What extract rate does a bathroom need?

Under Approved Document F (2021), a bathroom or shower room requires an intermittent extract rate of 8 litres per second (l/s). The fan should be capable of achieving this rate at the point of extract, accounting for duct resistance. If ducted more than 1.5m, you may need a higher-rated fan to overcome duct losses. A 15-minute overrun timer is also required.

What is the difference between intermittent and continuous ventilation?

Intermittent (or boost) ventilation is the higher rate that operates when the room is in use (e.g., while showering or cooking). Continuous (or background) ventilation runs at a lower rate 24/7 to maintain air quality throughout the dwelling. Most homes use a combination: trickle vents or MEV for continuous ventilation, and extract fans for intermittent boost.

What size trickle vents do I need?

Under Approved Document F (2021), habitable rooms (bedrooms, living rooms) need trickle vents with an equivalent area of 8000 mm², while wet rooms (kitchens, bathrooms, WCs, utility rooms) need 4000 mm². These are total equivalent areas per room and can be achieved with one or more trickle vents in the windows.

What is the continuous ventilation rate for my home?

The whole-dwelling continuous ventilation rate depends on the number of bedrooms: 1 bed = 13 l/s, 2 bed = 17 l/s, 3 bed = 21 l/s, 4 bed = 25 l/s, 5 bed = 29 l/s. Alternatively, calculate as 0.3 l/s per m² of internal floor area and use whichever is higher. This can be achieved through MEV, MVHR, or passive stack ventilation.

Do I need a fan with a timer or humidistat?

Approved Document F requires bathroom extract fans to have a 15-minute overrun timer to continue extracting moisture after use. A humidistat (humidity sensor) is an alternative that runs the fan when moisture is detected and stops when humidity drops. For kitchens, the fan should operate while cooking is in progress. Cooker hoods should extract at 30 l/s when positioned directly over the hob.

TradeCalcs

Ventilation Rate Calculator — Extract Rates per Document F

Calculate required extract ventilation rates for kitchens, bathrooms, WCs and habitable rooms per UK Building Regulations Approved Document F (2021).

Room Type

Select the room you are calculating ventilation for

Property Type

For whole-dwelling continuous ventilation rate

Room Length

Internal room length

Room Width

Internal room width

Room Height

Floor to ceiling height

Number of Wet Rooms

Total wet rooms in the property (for context)

How We Calculate This

This calculator determines the required ventilation rates for individual rooms based on UK Building Regulations Approved Document F (2021 edition). It covers both intermittent extract rates and whole-dwelling continuous ventilation.

Intermittent extract rates (Document F 2021)

Kitchen (general): 13 l/s
Kitchen (cooker hood): 30 l/s
Bathroom / shower room: 8 l/s
WC / cloakroom: 6 l/s
Utility room: 8 l/s
Habitable rooms: No extract required (trickle vents only)

Whole dwelling continuous rates

1 bedroom: 13 l/s
2 bedrooms: 17 l/s
3 bedrooms: 21 l/s
4 bedrooms: 25 l/s
5+ bedrooms: 29 l/s
Or 0.3 l/s per m² of floor area (whichever is higher)

Trickle vent equivalent areas

Habitable rooms: 8000 mm²
Wet rooms: 4000 mm²

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.