Cooker Hood Extraction Calculator — Kitchen Ventilation

This calculator determines the required cooker hood extraction rate based on Approved Document F minimum rates, adjusted for hob type, kitchen layout, and hood type. It also calculates the duct size and make-up air requirements.

Base extraction rates (ADF 2021)

Intermittent extract rates come from Table 1.1; the continuous (background) extract rate comes from Table 1.2 — the two are separate tables, so the figures are not interchangeable.

• Ducted cooker hood directly above hob (Table 1.1, adjacent-to-hob row): 30 l/s (108 m³/h) intermittent minimum. Manufacturers typically oversize to 60+ l/s to allow for duct losses.
• Recirculating cooker hood or no hood (Table 1.1, elsewhere-in-kitchen row): 60 l/s (216 m³/h) intermittent minimum. A recirculating hood does not satisfy ADF on its own — a separate extract path is still required.
• Continuous mechanical extract (Table 1.2, minimum high rate): 13 l/s minimum.

Adjustment factors (design margins — not ADF requirements)

Above the ADF compliance floor, the base rate is adjusted by industry rules of thumb for hob type (gas +20%, induction −10%) and kitchen layout (open-plan +30–50%). These are design margins, not regulatory uplifts — ADF specifies fixed minima, not hob-type or layout multipliers, so the un-multiplied 30/60 l/s figure is the legal floor. Gas hobs generate combustion by-products and need adequate combustion/replacement air; see the make-up air notes and the BS 5440-2 caveat for open-flued appliances.

Duct sizing

The duct diameter is calculated for an assumed conservative target velocity of 4 m/s and rounded up to the next standard size, with a floor of 150mm — the typical domestic hood outlet, which the duct must never be smaller than. Pressure drop is estimated from duct material, length, bends, grille resistance, and filter loss (for recirculating hoods); it is indicative only — confirm against the hood manufacturer’s static-pressure data.