What Size Bathroom Exhaust Fan Do I Need?
If you're wondering what size bathroom exhaust fan you need, the answer comes down to CFM — cubic feet per minute of air the fan moves. Enter your bathroom's floor area, ceiling height, and whether it has a shower or tub, and this calculator will estimate the CFM you need and pick the closest standard fan size to shop for.
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How to use this calculator
Measure your bathroom's floor area in square feet (length × width) and enter it above. If your ceilings are standard 8-foot height, you can leave the ceiling height alone — it only matters for bathrooms over 100 square feet, where the calculator switches to an air-changes method that uses room volume.
Tell the calculator whether the bathroom has a shower or tub. Any bathing fixture sets a practical floor of 50 CFM, since steam is what a bathroom fan is really fighting. A half bath with only a toilet skips that minimum. Your results update instantly as you type.
The headline number is the standard fan size to shop for — bathroom fans are sold in fixed CFM ratings (50, 80, 90, 110, 150, 200), so the calculator rounds your requirement up to the next available size.
How it works
For bathrooms up to 100 sq ft, this calculator uses the Home Ventilating Institute (HVI) rule of 1.1 CFM per square foot. An 80 sq ft bathroom, for example, needs about 80 × 1.1 = 88 CFM, which rounds up to a standard 90 CFM fan.
For bathrooms larger than 100 sq ft, the per-square-foot rule under-sizes the fan, so the calculator switches to the HVI 8 air changes per hour (ACH) method, which accounts for ceiling height: CFM = (square feet × ceiling height × 8) ÷ 60. A 150 sq ft bathroom with 8-foot ceilings needs (150 × 8 × 8) ÷ 60 = 160 CFM, which rounds up to a standard 200 CFM fan.
Finally, if the bathroom has a shower or tub, the required CFM is raised to a minimum of 50 CFM regardless of what the math says. The recommended fan is then the smallest standard size (50, 80, 90, 110, 150, or 200 CFM) that covers the requirement. If the requirement exceeds 200 CFM, two fans are the practical answer.
These are industry planning guidelines from the Home Ventilating Institute, not building code requirements. Duct length, bends, and the fan's sones (noise rating) also matter — always check your local code and the fan's installed performance, and consider a licensed contractor for final sizing.
Frequently asked questions
What size bathroom exhaust fan do I need for a 5x8 bathroom?
A 5x8 bathroom is 40 sq ft, which needs about 44 CFM by the 1.1-CFM-per-square-foot rule. A standard 50 CFM fan covers it comfortably — and 50 CFM is the practical minimum for any bathroom with a shower or tub.
How many CFM do I need per square foot of bathroom?
For bathrooms up to 100 square feet, the Home Ventilating Institute (HVI) guideline is 1.1 CFM per square foot. For larger bathrooms, sizing switches to an air-changes method: 8 air changes per hour, which is CFM = (square feet x ceiling height x 8) / 60.
What is the 50 CFM minimum for bathroom fans?
If the bathroom has a shower or tub, use at least a 50 CFM fan even if the square-footage math says less. A half bath with only a toilet can use a smaller fan, but any bathing fixture produces enough steam to justify the 50 CFM minimum.
Are these bathroom fan sizes building code requirements?
No. These are HVI industry guidelines for good ventilation, not code requirements. Local building codes set minimum ventilation rules, which can differ — always check your local code or a licensed contractor before buying.
Can a bathroom exhaust fan be too powerful?
Oversizing a little is fine — a 110 CFM fan in a room that needs 88 CFM clears steam faster. But a fan far larger than needed wastes energy, can be loud, and may pull conditioned air out of your home. Sizing up one standard size is sensible; sizing up three is not.