What are air changes per hour (ACH)?

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 the standard measure used in UK Building Regulations and CIBSE guides to specify ventilation requirements for different room types.

What ACH does a UK kitchen require?

UK domestic kitchens typically require 10-15 air changes per hour per CIBSE Guide A, with 12 ACH being a common design value. However, Approved Document F specifies minimum extract rates rather than ACH: 13 l/s intermittent (or 60 l/s for a cooker hood) for a domestic kitchen. For commercial kitchens, much higher rates of 20-30 ACH are common to handle heat and grease-laden air.

How does Part F specify ventilation for bathrooms?

Approved Document F (2021 edition) Table 5.1a specifies 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.

What is the difference between CIBSE Guide A and Part F?

Approved Document F is a legal requirement under UK Building Regulations and sets minimum ventilation rates for dwellings. CIBSE Guide A provides recommended rates for both domestic and commercial buildings and is used as design guidance by building services engineers. For domestic work, Part F is the legal minimum; for commercial buildings, CIBSE Guide A rates are the primary reference. This calculator provides rates from both sources where applicable.

How do I convert ACH to litres per second?

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.

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Air Changes Calculator — Required ACH Per Room Type

Calculate the required air changes per hour (ACH) for any room type in the UK. Based on Approved Document F and CIBSE Guide A recommendations.

Room Type

Select the room type for ACH lookup

Room Length

Internal room length

Room Width

Internal room width

Ceiling Height

Typical UK domestic: 2.4m

How We Calculate This

This calculator determines the required ventilation rate for any room type by looking up the recommended air changes per hour (ACH) from CIBSE Guide A and Approved Document F, then multiplying by the room volume.

Calculation formula

Total air volume (m3/h) = Room volume (m3) x Air changes per hour (ACH)

Total air volume (l/s) = Total air volume (m3/h) / 3.6

Data sources

ACH rates are taken from CIBSE Guide A Table 1.5 for commercial and institutional buildings, and from Approved Document F (2021 edition) Tables 5.1a and 5.1b for domestic dwellings. For specialist spaces such as server rooms, hospital wards, and laboratories, rates are drawn from the relevant HTM, BB, or CIBSE TM documents.

The calculator provides minimum, typical, and maximum ACH values for each room type. The typical value is used by default, but you can override it with a custom ACH rate in the advanced options.