Bathroom Exhaust Fan Size Calculator
Up to 100 square feet, a bathroom fan needs 1 CFM per square foot of floor and never less than 50 CFM. A bigger bathroom is sized by its fixtures: 50 CFM each for the toilet, shower and tub, 100 for a jetted tub. Then buy a little above that, because the duct takes its share.
Size your bathroom fan
The room and its fixtures give the airflow the fan must move, the rating to buy and the most noise to accept, and a check that the duct can carry it.
Feet with decimals: 7 ft 6 in is 7.5. The floor of the whole bathroom, wall to wall.
Fixtures (they set the size over 100 sq ft)The duct
From the spec sheet, the airflow at 0.25 in. of static pressure. Blank checks the rating to buy.
- Floor area
- Sized by
- Buy a fan rated at least
- Loudest to accept
How big a fan
These are the rates the Home Ventilating Institute (HVI) recommends for a fan you switch on when the room is used:
| Bathroom | Fan size |
|---|---|
| Up to 100 sq ft | 1 CFM per square foot of floor, at least 50 CFM |
| Over 100 sq ft | Add up the fixtures: toilet 50, shower 50, bathtub 50, jetted tub 100 CFM |
| Fan running all the time | 20 CFM, in place of a 50 CFM switched fan |
So a 5 by 8 ft hall bath, 40 sq ft, needs the 50 CFM minimum; an 8 by 10 ft bath needs 80; and a 12 by 12 ft primary bath with a toilet, a shower and a tub needs 150. HomeAdvisor's sizing table agrees: 50 to 60 CFM for a 40 sq ft bath, 150 to 250 for a 144 sq ft one. HVI also says an enclosed toilet should have its own fan, and that a fan rated for wet areas should go over the shower or tub where possible.
The code minimums are lower. The Department of Energy's Building America Solution Center gives the International Residential Code's and ASHRAE 62.2's figures as at least 50 CFM for a switched fan or 20 CFM for one that runs continuously. HVI's rates are what keep a bigger room clear.
What to buy
The rating on the box is measured in a lab, and once the fan is ducted it moves less. The Building America guide says ENERGY STAR recommends a fan rated at least 70 CFM to deliver the 50 CFM a switched fan needs, and one rated over 50 CFM to deliver a continuous 20. The calculator uses 70 as the floor for any switched fan whose need is under 70; that extension to needs between 50 and 70 is our reading. Above 70 it gives the need itself, and the duct check below tells you whether your duct can deliver it.
For noise, the same guide says ASHRAE 62.2 requires 3 sones or less for a fan you switch on and 1 sone or less for one that runs all the time, and that ENERGY STAR recommends the same limits. A quieter fan gets used. HVI recommends a timer or control that keeps the fan running 20 minutes after each use, and a 3/4-inch gap under the door so replacement air can get in. Is a humidity sensing fan worth it? looks at the automatic option.
Is the duct short and wide enough?
Most weak bathroom fans are a duct problem. The building code limits how long an exhaust duct can be for the fan it serves, in Table M1504.2 of the International Residential Code (as UpCodes publishes it). The table assumes no elbows; take 15 ft off the allowance for each one.
| Duct | 50 CFM fan | 80 CFM | 100 CFM | 125 CFM | 150 CFM |
|---|---|---|---|---|---|
| 3 in smooth | 5 ft | Not allowed | Not allowed | Not allowed | Not allowed |
| 4 in flexible | 56 ft | 4 ft | Not allowed | Not allowed | Not allowed |
| 4 in smooth | 114 ft | 31 ft | 10 ft | Not allowed | Not allowed |
| 5 in flexible | No limit | 81 ft | 42 ft | 16 ft | 2 ft |
| 5 in smooth | No limit | 152 ft | 91 ft | 51 ft | 28 ft |
| 6 in flexible | No limit | No limit | 158 ft | 91 ft | 55 ft |
| 6 in smooth | No limit | No limit | No limit | 168 ft | 112 ft |
The fan's rating in the table is its airflow at 0.25 inches of static pressure, which a good spec sheet lists alongside the usual figure. A 3-inch flexible duct is not allowed at any size, and the 4-inch flexible duct so many fans are hung on carries an 80 CFM fan only 4 ft: one elbow uses that up. That is the commonest reason a new fan still leaves the mirror fogged. The calculator reads a fan between two columns in the next one up, and the code also accepts a duct that meets the fan maker's design criteria, or one whose airflow is measured once installed.
The Building America guide adds the rest: duct to the outside, never into the attic, crawlspace or a wall; use smooth duct where you can and pull flex tight; keep the first two to three feet straight before any elbow; prefer gradual bends to sharp ones; insulate the duct against condensation; and end it at a cap with a damper, at least 3 ft from a gravity air inlet and 10 ft from a mechanical one (IRC M1504.3).
From a number to a quote
With a CFM and a sone limit you have what an installer asks for. What size bathroom exhaust fan you need walks through measuring, what a bathroom fan replacement costs prices the job, and the quote checker puts a bid against the published figures. Who to call is in electrician or handyman for a bathroom fan, whether you need a permit in permits for replacing a bathroom exhaust fan, and when it is time in signs your bathroom fan needs replacing.
Sources, all read on 19 October 2026: Home Ventilating Institute, How much ventilation do I need?; US DOE Building America Solution Center, Bathroom exhaust fans; UpCodes, IRC M1504.2 and Table M1504.2 Duct length; HomeAdvisor, Bathroom fan replacement cost. A planning estimate: the fan's instructions and your local code decide.