The short answer
A 10 by 8 ft bathroom with an 8 ft ceiling needs a 80 CFM exhaust fan (80 CFM calculated). A 3 ft wall-mount range hood over 50,000 BTU/h of gas burners needs 600 CFM; over an electric cooktop of the same width it needs only 300.
Fan size chart
Bathroom airflow from the Home Ventilating Institute (HVI) rule, and range hood airflow by cooktop width. Numbers are calculated for an 8 ft ceiling and an electric cooktop. Gas burners can raise the hood number.
Bathroom fans
| Bathroom | CFM | Fan |
|---|---|---|
| 5×6 ft | 50 | 50 |
| 5×8 ft | 50 | 50 |
| 8×8 ft | 64 | 80 |
| 10×8 ft | 80 | 80 |
| 10×10 ft | 100 | 110 |
| 12×10, toilet + shower | 100 | 110 |
| 12×10, + bathtub | 150 | 150 |
| 14×12, + tub + jetted | 250 | 2 × 150 |
Rooms over 100 sq ft add up the fixtures' CFM (the last three rows). Multiply by ceiling height ÷ 8 when the ceiling is higher than 8 ft.
Range hoods
| Cooktop | Wall CFM | Island CFM |
|---|---|---|
| 24 in | 200 | 300 |
| 30 in | 250 | 375 |
| 36 in | 300 | 450 |
| 48 in | 400 | 600 |
Gas: also at least 1 CFM per 100 BTU/h of burner output, and take the larger. A 3 ft (36 inch) gas cooktop with 36,000 BTU/h of burners needs 360 CFM by that rule, more than the 300 CFM by width.
How it works
The formula
For a bathroom we follow the Home Ventilating Institute (HVI) guidance. For a range hood we use the common width and burner rules, and check the result against the International Residential Code (IRC).
Bathroom, over 100 sq ft: CFM = 50 × toilets + 50 × showers + 50 × bathtubs + 100 × jetted tubs
Ceiling over 8 ft: CFM × height ÷ 8
Hood: CFM = 100 (wall) or 150 (island) × cooktop feet
Gas hood: CFM = burner BTU/h ÷ 100; take the larger of the two
Assumptions
- Bathroom fans are sold at 50, 80, 110, 150 CFM and range hoods at 300, 400, 600, 900, 1,200 CFM. We pick the smallest size that meets the need. Past the largest size we show how many fans or hoods you would need.
- The IRC minimum for a bathroom fan is 50 CFM if it runs when needed, or 20 CFM if it runs all the time. Many codes also accept a window.
- Fan loudness is measured in sones. Look for 1 or less for a quiet fan; 2 is acceptable. HVI-certified fans list both CFM and sones.
- Range hoods over 400 CFM need makeup air under IRC M1503.6, which means a way to bring outside air into the house. Check local code: it varies.
- CFM ratings are measured with a short, straight duct. A long run, tight bends or a duct smaller than the fan's outlet cuts the real airflow. Follow the manufacturer's duct size.
Worked example: a 10 by 8 ft bathroom and a 3 ft gas cooktop
Bathroom with an 8 ft ceiling, one toilet and one shower.
- Area: 10 × 8 = 80 sq ft, which is under 100.
- Rule: 1 CFM per sq ft = 80, and the 50 CFM minimum doesn't apply. Ceiling factor 8 ÷ 8 = 1. So 80 CFM, a 80 CFM fan.
- Range hood: 3 ft × 100 = 300 CFM by width. Gas: 50,000 ÷ 100 = 500 CFM by burners. Take the larger: 500 CFM, a 600 CFM hood.
- That is over 400 CFM, so ask your building department about makeup air.
Too big or too small
- Too big (bathroom): noise and drafts. A fan much bigger than the room is loud, and it pulls cold air in through every gap.
- Too small (bathroom): fog and mold. Steam stays in the room, the mirror stays fogged and moisture soaks into paint and drywall.
- Too big (range hood): it pulls conditioned air out of the house. Above 400 CFM you may need a makeup air system, which costs extra.
- Too small (range hood): smoke gets away. Smoke, grease and (with gas) combustion gases escape into the kitchen.
- Between sizes? In the bathroom, take the larger size, and pick a low-sone model so it stays quiet. For a hood, pick a model with variable speed so you can run it low most of the time.
Questions people ask
What size bathroom exhaust fan do I need?
For a bathroom of 100 sq ft or less, use 1 CFM per square foot, and never less than 50 CFM. A 10 by 8 ft bathroom (80 sq ft) needs 80 CFM, so a 80 CFM fan. Bigger bathrooms add 50 CFM for the toilet, 50 for the shower, 50 for a bathtub and 100 for a jetted tub.
How many CFM for a 5x8 bathroom?
A 5 by 8 ft bathroom is 40 sq ft. The rule gives 40 CFM, but the minimum is 50, so buy a 50 CFM fan.
How loud should a bathroom fan be?
The loudness is rated in sones. One sone is about a quiet refrigerator. Choose 1 sone or less for a fan you will hardly hear, and 2 sones or less for an acceptable one. A fan that is too loud is a fan people stop using.
How many CFM for a range hood?
Two rules, and you take the larger. Width: 100 CFM per foot of cooktop for a wall hood, 150 for an island. Gas: 1 CFM for every 100 BTU/h of total burner output. A 3 ft wall hood over 50,000 BTU/h of gas burners needs 500 CFM by the gas rule, so a 600 CFM hood. The same cooktop with electric burners needs 300 CFM.
Do I need makeup air for a range hood?
The International Residential Code (M1503.6) calls for makeup air when a kitchen exhaust fan moves more than 400 CFM. That covers many 600 CFM hoods. Local codes vary, and some towns change this limit, so check local code before you buy.
Is a ceiling height over 8 feet a problem for a bathroom fan?
It adds air to clear. Multiply the CFM by the ceiling height divided by 8. A 10 by 8 ft bathroom with a 10 ft ceiling needs 100 CFM instead of 80, so a 110 CFM fan.