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CFM Calculator

Work out how many cubic feet per minute (CFM) of airflow a room needs. Enter the room size and a target air change rate, or pick a room type such as bathroom or kitchen to apply common fan and range hood sizing rules.

Last updated: October 24, 2026

Room

Units

FeetMeters

Length

Width

Ceiling height

Room type

Air changes per hour

ACH

Toilets

Bathrooms over 100 sq ft are sized by fixtures.

Showers

Bathtubs

Jetted tubs

Cooktop width

For sizing the range hood.

Hood placement

Gas burners total (optional)

BTU

Sum of all burner ratings; leave empty for electric.

Check a fan (optional)

Fan rating you have or are considering

CFM

Enter your values to see the result.

How to use the CFM Calculator

Enter the room size: length, width and ceiling height in feet or meters.
Pick a room type to use a typical air change rate, or choose Custom and type your own ACH.
For bathrooms and kitchens, add fixtures or the cooktop width so the fan and hood rules can be applied.
Read the CFM and the fan size to look for, plus metric airflow and time per air change.
Optionally enter a fan's rating to see whether it meets the target.

The CFM formula

CFM = room volume (ft³) × ACH ÷ 60, where room volume = length × width × ceiling height. Rearranged, ACH = CFM × 60 ÷ volume tells you how often a given fan replaces the room's air.

Example: a 14 × 12 ft kitchen with an 8 ft ceiling is 1,344 ft³. At 15 ACH it needs 1,344 × 15 ÷ 60 = 336 CFM of general ventilation. In metric, multiply m³ by ACH to get m³/h directly, then divide by 1.699 for CFM.

Typical air change rates by room

Room	Typical ACH	Source / note
Bathroom	8	HVI recommendation; or 1 CFM per sq ft (min. 50)
Kitchen	15	HVI recommendation; size the hood separately
Laundry room	6–10	Rule of thumb for moisture and lint
Bedroom, living room	4–6	Rule of thumb for comfort ventilation
Home office	4–6	Rule of thumb; more with many occupants
Basement	3–4	Rule of thumb; pair with a dehumidifier if damp
Garage / workshop	4–10	Higher when using finishes, solvents or engines

Bathroom fan sizing by fixture (HVI)

Fixture	CFM
Toilet	50
Shower	50
Bathtub	50
Jetted tub	100

Use the fixture method for bathrooms over 100 sq ft. An enclosed toilet room usually gets its own 50 CFM fan. Duct the fan outdoors, never into the attic, and run it for about 20 minutes after a shower to clear moisture.

Frequently asked questions

How do I calculate CFM for a room?

Multiply the room volume in cubic feet by the air changes per hour you want, then divide by 60. A 12 × 10 ft room with an 8 ft ceiling is 960 ft³; at 6 air changes per hour it needs 960 × 6 ÷ 60 = 96 CFM.

What size bathroom fan do I need?

The Home Ventilating Institute (HVI) guideline is 1 CFM per square foot of floor for bathrooms up to 100 sq ft, with a minimum of 50 CFM, increased for ceilings over 8 ft. For larger bathrooms, add 50 CFM each for a toilet, shower and bathtub and 100 CFM for a jetted tub. An 8 × 5 ft bathroom needs at least 50 CFM.

How many CFM does a range hood need?

A common HVI guideline is 100 CFM per linear foot of cooktop width for a wall hood and 150 CFM per foot for an island hood, so a 30 in range under a wall hood needs about 250 CFM. For gas ranges, another rule of thumb is 1 CFM per 100 BTU of total burner output. Very large hoods may require make-up air under local codes.

What is ACH (air changes per hour)?

ACH is how many times the entire volume of air in a room is replaced in one hour. At 8 ACH, the room's air is exchanged every 7.5 minutes. ACH = CFM × 60 ÷ room volume.

Are the room type ACH values a building code?

No. The bathroom (8 ACH) and kitchen (15 ACH) values follow commonly cited HVI recommendations, and the others are general rules of thumb. Building codes such as ASHRAE 62.2 and local codes set their own minimum rates, for example 50 CFM intermittent or 20 CFM continuous for a bathroom, so check what applies where you live.

Why buy a fan larger than the calculated CFM?

Fan ratings are measured at low static pressure. Long duct runs, elbows, small duct diameters and roof or wall caps all reduce real airflow, so choosing the next size up and keeping ducts short and straight helps the fan deliver close to its rating.

How do I convert CFM to m³/h?

Multiply CFM by 1.699 to get cubic meters per hour, or by 0.4719 to get liters per second. 100 CFM ≈ 170 m³/h ≈ 47.2 L/s.

Are all the tools on FreeToolCabin tested for quality and accuracy?

Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.

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