Bathroom Fan Size Calculator

How to use this calculator

Enter the bathroom's length, width, and ceiling height, and tick the fixtures in the room. Fixtures only matter for bathrooms over 100 square feet. The result is the minimum fan rating, in cubic feet per minute (CFM), to look for when you buy a fan.

How this is calculated

The Home Ventilating Institute (HVI) recommends 1 CFM per square foot of floor for bathrooms up to 100 square feet, which is about eight air changes an hour with an 8-foot ceiling. For ceilings taller than 8 feet, the calculator scales that up so the room still gets the same air-change rate; that adjustment is this site's, not HVI's. For bathrooms over 100 square feet, HVI sizes by fixture instead: 50 CFM each for a toilet, shower, and tub, and 100 CFM for a jetted tub. The result is rounded up to the next 10 CFM and is never below 50 CFM, the building-code minimum for an intermittent bathroom fan.

Example

A 9 × 10 foot bathroom with an 8-foot ceiling needs a 90 CFM fan. With a 10-foot ceiling, it needs 120 CFM. A 12 × 10 foot bathroom with a toilet, shower, and tub needs 150 CFM, either as one fan or as separate fans over each fixture.

Choosing the fan

Look for the CFM figure on the box, and choose a model that is HVI-certified so the rating has been tested. Long or bent duct runs reduce how much air a fan actually moves, so if the duct to the outside is long, size up. Run the fan for about 20 minutes after a shower, as HVI recommends; a timer switch makes that automatic.

Planning estimate only. Local codes can require more, and an enclosed toilet room needs its own fan or an opening window.