🚿What CFM does this bathroom's extractor fan need?

✓ Tested & verified Updated: Aug 8, 2026 How we calculate this

Bathroom fan CFM calculator

Room length (ft)

Room width (ft)

50 CFM

The room works out at 50 CFM — 50 is the nearest size actually sold.

Floor area: 48 sq ft
Working rate: 1 CFM per sq ft

Over 100 sq ft, size by fixtures instead: 50 CFM each for the toilet, shower and bath, 100 for a jetted tub — add them up.

Try also

What size extractor fan does a bathroom need?

One CFM per square foot of floor, and never below 50. An 8 × 6 ft bathroom is 48 sq ft, so the rule gives 48 and the floor gives 50 — the smallest fan sold, and the right one.

That is the Home Ventilating Institute’s recommendation and it works out at roughly eight air changes an hour in a standard-height bathroom, which is the rate that clears steam before it condenses on cold surfaces. Above 100 sq ft the rule changes to fixtures: 50 CFM each for the toilet, the shower and the bath, 100 for a jetted tub, added together.

The duct decides whether you get the CFM you paid for. A fan rated at 80 CFM on a test rig delivers far less through a long, flexible, kinked duct with a restrictive flap at the end. Use rigid duct at the fan’s full diameter, keep the run short, avoid sharp bends, and check the fan’s rating at 0.25 in w.g. of static pressure rather than the headline figure.

Bathrooms come in a handful of sizes and so do fans. Here is the whole decision in one table.

Bathroom	Floor area	Fan by area	Fan by fixtures
Cloakroom, 4 × 5 ft	20 sq ft	50 CFM (minimum)	50 CFM (toilet)
Small, 6 × 8 ft	48 sq ft	50 CFM	100 CFM (toilet + shower)
Standard, 8 × 10 ft	80 sq ft	80 CFM	150 CFM (toilet + shower + bath)
Large, 10 × 12 ft	120 sq ft	use fixtures	150 CFM
Ensuite with jetted tub	140 sq ft	use fixtures	200 CFM

Where the two columns disagree, the fixture method wins — it is what HVI recommends above 100 sq ft, and it is the one that notices a separate shower and bath in the same room.

Frequently asked questions

How many CFM for a 50 square foot bathroom?

Fifty. The one-CFM-per-square-foot rule gives 50 exactly, and 50 CFM is also the minimum HVI recommends for any bathroom regardless of size — so bathrooms under 50 sq ft get the same fan. Below that the fan cannot clear a shower’s steam quickly enough for it to matter how small the room is.

Is a stronger bathroom fan better?

Only up to a point. An oversized fan pulls conditioned air out of the house faster than needed, is louder, and in a tightly sealed home can create enough negative pressure to backdraught a water heater or fireplace flue. If the bathroom clears slowly, look at duct length, duct diameter and the exterior flap before buying a bigger fan.

How long should the fan run after a shower?

About twenty minutes, which is the HVI recommendation. The steam that causes mould is the film left on cold surfaces after you leave, not the cloud you can see. A timer switch or a humidity-sensing fan does this without anybody remembering to.

Does the fan have to vent outside?

Yes. Venting into an attic or a soffit moves the moisture from a room you can see into a space you cannot, where it condenses on cold framing and insulation. Every code requires termination to the outdoors through a proper hood with a backdraught damper — and a surprising number of houses do not have it.

What about sones — how loud is too loud?

One sone is about as loud as a quiet fridge, 4 sones is a noisy fan nobody leaves running. Anything at or below 1.5 sones is quiet enough that people use it as intended, which is the whole point: the quietest fan that meets the CFM will do more for the bathroom than a powerful one that gets switched off.

Does ceiling height change the CFM?

The published rule is per square foot at a standard ceiling, so a room with a very high ceiling has more air to move than the rule assumes. In practice the fixture-based method handles this better for large or tall bathrooms, and a fan with a humidity sensor covers the difference by simply running longer.

How we calculate this

Fan sizing uses the Home Ventilating Institute recommendation of 1 CFM per square foot of floor for bathrooms up to 100 sq ft, with a 50 CFM minimum for any bathroom. That rate corresponds to approximately eight air changes an hour at a standard ceiling height. Above 100 sq ft, HVI recommends fixture-based sizing instead — 50 CFM each for a toilet, shower or bath and 100 CFM for a jetted tub — which is described on the page but not computed by the tool. Results round up to the CFM ratings fans are built in. Rated airflow is measured at a standard static pressure; real duct runs reduce it, so the figure to compare when buying is the delivered CFM at 0.25 in w.g., not the headline number.

Worked example

With Room length (ft) 8, Room width (ft) 6, this page works out 50 CFM. The room works out at 50 CFM — 50 is the nearest size actually sold.

Floor area48 sq ft
Working rate1 CFM per sq ft

Sources: