Ventilation · Bathroom
Bathroom exhaust fan size
The quick rule is 1 CFM per square foot, minimum 50 CFM — a 5 × 8 ft bathroom takes a 50 CFM fan. By air changes, which is what actually clears steam, the same room wants 15–25 changes per hour and lands closer to 90–150 CFM. Both numbers are below.
Room
- Minimum airflow
- 136 m³/h
- Maximum airflow
- 227 m³/h
- Room area
- 3.72 m²
- Room volume
- 9.06 m³
Exhaust fan · Bathroom with Shower
80–133CFM
15–25 air changes per hour
Fan size by bathroom size
At an 8 ft ceiling. The rule column is the 1 CFM per sq ft minimum; the other two come from the air-change rates for each room type.
| Bathroom | Area | 1 CFM/sq ft | Toilet only | With shower |
|---|---|---|---|---|
| 5 × 5 ft | 25 sq ft | 50 | 20–50 | 50–83 |
| 6 × 6 ft | 36 sq ft | 50 | 29–72 | 72–120 |
| 8 × 5 ft | 40 sq ft | 50 | 32–80 | 80–133 |
| 8 × 6.5 ft | 52 sq ft | 52 | 42–104 | 104–173 |
| 10 × 8 ft | 80 sq ft | 80 | 64–160 | 160–267 |
| 12 × 10 ft | 120 sq ft | 120 | 96–240 | 240–400 |
All figures in CFM. Fans are sold in steps — 50, 80, 110, 150 CFM — so round up to the next available size.
The two methods
Area rule CFM = floor area (sq ft), minimum 50 — bathrooms under 100 sq ft
Fixture CFM = 50 per toilet + 50 per shower + 50 per tub + 100 per jetted tub
Air change CFM = volume(m³) × ACH ÷ 1.698 — shower 15–25 ACH, toilet 6–15The area rule is the code minimum most inspectors check. The air-change method is what ventilation design actually uses, and it is the one built into the CFM calculator for all 16 room types.
FAQ
- How many CFM does a bathroom fan need?
- For a bathroom under 100 sq ft the common rule is 1 CFM per square foot, with 50 CFM as the floor — so a 5 × 8 ft bath takes a 50 CFM fan. The air-change method used by the calculator on this page is stricter for showers: it asks for 15–25 air changes per hour, which puts the same bathroom closer to 90–150 CFM. Use the rule as the minimum a code inspector expects, and the higher figure when the room actually gets steamy.
- How do I calculate exhaust fan size for a large bathroom?
- Above 100 sq ft, size by fixture instead of by area: 50 CFM each for the toilet, the shower and the bathtub, and 100 CFM for a jetted tub. Add them up — a large bath with toilet, shower and tub needs 150 CFM.
- How do I calculate exhaust fan CFM from room volume?
- Multiply length × width × height in metres to get the volume, multiply by the air changes per hour the room needs, and divide by 1.698 to convert m³/h to CFM. A bathroom with a shower needs 15–25 air changes per hour; a toilet-only room needs 6–15.
- Is a bigger bathroom fan always better?
- No. An oversized fan pulls conditioned air out of the house faster than it clears moisture, which wastes energy and can backdraft combustion appliances in a tight home. Size it to the room and run it long enough — 20 minutes after the shower does more than an oversized fan running for five.
- Where does the duct matter?
- A fan's rating is measured on a test rig, not on your ductwork. A 110 CFM fan on a long flexible duct with elbows may move well under its rating. Keep the run short and straight, use smooth duct where possible, and size it to the fan's outlet rather than pinching it down.