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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
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.
With these inputs:
| Result | Value |
|---|---|
| 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 |
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.
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 | 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.
| 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.
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.
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.
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.
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.
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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