Bathroom Exhaust Fan Size Calculator
HVI-method fan size from floor area and fixtures, with duct-length reality check.
Rated CFM assumes 0.1 in wg — a long flex run can halve it. Buy the 0.25 in wg rating if the duct exceeds ~5 m, and vent OUTSIDE, never into the attic.
Formula
Bathroom Exhaust Fan Size Calculator is a free bathroom fan size for HVAC designers, technicians and energy auditors — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Bathroom Exhaust Fan Size Calculator
HVI-method fan size from floor area and fixtures, with duct-length reality check. The calculation implements Small rooms: 1 CFM/ft² (min 50); large rooms: 50 CFM per fixture (HVI 2016 ventilation guidelines; IRC M1505). Rated CFM assumes 0.1 in wg — a long flex run can halve it. Buy the 0.25 in wg rating if the duct exceeds ~5 m, and vent OUTSIDE, never into the attic.
How to use Bathroom Exhaust Fan Size Calculator
- 1Enter Bathroom area in m².
- 2Enter Fixtures (toilet+shower+tub counts).
- 3Enter Larger than 9.3 m² (100 ft²)? (1 = yes → fixture method).
- 4Read Required fan instantly — no submit button needed.
- 5Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Bathroom Exhaust Fan Size Calculator?
- ✓Implements the standard formula — Small rooms: 1 CFM/ft² (min 50); large rooms: 50 CFM per fixture
- ✓Reference cited on-page: HVI 2016 ventilation guidelines; IRC M1505
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Bathroom Exhaust Fan Size Calculator use?+
It computes Small rooms: 1 CFM/ft² (min 50); large rooms: 50 CFM per fixture, per HVI 2016 ventilation guidelines; IRC M1505. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
Rated CFM assumes 0.1 in wg — a long flex run can halve it. Buy the 0.25 in wg rating if the duct exceeds ~5 m, and vent OUTSIDE, never into the attic.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Bathroom Exhaust Fan Size Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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