Bathroom exhaust fan sizing
Calculator
Calculator
Required airflow from room volume and air changes, including minimum bathroom or kitchen airflow.
Net airflow is the greater of volume × ACH and the reference minimum; selection airflow = net airflow × (1 + reserve).
Hazardous fumes, heat, commercial kitchens, and process ventilation require specialist design.
This is a planning and comparison estimate. Final protection and conductor selection requires review by a licensed engineer or electrician, site-condition checks, manufacturer instructions, and the applicable code.
Calculation guide
The calculator finds room volume and multiplies it by the selected air changes per hour. It then compares the result with a bathroom reference minimum and uses the larger airflow before adding an editable selection reserve.
Free-air capacity on a box is not enough. Duct length, bends, backdraft dampers, and external grilles add pressure, so the selected fan must deliver the required airflow at the actual operating pressure.
Frequently asked questions
The calculator checks dimensions and duty and applies the HVI reference minimum of 50 CFM for intermittent exhaust in smaller bathrooms.
Nominal diameter does not prove capacity. Compare certified CFM or m³/h at operating pressure, sound level, and duct resistance.
Common causes include long or narrow ducting, too many bends, a blocked outlet, insufficient make-up air, or using free-air rather than pressure-rated airflow.
Use the shortest practical outdoor discharge path. Ceiling fans suit overhead ducts; direct wall discharge can reduce resistance where a safe outlet is available.
