Bathroom Exhaust Fan Calculator — What Size Fan Do I Need?
Enter your room size and fixtures to get the CFM rating for your exhaust fan. No sign-up, no data stored.
Fan size (CFM)
How exhaust fan size is estimated
- Room area (sq ft) = length x width (or area directly).
- Base CFM = 1 CFM per sq ft (8 air changes per hour).
- Fixtures add 50 CFM per shower/tub and 100 CFM per jetted tub.
- Shop size rounds up to the nearest 10/50 CFM, plus ~20% for ducts over 15 ft.
The 1-CFM-per-square-foot rule comes from the HVI (Home Ventilating Institute) sizing method " "and is what most code officials expect. A taller ceiling needs slightly more air, so the " "calculator scales base CFM by ceiling height over the standard 8 ft.
Room size → fan CFM reference
8-ft ceiling, standard shower included.
| Room size | Area | Base CFM | With shower | Shop for |
|---|---|---|---|---|
| 5x7 | 35 | 35 | 85 | 90 CFM |
| 5x8 | 40 | 40 | 90 | 100 CFM |
| 6x9 | 54 | 54 | 104 | 110 CFM |
| 8x10 | 80 | 80 | 130 | 130 CFM |
| 10x12 | 120 | 120 | 170 | 170 CFM |
Last updated: October 2026 · HVI 1-CFM/sf method. Ducts over 15 ft and elbows cut airflow - size up accordingly.
Bathroom exhaust fan FAQ
Between the shower and the toilet, centered on the room if possible - steam from both spots " "gets pulled in. A fan directly over the shower works but misses toilet odors; a long duct run " "to the roof is worth the extra pipe.
Most codes still require one in any room with a toilet or bath, and a powder room without a " "window needs ventilation for moisture and odor. Even a small 50-CFM fan beats an open window in " "winter.
Under 2 sones is noticeably quiet; 1-1.5 sones is what most homeowners want. Budget fans run " "3-4 sones and sound like a vacuum. CFM and sones together tell you if a fan moves air without " "being annoying.
Yes, but keep the run short and slope the duct down slightly so condensation drains out. " "Venting into an attic or soffit cavity is a code violation - moist air has to reach outside, " "not into insulation.
15-20 minutes after the last shower, or until the mirror clears and the air feels dry. A " "timer switch or humidity-sensing fan removes the guesswork - the best upgrade is letting the " "fan outlive your shower by a quarter hour.
Related calculators & guides
Bathroom Exhaust Fan Sizing Guide
CFM rules, placement and quiet fans for bathrooms.
Read the guide →