Air Changes Calculator — Required ACH Per Room Type

Air changes per hour (ACH) is the number of times the entire volume of air in a room is replaced with fresh air in one hour. For example, 6 ACH in a 30m³ bathroom means 180m³ of air is extracted per hour (or 50 l/s). ACH is a common design measure used by building services engineers (CIBSE/ASHRAE guidance) to size ventilation for different room types. Note that UK Building Regulations Approved Document F sets dwelling ventilation as a litres-per-second extract rate, not as ACH.

For a UK domestic kitchen the legal minimum is set by Approved Document F 2021 as an extract rate, not as an air-change figure. Table 1.1 (intermittent) requires 30 l/s adjacent to the hob (a ducted cooker hood over the hob) or 60 l/s elsewhere in the kitchen (no hood, or a recirculating hood that does not extract to outside); Table 1.2 (continuous) requires 13 l/s. As a sizing rule of thumb, domestic kitchens are often designed around 10-15 ACH, but the l/s figure is what the regulations actually require. Commercial kitchens typically use much higher rates (around 20-40 ACH) to handle heat and grease-laden air — confirm against CIBSE/manufacturer guidance.

Approved Document F (2021 edition) Tables 1.1 and 1.2 specify that bathrooms and shower rooms require a minimum intermittent extract rate of 15 l/s, or a continuous rate of 8 l/s. At 15 l/s intermittent, a small ensuite (6m³) achieves about 9 ACH, while a larger family bathroom (15m³) achieves about 3.6 ACH. With continuous extract at 8 l/s, the ACH values are lower but run constantly.

Approved Document F is a legal requirement under UK Building Regulations and sets minimum ventilation rates for dwellings as litres-per-second extract figures (Table 1.1 intermittent, Table 1.2 continuous). CIBSE Guide A provides recommended design rates for both domestic and commercial buildings and is used as design guidance by building services engineers. For domestic work, Part F l/s extract rates are the legal minimum; for commercial buildings, CIBSE/ASHRAE design guidance is the primary reference. The ACH ranges in this calculator are typical engineering design values for sizing, while the Domestic Kitchen option returns the Part F 30 l/s legal figure directly.

To convert ACH to l/s: multiply the room volume (m³) by the ACH rate, then divide by 3600 (seconds in an hour), then multiply by 1000 (litres per m³). Or more simply: Volume x ACH / 3.6 = l/s. For example, a 40m³ room needing 6 ACH: 40 x 6 / 3.6 = 66.7 l/s. Our calculator does this conversion automatically.