What does CFM stand for?

CFM stands for cubic feet per minute — the volume of air moved by a fan or ventilation system each minute. A higher CFM rating means more airflow capacity.

What is ACH (air changes per hour)?

ACH is how many times per hour the full volume of air in a room is replaced. The presets here are common rules of thumb for fan sizing: 4 for bedrooms and living rooms, 6 for a home office, 8 for bathrooms and 15 for kitchens. They are not code minimums. The IRC and ASHRAE 62.2 set fixed airflows instead, such as 50 CFM for a bathroom fan run on demand or 100 CFM for a kitchen exhaust.

How is CFM calculated from room size and ACH?

CFM = (Room Volume in cubic feet × ACH) ÷ 60. Room volume = length × width × ceiling height. For example, a 10 × 12 ft room with an 8 ft ceiling and 6 ACH needs (10 × 12 × 8 × 6) ÷ 60 = 96 CFM.

Should I add a safety margin to the calculated CFM?

The calculator shows the result plus 15% as a buffer. The larger gap is often static pressure. Bathroom fans are usually rated at 0.1 inch of water column, and a long or bent duct pushes back harder, so the fan moves less air than its label says. If the maker lists airflow at 0.25 inch, size from that number.

Does duct length affect the required CFM rating?

No. The room needs the same CFM whatever the duct. Duct length, elbows and the wall or roof cap add static pressure, which cuts how much air a given fan delivers. Pick a fan whose airflow at your duct's static pressure, from the maker's performance table, meets the number here, and keep the run short and straight.

How many CFM does a bathroom fan need?

For a bathroom up to 100 sq ft, HVI recommends 1 CFM per square foot of floor with an 8 ft ceiling, so a 5 × 8 ft bathroom needs 40 CFM and an 8 × 10 ft one 80 CFM. The code minimum is 50 CFM for a fan run on demand. Larger bathrooms are sized by fixture: 50 CFM each for a toilet, shower and tub, and 100 for a jetted tub. The Bathroom preset (8 ACH) gives 43 and 85 CFM for those two rooms.

How many CFM per ton of air conditioning?

Central air is usually designed for about 400 CFM per ton of cooling, lower (around 350) in humid climates and up to about 450 in dry ones. A 3-ton system moves roughly 1,200 CFM. This calculator does not size by tonnage. It works from room volume and air changes, so use the CFM from Tonnage calculator for that.

CFM Calculator

Enter room dimensions and required air changes per hour to calculate the CFM needed for proper ventilation.

CFM = room volume × air changes per hour (ACH) ÷ 60, where volume is length × width × ceiling height in feet.

Room Dimensions

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Room Type (sets ACH)

Enter room dimensions and select a room type, then click Calculate CFM.

Quick answer

CFM = room volume × air changes per hour (ACH) ÷ 60, where volume is length × width × ceiling height in feet. A 10 × 12 ft room with an 8 ft ceiling at 6 ACH needs 960 × 6 ÷ 60 = 96 CFM. The calculator also shows that figure plus 15% (111 CFM) as a margin for picking a fan.

Examples

Room, 8 ft ceiling	ACH	CFM
Bathroom 5 × 8 ft	8	43
Bedroom 12 × 12 ft	4	77
Office 10 × 12 ft	6	96
Kitchen 12 × 15 ft	15	360
Conference 15 × 20 ft	10	400
Garage 20 × 20 ft	30	1,600

Enter the room size and pick a room type.

Summary

The CFM Calculator determines how much airflow (in cubic feet per minute) a room needs to achieve a target number of air changes per hour (ACH). It works by computing room volume from your floor dimensions and ceiling height, then dividing by the minutes required per air change cycle. Use it to size HVAC equipment, exhaust fans, or fresh-air ventilation systems for any residential or commercial space.

How it works

Enter the room length and width in feet.
Enter the ceiling height in feet.
Select or enter the required air changes per hour (ACH) for your space type.
The calculator multiplies length × width × height to get room volume in cubic feet.
It divides volume by (60 ÷ ACH) to produce the required CFM.
Use the result to select a fan or HVAC unit rated at or above that CFM.

Use cases

Sizing a bathroom exhaust fan, then checking it against the 50 CFM code minimum.
Determining fresh-air ventilation needs for home offices and bedrooms.
Estimating kitchen exhaust by room size (range hoods are usually sized by cooktop width or burner output).
Sizing commercial HVAC units for offices and retail spaces.
Verifying adequate ventilation for server rooms and equipment closets.
Designing industrial exhaust systems for workshops and garages.
Checking if an existing fan meets minimum ACH standards for a given room.
Comparing multiple room configurations side by side.

Frequently Asked Questions

Last updated: 2026-05-25 · Reviewed by Nham Vu