fitting to equivalent straight length.

---

Mistake 5: Exhausting Into the Attic

"Bathroom air goes into the attic. It vents out the soffit."

No. It condenses on roof decking. It grows mold. It rots sheathing. It violates every code.

All exhaust must terminate outdoors, not into attics, crawl spaces, or wall cavities.

---

Mistake 6: Undersizing Commercial Kitchen Hoods

"1,200 CFM should handle this 8-foot hood."

But you are cooking with woks at 30,000 BTU each. The code requires 150 CFM per foot = 1,200 CFM minimum, plus filter area, plus makeup air.

You hit the minimum and ignore the system effect. The hood fails.

Calculate total system, not just fan size.

---

Mistake 7: No Timer or Humidistat Control

You run the bathroom fan until the mirror clears. Then you turn it off.

But the shower walls are still wet. The grout holds moisture for hours.

ASHRAE 62.2 requires a timer: 20 minutes minimum after shower use. Or a humidistat that runs until relative humidity drops below 60%.

---

PRO TIPS TO USE AIRFLOW EFFECTIVELY

Tip 1: Size Duct One Size Larger Than Minimum

The calculator says 6-inch duct works. Use 8-inch.

The marginal cost is small. The noise reduction is massive. The future upgrade headroom is valuable.

---

Tip 2: Use Rigid Metal Duct Wherever Possible

Flex duct has 1.5×–2× the pressure loss of rigid. It sags, creating traps. It tears.

Use rigid metal for all primary runs. Use short flex connections only at terminal devices.

---

Tip 3: Install Dampers for Balancing

Every branch needs a balancing damper. Without it, the path of least resistance steals all the air.

The calculator sizes each branch, but field balancing with a hood and anemometer is still required.

---

Tip 4: Plan Makeup Air Before You Exhaust

Design the makeup air system at the same time as exhaust.

If you cannot provide 80% makeup, reduce exhaust or add a passive vent.

Do not create a vacuum in a house with combustion appliances.

---

Tip 5: Use ERVs in Cold Climates

An Energy Recovery Ventilator exhausts stale air and brings in fresh air while transferring heat and moisture.

In winter, it saves 70% of the energy that simple exhaust would waste.

The calculator sizes ERVs to match whole-house ventilation requirements.

---

Tip 6: Measure With a Balometer

After installation, verify with an instrument.

A flow hood (balometer) measures actual delivered CFM at grilles and diffusers.

If the calculator said 80 CFM and you measure 55, find the restriction.

---

Tip 7: Document Your Ventilation Design

Save the calculator output. Show CFM per room, duct sizes, fan specs, and makeup air sources.

Inspectors, appraisers, and future buyers value documented engineering.

---

QUICK SUMMARY

Before you use the calculator, remember these key points:

• CFM is volume per minute — but effective CFM depends on duct resistance, not just fan rating

• Required CFM = (Volume × ACH) / 60 — know your room volume and target air changes

• ASHRAE 62.2: Bathrooms need 50 CFM intermittent or 20 CFM continuous — kitchens need 100 CFM or 5 ACH

• Duct velocity matters — below 500 FPM for quiet residential, below 2,500 FPM for grease exhaust

• 4-inch duct is almost always too small — use 6" for baths, 8"+ for kitchens, 10"+ for commercial

• Every fitting adds equivalent length — a 90° elbow = 10–30 feet of straight duct resistance

• Makeup air is mandatory — exhaust without replacement backdrafts combustion appliances

• Fan curves show real performance — rated CFM at 0" w.c. is fantasy; check CFM at operating pressure

• Flex duct has 1.5×–2× the loss of rigid — use rigid metal for primary runs

• Filter loading reduces CFM over time — size system with dirty filter in mind

• Timers and humidistats are required — running a fan while showering is not enough

• Altitude reduces air density — at 5,000 feet, mass flow is 17% lower for same CFM

• Exhaust must terminate outdoors — never into attics, soffits, or wall cavities

• Verify with a balometer — instruments reveal what guesswork hides

---

FREQUENTLY ASKED QUESTIONS

Q1: What is the difference between CFM and ACH?

CFM (Cubic Feet per Minute) is the rate of airflow.

ACH (Air Changes per Hour) is how many times the room's total volume is replaced in one hour.

Conversion: ACH = (CFM × 60) / Room Volume

---

Q2: Can I use a bigger fan on a small duct?

No. A bigger fan on a small duct creates more velocity, more noise, more resistance, and barely more airflow.

The duct becomes the bottleneck. Upgrade the duct first, then the fan.

---

Q3: Why does my heat pump freeze up?

Low airflow across the indoor coil. Causes:

• Undersized return duct

• Dirty filter

• Closed registers

• Fan speed set too low

The heat pump needs 350–450 CFM per ton. Below 300 CFM/ton, the coil ices.

---

Q4: How do I calculate makeup air for a kitchen hood?

Makeup air = 80–90% of exhaust CFM.

A 1,000 CFM hood needs 800–900 CFM of tempered replacement air.

If the house is tight (below 3 ACH50), you must provide mechanical makeup air. Natural infiltration is insufficient and unsafe.

---

Q5: Is a higher CFM fan always better?

No. Oversized fans:

• Create excessive noise

• Waste energy

• Over-dry air in winter

• Can depressurize the house

• Short-cycle if on a humidistat

Right-size using the calculator. Do not oversize "to be safe."

---

Q6: What is static pressure and why does it matter?

Static pressure is the resistance the fan must overcome, measured in inches of water column (" w.c.).

Typical residential systems: 0.3–0.7" w.c.

High-velocity systems: 1.0–2.0" w.c.

As static pressure rises, fan output drops. The calculator matches fan curve to system resistance.

---

Q7: Can I vent a bathroom fan into the attic?

Never. Moist air condenses on cold roof decking, causing mold and rot.

Code requires termination to the outdoors, typically through a roof cap or sidewall vent, with insulated duct in unconditioned spaces.

---

RELATED CALCULATORS

Explore our full suite of free HVAC and ventilation tools:

• Duct Size Calculator

• Static Pressure Calculator

• BTU Calculator

• Heat Load Calculator

• CFM to m³/h Converter

• Air Velocity Calculator

• FPM to MPH Converter

• Kitchen Hood Calculator

• Bathroom Fan Calculator

• ERV/HRV Sizing Calculator

• Filter Pressure Drop Calculator

---

FINAL THOUGHTS

Air is invisible. You cannot see 50 CFM drifting through a duct. You cannot smell the CO₂ at 1,200 ppm until your head aches. You cannot feel the grease particle that escapes the hood and lands on your ceiling.

But the consequences are visible. Mold on drywall. Grease on tiles. Ice on coils. CO alarms at 3 AM.

The Airflow (CFM) Calculator does not turn the fan.

It guides you.

It tells you: "This is the CFM. This is the duct size. This is where 4 inches ends and 6 inches begins. This is where guessing ends and ventilation engineering begins."

Below the right calculation, you are not ventilating. You are moving air uselessly while moisture, grease, and CO₂ accumulate.

At the right calculation, with properly sized ducts and balanced makeup air, you are ventilating.

Bathrooms dry. Kitchens clear. Bedrooms stay below 800 ppm CO₂. Heat pumps operate at rated efficiency.

Before you buy another bathroom fan, calculate the CFM.

Before you install another kitchen hood, calculate the CFM.

Before you wonder why your house smells, your paint bubbles, and your alarm chirps, calculate the CFM.

Know your airflow. Respect the duct. Breathe from a place of precision, not guesswork.

That is how you ventilate without regret.

That is how you build without mold.

That is how you create indoor air that sustains life.

---

DISCLAIMER

This article is for educational and informational purposes only.

Airflow calculations, ventilation requirements, and duct sizing are general estimates and vary significantly by local mechanical codes, climate zones, building tightness, and occupancy patterns.

The examples provided are illustrative and based on standard engineering practices (ASHRAE 62.1, ASHRAE 62.2, ACCA Manual D, SMACNA, NFPA 96).

Actual ventilation requirements depend on:

• Local mechanical and building codes (IMC, IRC, UMC)

• Building envelope tightness and blower door results

• Actual occupancy and activity levels

• Combustion appliance types and venting configurations

• Duct material, installation quality, and sealing

• Altitude, temperature, and humidity conditions

• Professional HVAC design and commissioning

Always consult a licensed HVAC contractor, mechanical engineer, or ventilation specialist before designing or modifying ventilation systems, especially for commercial kitchens, laboratories, healthcare facilities, and tight residential construction.

Numovix does not provide HVAC design, installation, or code compliance advice.

Our calculator results are estimates and should not replace professional load calculations, duct design, or commissioning with airflow measurement.

If you are experiencing mold, condensation, CO alarms, or inadequate ventilation, hire a licensed professional to diagnose and remedy the system. Do not rely on guesswork for life-safety systems.

Airflow (CFM) Calculator | Calculate HVAC Duct Size, Fan Speed & Ventilation Requirements | Numovix

Free CFM calculator. Calculate exact airflow for HVAC systems, exhaust fans, kitchen hoods, and bathroom ventilation. Convert CFM to m³/h, find duct size, and check ACH compliance. No signup needed.