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Extractor Fan Size Calculator

Work out the extraction airflow an exhaust fan needs for a room from its size and a target air-change rate. Handy for sizing bathroom, toilet, kitchen or utility-room extractor fans in both m³/h and L/s.

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How to use this calculator

Enter Values

Minimum 0

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the floor area of the room in m² and the ceiling height in m (2.4 m is a common default).
  2. Enter the air changes per hour (ACH) for the room type — about 15 for a bathroom, 15–30 for a kitchen, 6–10 for a toilet.
  3. Read the required airflow in m³/h and L/s, plus the room volume, and pick a fan rated at or above that figure.

How it works

The room volume is V = floor area × ceiling height. The required airflow is that volume multiplied by the number of times per hour you want the air replaced: airflow (m³/h) = V × ACH. Fan and grille ratings are often quoted in litres per second, so the tool also divides by 3.6 to convert (1 m³/h = 1/3.6 L/s ≈ 0.2778 L/s).

Worked example

Worked example. A 6 m² bathroom with a 2.4 m ceiling has a volume of 14.4 m³. At 15 air changes per hour that is 14.4 × 15 = 216 m³/h, which is 216 ÷ 3.6 = 60 L/s. So an extractor rated at 60 L/s or more suits the room.

Common mistakes

  • Using the fan's free-air (rated) flow as the delivered flow — duct runs, bends and grilles cut it, so size a little above the calculated requirement.
  • Applying a bathroom ACH to a kitchen or vice versa — the required rate depends heavily on the room type, so pick the right ACH.
  • Ignoring a code minimum: some standards set a fixed minimum extraction (e.g. L/s per room or per fixture) that can exceed the ACH-based figure.

Frequently asked questions

What ACH should I use for a bathroom?

Around 15 air changes per hour is a common design value for a bathroom or shower room. Kitchens need more (roughly 15–30 because of cooking moisture and odours) and separate WCs less (about 6–10). Always check your local building regulations, which may set a minimum extraction rate directly.

Why convert m³/h to L/s?

Extractor fans and ventilation grilles are frequently rated in litres per second, especially under UK/Australian conventions, while the air-change calculation naturally lands in m³/h. Dividing by 3.6 converts between them (1 m³/h = 0.2778 L/s) so you can match the calculated duty to a fan's quoted L/s rating.

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Cite this tool

Zerdly. (2026). Extractor Fan Size Calculator. Retrieved from https://www.zerdly.com/tools/extractor-fan-size-calculator/

Tip: Enter any known values to calculate the remaining results.

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