Extractor Fan Sizing Calculator

About this calculator

Extractor fan sizing is driven by air changes per hour (ACH) — the number of times per hour a room's entire volume of air needs to be replaced, which building regulations and good-practice guides set per room type (kitchens and bathrooms typically need more changes per hour than bedrooms or living rooms). The required airflow is simply fan capacity (m³/h) = room volume (m³) × required air changes per hour.

Electricians, builders and homeowners size extractor fans this way because buying by fan diameter alone ignores the room it's actually meant to ventilate — a fan rated for a small en-suite bathroom will underperform in a larger kitchen or a bathroom with a shower enclosure, leaving condensation, odors and moisture problems that a correctly sized fan would have prevented.

This calculator takes your room's dimensions and the air changes per hour required for that room type and returns the fan airflow in m³/h you need, so you can shop for an extractor fan rated to actually clear the air in that specific room.