CFM Calculator: Airflow for a Room or Bath Fan
CFM is cubic feet of air per minute. Enter the room and what it is, and the calculator works out the airflow: for a bathroom it applies the Home Ventilating Institute's sizing rule and checks it against the ASHRAE 62.2 minimum; for a kitchen it gives the 62.2 range hood and continuous rates; for any other room it turns air changes per hour into CFM.
Your project
Used for 'Other room': how many times an hour the room's air should be replaced.
Bathrooms over 100 sq ft are sized by fixture.
Your results
- Bath fan size
- 80 CFM
- HVI rule (1 CFM per sq ft)
- 80 CFM
- ASHRAE 62.2 minimum
- 50 CFM on demand or 20 CFM continuous
- Air changes per hour at this size
- 7.5 ACH
- Bathroom area
- 80 sq ft
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
Gear for this job
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How this calculator works
- CFM = room volume (cu ft) × air changes per hour ÷ 60
- Bathroom ≤ 100 sq ft: CFM = floor area × 1 (HVI)
- Bathroom > 100 sq ft: 50 CFM per toilet, shower or tub + 100 per jetted tub (HVI)
- ASHRAE 62.2: bath 50 CFM on demand or 20 continuous; kitchen 100 CFM on demand or 5 ACH continuous
- Whole house (ASHRAE 62.2): CFM = 0.03 × floor area + 7.5 × (bedrooms + 1)
Bathroom fan size by room (HVI rule, 50 CFM minimum)
| Bathroom | Area | Fan size |
|---|---|---|
| 5 × 7 powder room | 35 sq ft | 50 CFM |
| 5 × 8 hall bath | 40 sq ft | 50 CFM |
| 8 × 10 bath | 80 sq ft | 80 CFM |
| 10 × 10 primary bath | 100 sq ft | 100 CFM |
Above 100 sq ft, size by fixture instead: a toilet, a shower and a tub need 150 CFM; add 100 for a jetted tub.
Whole-house ventilation (ASHRAE 62.2 fan flow)
| Floor area | 2 bedrooms | 3 bedrooms | 4 bedrooms |
|---|---|---|---|
| 1,000 sq ft | 53 CFM | 60 CFM | 68 CFM |
| 1,500 sq ft | 68 CFM | 75 CFM | 83 CFM |
| 2,000 sq ft | 83 CFM | 90 CFM | 98 CFM |
| 2,500 sq ft | 98 CFM | 105 CFM | 113 CFM |
| 3,000 sq ft | 113 CFM | 120 CFM | 128 CFM |
3 CFM per 100 sq ft + 7.5 CFM × (bedrooms + 1), the formula in ASHRAE 62.2 since 2013. Local code may use a different edition.
Worked example: an 8 × 10 ft bathroom
8 × 10 = 80 sq ft, under HVI's 100 sq ft limit, so 1 CFM per sq ft gives 80 CFM. That is above ASHRAE 62.2's 50 CFM minimum, so an 80 CFM fan is the size to buy. With an 8 ft ceiling the room holds 640 cu ft, and 80 CFM replaces it 80 × 60 ÷ 640 = 7.5 times an hour — HVI's "about eight air changes".
A 12 × 10 ft primary bath is 120 sq ft, so HVI sizes it by fixture instead: a toilet, a shower and a tub at 50 CFM each make 150 CFM.
Airflow, ducts and insulation
A fan's rating is measured against a set static pressure; a long or undersized duct run delivers less. Keep bath fan ducts short and smooth, and size them with the duct size calculator. Where a duct runs through an unconditioned attic, insulate it so warm, moist air doesn't condense inside.
Tighter houses need deliberate ventilation: as you air-seal and insulate, the ASHRAE 62.2 whole-house rate is the minimum fresh-air flow to plan for.
Keep going
- Size the duct that carries this airflow: duct size calculator
- Fan duct through a cold attic? Wrap it: duct insulation calculator
- Will moist air condense on a cold surface? dew point calculator
Frequently asked questions
How do I calculate CFM for a room?
Multiply length × width × ceiling height for cubic feet, multiply by the air changes per hour you want, and divide by 60. A 12 × 15 ft room with an 8 ft ceiling is 1,440 cu ft; at 6 air changes an hour that is 1,440 × 6 ÷ 60 = 144 CFM.
What size bathroom fan do I need?
The Home Ventilating Institute recommends 1 CFM per square foot for bathrooms up to 100 sq ft — 80 CFM for an 8 × 10 ft bath — and ASHRAE 62.2 sets a 50 CFM minimum for a fan run on demand. Over 100 sq ft, add 50 CFM for each toilet, shower and tub and 100 for a jetted tub.
How many CFM does a range hood need?
ASHRAE 62.2 sets 100 CFM for a kitchen hood run on demand, or continuous exhaust of 5 air changes an hour of the kitchen's volume. Those are ventilation minimums; a powerful cooktop may call for more, and local code can require higher rates.
How much ventilation does a whole house need?
ASHRAE 62.2's whole-dwelling rate is 3 CFM per 100 sq ft of floor area plus 7.5 CFM per bedroom-plus-one. A 2,000 sq ft, 3-bedroom home needs 60 + 30 = 90 CFM of continuous fan flow.
How do I convert air changes per hour to CFM?
CFM = volume × ACH ÷ 60, and ACH = CFM × 60 ÷ volume. A 640 cu ft bathroom with an 80 CFM fan gets 80 × 60 ÷ 640 = 7.5 air changes an hour.
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Bath fan size
80 CFM