Free UK construction calculators Metric and imperial 2026 UK prices
Work out the ventilation a room or home needs under Approved Document F. Choose an extract fan for one room or the whole home, enter the size, and press Calculate to see the rate in litres per second, cubic metres per hour and air changes per hour.
Minimum Ventilation Rate
0l/s
Check the box above. The result updates once it looks right.
| Room | Intermittent | Continuous | m³/h |
|---|
Minimum rates from Approved Document F 2021, Volume 1: Dwellings. 1 l/s = 3.6 m³/h.
You get the minimum rate in litres per second (l/s), the same in cubic metres per hour (m³/h), which is how most fans are sold, and the air changes per hour for the room or home. A drawing shows the air in and out, a table lists the Approved Document F rates, and you can copy the result or save it as a PDF.
In England, Approved Document F (2021 edition, Volume 1: Dwellings) sets the minimum extract rates for kitchens, bathrooms, utility rooms and WCs:
| Room | Intermittent Fan | Continuous Fan | Intermittent In m³/h |
|---|---|---|---|
| Kitchen, cooker hood over the hob | 30 l/s | 13 l/s | 108 m³/h |
| Kitchen, fan elsewhere | 60 l/s | 13 l/s | 216 m³/h |
| Utility room | 30 l/s | 8 l/s | 108 m³/h |
| Bathroom or shower room | 15 l/s | 8 l/s | 54 m³/h |
| WC only | 6 l/s | 6 l/s | 22 m³/h |
A kitchen fan away from the hob needs twice the rate of a cooker hood over it, because a hood catches steam and fumes at the source. The hood must extract to outside: a recirculating hood filters the air and puts it back, so it does not count as extract ventilation. A continuous fan runs at the lower rate all the time and needs a boost setting for when the room is in use.
The whole home rate is the fresh air the home needs all the time to keep moisture and pollutants down. Under Approved Document F 2021 it is the greater of a figure based on the number of bedrooms and 0.3 l/s for each square metre of internal floor area, counting every floor.
| Bedrooms | Rate (l/s) | Rate (m³/h) |
|---|---|---|
| 1 | 19 | 68 |
| 2 | 25 | 90 |
| 3 | 31 | 112 |
| 4 | 37 | 133 |
| 5 | 43 | 155 |
The table assumes two people in the main bedroom and one in each other bedroom. For more than five bedrooms, the calculator continues the table’s pattern of 6 l/s per bedroom; check Approved Document F for larger homes. Zehnder’s summary of the regulations compares the 2021 figures with the older ones, which were lower.
m³/h = l/s × 3.6 · air changes per hour = m³/h ÷ room volume (m³)
Building Regulations give rates in litres per second, but most fans are sold in cubic metres per hour, so multiply by 3.6. A bathroom fan of 15 l/s must move at least 54 m³/h. Air changes per hour show how many times the air in the room is replaced each hour: 54 m³/h in a 12 m³ bathroom is 4.5 air changes an hour. Approved Document F does not set air changes for homes, but they are a useful check on how hard a fan is working in a large or small room.
In whole home mode, the calculator gives the total continuous extract and the set of intermittent fans for your home, so you can compare the two approaches.
Ventilation affects heat loss too: see our heat loss calculator for the heat a room needs, including the air leaving through fans and vents.
Bathroom fan: a 2.5 × 2 m bathroom, 2.4 m high, with an intermittent fan.
Whole home: a three-bedroom house of 90 m² with one bathroom and a separate WC.
Room rates come straight from the Approved Document F 2021 tables for intermittent and continuous extract. The whole home rate is the larger of the bedroom figure and 0.3 l/s per m² of floor area. Continuous extract adds 13 l/s for the kitchen, 8 for each bathroom and utility room and 6 for each separate WC, and the system total is the larger of that and the whole home rate. Rates are converted at 3.6 m³/h per l/s, and air changes use the room or home volume from your ceiling height. These are minimum rates for England; Wales, Scotland and Northern Ireland have their own guidance.
At least 15 l/s, or 54 m³/h, for an intermittent fan under Approved Document F 2021, or 8 l/s for a continuous fan.
30 l/s (108 m³/h) for a cooker hood over the hob that extracts to outside, or 60 l/s (216 m³/h) for a fan elsewhere in the kitchen.
Multiply by 3.6. 15 l/s is 54 m³/h and 30 l/s is 108 m³/h.
At least 31 l/s, or 0.3 l/s per m² of floor area if that is more.
No. It does not extract air to outside, so the kitchen still needs extract ventilation.
Where the room has no openable window, an intermittent fan should run on for 15 minutes after use.
Approved Document F sets rates in l/s rather than air changes. A 15 l/s fan in a typical 12 m³ bathroom gives about 4.5 air changes per hour.
30 l/s intermittent or 8 l/s continuous.
Extract and whole home rates from Approved Document F 2021, Volume 1: Dwellings, as summarised by Zehnder and Nuaire. Reviewed November 2026.