Bathroom Exhaust Fan Size Calculator (Large Bathroom, Multiple Fixtures)

Per HVI's own guidance, ONE sizing method is selected — not compared and maxed — based on room size and ventilation type: for intermittent ventilation in a bathroom of 100 sq ft or less, the square-footage method (1 CFM per sq ft) applies; for intermittent ventilation in a bathroom over 100 sq ft, the fixture-based method applies instead (50 CFM each for a toilet — always assumed present — a separate shower, and a separate standard tub; 100 CFM for a jetted/whirlpool tub, which counts as ONE fixture, not an addition on top of a separate tub). Continuous ventilation uses a flat 20 CFM code-compliant rate regardless of room size or fixtures, per ASHRAE 62.2/IRC. The raw requirement (floored at the 50 CFM intermittent / 20 CFM continuous code minimum) is rounded UP to the next standard retail bathroom fan size (50, 80, 100, 110, 150, 190, 250, or 300 CFM) — bathroom fans are commonly sold in sizes across this range, the same way central AC is sold in standard tonnage sizes, though exact model availability still varies by manufacturer. A raw requirement beyond 300 CFM exceeds what a single residential fan typically covers and is flagged rather than capped. A known limitation of HVI's own method: once a bathroom exceeds 100 sq ft, its actual floor area no longer factors into the result at all — only fixture count does, so an unusually large, sparsely-fixtured bathroom may need more capacity than shown here. This is a planning estimate — actual duct length and elbow count reduce a fan's delivered CFM below its rated CFM; confirm your chosen unit's actual (ideally HVI-certified) performance with a licensed contractor.