Bathroom and Kitchen Exhaust Fan Sizing
What size bath fan a bathroom needs, and the code floor for a kitchen.
Example
You enter
- Room bathroom
- Room length (ft) 8
- Room width (ft) 10
- Ceiling height (ft, 6 to 20) 8
- Operation intermittent
- Bathroom sizing rule ach8
- Toilets (fixture rule) 0
- Showers (fixture rule) 0
- Bathtubs (fixture rule) 0
- Jetted tubs (fixture rule) 0
You get
- Rule of thumb 85.3 cfm
- Recommended (CFM) 90
- ACH at recommended 8.4375
Checked against published worked examples from: ICC.
Details, formula, and sources
ScopeIRC 2024 Table M1505.5 sets the minimum: 50 cfm intermittent or 20 cfm continuous for a bathroom or toilet room, 100 cfm intermittent or 25 cfm continuous for a kitchen, and a bath fan's listed rate must be met at 0.25 in wc, not in free air. Rules of thumb set the size above that floor: the Home Ventilating Institute's 1 cfm per sq ft for a bathroom of 100 sq ft or less (scaled for a taller ceiling), its 50 cfm per toilet, shower or tub and 100 cfm per jetted tub over 100 sq ft, or the common eight air changes an hour, which runs a little higher. The larger governs, rounded up to the next 10 cfm the way fans are sold. An 8 by 10 ft bathroom at 8 ft holds 640 cu ft and wants 85.3 cfm by eight air changes, so a 90 cfm fan, or 80 cfm by HVI's square footage, which is why the 50 cfm fan in it never clears the mirror; a 5 by 8 ft powder room at 9 ft computes 48 cfm and the 50 cfm floor governs. A continuous fan meets the code at its continuous rate and is shown the boost size too. Duct sizing, makeup air and the whole-house rate are separate calculators.
Formula or tablecode minimum from IRC 2024 Table M1505.5 by room and operation; 8 ACH rule cfm = L x W x H x 8 / 60; HVI area rule cfm = L x W x (H / 8); fixture rule cfm = 50 x (toilets + showers + tubs) + 100 x jetted tubs; intermittent size = ceil(max(intermittent minimum, rule) / 10) x 10; recommended = the continuous minimum for a continuous fan, else the intermittent size; air changes = cfm x 60 / volume.
Source and editionIRC 2024 (International Residential Code). Section M1505.5 and Table M1505.5 (M1505.4.4 and Table M1505.4.4 in IRC 2021), read on the IRC 2024 text on 2026-10-19 and the IRC 2021 text on 2026-10-20. Home Ventilating Institute sizing practice by name.
Free accessThe IRC is free to read at codes.iccsafe.org; the Home Ventilating Institute publishes its sizing guidance free at hvi.org.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesroom_type, length_ft, width_ft, ceiling_ft, operation, rule, toilets, showers, tubs, jetted_tubs, rule_cfm, recommended_cfm, ach_at_recommended
- Rule of thumb above the floor 1 cfm per sq ft scaled for the ceiling (100 sq ft or less) or 50 cfm per toilet, shower or tub and 100 per jetted tub (over 100 sq ft); 8 air changes an hour as a volume alternativeHome Ventilating Institute sizing practice
- Fans sold in 10 cfm steps the recommended rate is rounded up to the next 10 cfmstated assumption
- Rated at 0.25 in wc compare the fan's listed rate at 0.25 in wc, not its free-air rateIRC 2024 M1505.5