How the bathroom fan calculator works
A bathroom fan has one job: pull the moist air from a shower out of the house before it condenses on cold walls, windows and the ceiling. Two sizing rules are in common use, and the calculator applies both. The first replaces the whole volume of air eight times an hour. The second, used by the Home Ventilating Institute (HVI) for bathrooms up to 100 ft² with ordinary ceilings, asks for 1 CFM of airflow per square foot of floor. A floor of 50 CFM applies in every case, because that is the minimum intermittent rate in the ASHRAE 62.2 ventilation standard.
Airflow (CFM) = max(volume in ft³ × air changes ÷ 60, floor area in ft², 50)In metric, the volume rule is simply volume in m³ × air changes = m³/h. One CFM equals 1.699 m³/h or 0.472 L/s, so the area rule becomes about 18.3 m³/h per square metre of floor. The volume rule wins in rooms with high ceilings; the area rule wins in low, wide rooms.
Worked example
A 2.5 m × 2 m bathroom with a 2.4 m ceiling has 12 m³ of air. Eight changes an hour means 12 × 8 = 96 m³/h, or 56.5 CFM. Its 5 m² of floor is 53.8 ft², so the area rule asks for 53.8 CFM (91 m³/h). The larger figure, 96 m³/h (26.7 L/s), is the minimum; in a US catalogue that points to a 70 or 80 CFM model once duct losses are allowed for. For comparison, the English building regulations ask for at least 15 L/s from an intermittent bathroom fan, so this fan clears that comfortably.
Fan size by bathroom
| Bathroom floor | Ceiling | Required airflow | Common fan size |
|---|---|---|---|
| 5 × 8 ft (1.5 × 2.4 m) | 8 ft | 50 CFM (85 m³/h), minimum applies | 50 CFM |
| 7 × 9 ft (2.1 × 2.7 m) | 8 ft | 67 CFM (114 m³/h) | 70 CFM |
| 8 × 10 ft (2.4 × 3.0 m) | 8 ft | 85 CFM (145 m³/h) | 90 CFM |
| 10 × 10 ft (3.0 × 3.0 m) | 8 ft | 107 CFM (181 m³/h) | 110 CFM |
| 10 × 12 ft (3.0 × 3.7 m) | 9 ft | 144 CFM (245 m³/h) | 150 CFM |
For bathrooms over 100 ft², HVI suggests adding up the fixtures instead: 50 CFM each for a toilet, a shower and a bathtub, and 100 CFM for a jetted tub. An enclosed toilet or steam shower is best served by its own fan.
Why a correctly sized fan still underperforms
- Duct resistance. Fan ratings are usually measured at 0.1 in of static pressure, a short and straight duct. Long runs, several elbows and flexible hose can cut real airflow by a third or more; choose a fan whose rating at 0.25 in is still above your target.
- Duct size. Keep the duct at least the diameter of the fan outlet, typically 4 in (100 mm) on small fans and 6 in (150 mm) on larger ones, and use smooth rigid duct where possible.
- Venting into the loft. Always terminate outside through a wall or roof cap with a backdraft damper; moist air dumped into an attic causes mould and rot on the roof timbers.
- No make-up air. A fan cannot extract air that cannot get in. Leave a 10–20 mm (½–¾ in) gap under the door or fit a transfer grille.
- Switching off too soon. Run the fan for about 20 minutes after a shower; a humidity sensor or timer does this automatically. Look for a noise level of 1 sone or less if you want it used.
Fans in shower zones must be suitable for that location and wired to local electrical rules; have an electrician connect anything above a bath or shower.
Frequently asked questions
What CFM bathroom fan do I need?
Use 1 CFM per square foot of floor for bathrooms up to 100 ft² with 8 ft ceilings, with a minimum of 50 CFM. An 8 × 10 ft bathroom needs about 80–90 CFM.
How do I convert CFM to m³/h?
Multiply CFM by 1.699. A 50 CFM fan moves about 85 m³/h, an 80 CFM fan about 136 m³/h and a 110 CFM fan about 187 m³/h.
How many air changes per hour for a bathroom?
The common rule is 8 air changes per hour. Some guides use 6 for a toilet-only room and up to 10 for bathrooms with long, hot showers.
Can a bathroom fan be too powerful?
An oversized fan is noisier and can pull warm air from the house or backdraft a fuel-burning appliance in a tight home. Size close to the result and use a humidity-controlled switch.
How long should a bathroom fan run after a shower?
About 20 minutes clears the moisture from a typical bathroom. A timer or humidistat switch makes sure it happens.
Last reviewed: 2026-09-27 · How we calculate