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Extractor Fan Sizing Calculator

Enter the room dimensions and required air changes per hour to size your extractor fan.

How it is calculated

The room is treated as a box: volume (m³) = length × width × height, with every dimension in metres. The airflow the fan must move is airflow (m³/h) = volume × ACH, where ACH is the number of complete air changes per hour you want. The CFM figure is the same flow converted to cubic feet per minute: CFM = m³/h × 0.5885778. The airflow is shown to one decimal, the volume to two. All four inputs must be above zero. The tool does not choose an ACH for you and does not allow for duct length, bends, grilles or fan pressure loss, so treat the result as the minimum free-air rating to compare with a fan's datasheet.

Worked example

A bathroom 3.5 m long, 2.4 m wide and 2.4 m high, aiming for 8 air changes per hour. Volume: 3.5 × 2.4 × 2.4 = 20.16 m³. Airflow: 20.16 × 8 = 161.3 m³/h (161.28 before rounding). In imperial units: 161.28 × 0.5885778 = 94.9 CFM. A fan whose rated capacity is at or above about 162 m³/h would meet this target.

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Common mistakes

  • Entering the room in centimetres or feet: the three dimension fields are metres, so 240 for the height gives a result a hundred times too large.
  • Mixing up m³/h and CFM when reading a fan datasheet; 100 CFM is only about 170 m³/h, so compare the same unit.
  • Using a fan's maximum free-air figure as if it were its real output: ducting, bends and external grilles reduce it, so choose a fan with some margin.
  • Entering the room's floor area instead of its volume dimensions, or leaving out the height, which changes the result directly.
  • Using one ACH value for every room: a kitchen and a bedroom need different rates, and the tool uses whatever number you type.

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.

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