Bathroom fan CFM, by the HVI rule.

Short answer

For a bathroom up to 100 sq ft, HVI recommends 1 CFM per square foot of floor area, so an 8 x 5 ft bathroom (40 sq ft) needs a fan rated at least 40 CFM. Above 100 sq ft, add 50 CFM each for a toilet, shower and tub, or 100 CFM for a jetted tub.

Worked example

HVI's own example is an 8 x 5 ft bathroom: 8 x 5 = 40 sq ft at 1 CFM per sq ft, so a 40 CFM fan. For a 20 x 12 ft bathroom (240 sq ft) with a tub, a shower and an enclosed toilet: 50 + 50 + 50 = 150 CFM, which HVI says can be one 150 CFM fan or three 50 CFM fans.

Formula, assumptions and sources

HVI: for bathrooms up to 100 sq ft, an exhaust fan should provide 1 CFM per square foot, which HVI describes as approximately eight air changes per hour. For bathrooms above 100 sq ft, HVI recommends a rate from the fixtures: toilet 50 CFM, shower 50 CFM, bath tub 50 CFM, jetted tub 100 CFM. The air-changes figure shown is your fan CFM x 60 / room volume using the ceiling height you enter. Fan labels are tested at specific duct conditions; long or bent ducts deliver less than the label. Local code can require more, so check it.

Source: Home Ventilating Institute (HVI), Bathroom Ventilation (opened 2026-10-08)

Safety and limits

This is a planning aid, not professional advice. Electrical and HVAC work can cause fire, shock and carbon monoxide hazards; have a licensed professional size, install and inspect equipment.

FAQ

What size exhaust fan do I need for a bathroom?

HVI recommends 1 CFM per square foot for bathrooms up to 100 sq ft, so a 40 sq ft bathroom needs at least 40 CFM.

How do I size a fan for a large bathroom?

Above 100 sq ft HVI sizes by fixtures: 50 CFM each for a toilet, shower and tub, and 100 CFM for a jetted tub. A bathroom with one of each of the first three needs 150 CFM.

Does this account for duct length?

No. The rating is a minimum for the room; a long or restricted duct reduces what the fan delivers, so check the fan's installed-airflow rating.