Skip to main content
Tool Belt Calc

HVAC

Bathroom Fan CFM Calculator

Extractor fan size for a bathroom, by both common rules.

Your measurementsEvery assumption editable01

Eight per hour is the usual residential bathroom target.

Jetted tubs and separate enclosed toilets are often given their own allowance.

Fan sizeCalculated live02

Result

70CFM

A 70 CFM fan or larger

Bathroom volume60 sq ft × 8 ft
480 cu ft
By air changes8 ACH
64 CFM
By floor area rule1 CFM per sq ft, minimum 50
60 CFM
Recommendedthe larger of the two rules, rounded up
70 CFM

The fan's rating is measured at zero static pressure and is not what it delivers in your ceiling. A long duct run, flexible ducting and a termination flap can easily cut real airflow by a third, so buying at the calculated figure rather than above it often leaves the room under-ventilated.

The fan needs somewhere to draw air from. A bathroom with a well-sealed door and no undercut will not move its rated airflow no matter what fan is fitted, because the room cannot replace the air being removed.

Duct to the outside, never into a loft or soffit. Warm, wet air discharged into a roof space is one of the most reliable ways to grow mould in a place nobody looks.

Enter the bathroom dimensions and this applies both standard rules — eight air changes an hour, and one CFM per square foot with a fifty CFM floor — then takes the larger. Fans are rated at zero static pressure, so buying at the calculated figure usually leaves a room under-ventilated.

Why use this tool?What it does differently03

Why use this tool?

Both rules applied

The air-changes method and the floor-area method, with the larger taken rather than whichever happens to suit.

Fixture allowances

Jetted tubs and enclosed toilets are conventionally given their own 50 CFM each.

Honest about ratings

Rated CFM is measured at zero static pressure. Real ducting can cut delivered airflow by a third.

Make-up air flagged

A sealed door with no undercut throttles any fan, whatever it is rated at.

How this worksThe method04

How this bathroom fan cfm calculator works

Two rules are calculated. The volume method multiplies floor area by ceiling height and applies eight air changes per hour, divided by sixty for CFM. The area method allows one CFM per square foot with a fifty CFM minimum.

The larger of the two is taken, extra fixture allowances are added, and the result rounds up to the nearest ten CFM.

What the arithmetic cannot capture is that a fan's rating is measured at zero static pressure. A long duct run, flexible ducting and a termination flap can easily cut real airflow by a third, which is why buying above the calculated figure rather than at it is sensible.

How to use itStep by step05

How to use it

  1. Step 1: Measure the bathroom

    Floor area and ceiling height.

  2. Step 2: Count extra fixtures

    Jetted tubs and separate enclosed toilets are conventionally given 50 CFM each.

  3. Step 3: Buy above the figure

    Duct losses are real. A fan rated at the calculated number will deliver less than it.

  4. Step 4: Check the make-up air

    The room needs an undercut or grille so air can replace what is being removed.

Example usageWorked figures06

Example usage

A standard bathroom
60 sq ft with 8 ft ceilings is 480 cubic feet — 64 CFM by air changes, 60 by floor area. The larger governs, rounding to 70 CFM.
A small cloakroom
20 sq ft gives 21 CFM by volume and 50 by the area rule's minimum. The 50 CFM floor governs, which is why it exists.
A large ensuite with a jetted tub
120 sq ft at 9 ft is 1,080 cubic feet — 144 CFM by air changes, plus 50 for the tub, rounding to 200 CFM.
Frequently asked questionsCommon questions07

Frequently asked questions

What size bathroom fan do I need?

For a standard bathroom, one CFM per square foot with a 50 CFM minimum, or eight air changes per hour — whichever is larger. A typical 60 square foot bathroom needs around 70 CFM.

Is a bigger fan always better?

Up to a point. Oversized fans are noisier and pull conditioned air out of the house faster than needed. Buying one size above the calculation covers duct losses without overdoing it.

Why does my fan not seem to work?

Usually duct losses or no make-up air. Ratings are measured at zero static pressure, and a sealed door with no undercut prevents air replacing what is removed.

Where should the fan duct terminate?

Outside, always — through the roof or a wall. Discharging into a loft or soffit puts warm wet air into a space nobody inspects, which is a reliable way to grow mould.

How long should the fan run after a shower?

Around twenty minutes, which is why timer switches and humidity-sensing fans exist. Switching it off with the light leaves most of the moisture in the room.

Related toolsElsewhere on the site08

Browse every tool in HVAC Calculators.