How this calculator works
Approved Document F asks for two kinds of ventilation. Purge ventilation must be able to clear a room at four air changes an hour, and for a window the document gives the opening you need as a fraction of the floor area: one tenth if the window opens 15 to 30 degrees, one twentieth if it opens 30 degrees or more. Background ventilation (trickle vents) is the small permanent opening for everyday air: 8,000 mm² equivalent area for a living room, bedroom or kitchen in a home with more than one floor (10,000 mm² in a single-storey one), and 4,000 mm² for a bathroom.
Worked example: a 14 m² room is 14.0 m² of floor, so a window opening 15 to 30 degrees needs 1.40 m² of opening and one opening 30 degrees or more needs 0.70 m². Four air changes an hour of the 33.6 m³ room is 37.3 l/s. It needs 8,000 mm² of background vents, which is 4 vents of 2,500 mm², or 10,000 mm² in a single-storey home. A bathroom needs 4,000 mm².
Assumptions and sources
- These are the Approved Document F Volume 1 (2021) rules for a dwelling. Purge ventilation is the extra airflow for removing steam, fumes and heat quickly, and is separate from the everyday background ventilation. A window that opens less than 15 degrees does not count for purge ventilation. The purge areas assume a single-sided room in an urban area with a light wind, and Part O (overheating) can ask for larger openings. The background vent figures apply to natural ventilation with extract fans in the wet rooms, and not to a home with only one exposed face or most of its openings on the same wall: ask an expert in that case. A vent must be marked with its equivalent area (EA), measured to BS EN 13141-1.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Purge opening and trickle vents for common rooms
A window opening 30 degrees or more, in a home with more than one floor, with 2,500 mm² vents. Ceiling height 2.4 m.
| Room | Floor area | Window opening | Four air changes an hour | Trickle vents |
|---|---|---|---|---|
| Bedroom or living room, 4 × 3.5 m | 14.0 m² | 0.70 m² | 37.3 l/s | 4 |
| Bedroom or living room, 5 × 4 m | 20.0 m² | 1.00 m² | 53.3 l/s | 4 |
| Kitchen, 3.5 × 3 m | 10.5 m² | 0.53 m² | 28.0 l/s | 4 |
| Bathroom, 2.5 × 2 m | 5.0 m² | 0.25 m² | 13.3 l/s | 2 |
Tips before you buy
- Count the opening, not the window. A window that opens 30 degrees gives a smaller opening than its frame size: measure the gap with the window at its stay.
- Trickle vents are marked with their equivalent area (EA) in mm². A slot vent of the same width can have a different EA, so read the box.
- Fit the vents in the window frame or the wall, high up to avoid draughts, and do not block them with curtains or paint.
- If you replace windows, vents must go into the new ones, or the room must be no worse ventilated than it was.
- Bathrooms and kitchens also need an extract fan: use the extract fan rate calculator.
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How big should a trickle vent be?
For a living room, bedroom or kitchen in a home with more than one floor, background vents totalling 8,000 mm² equivalent area, which is 4 vents of 2,500 mm² EA. A bathroom needs 4,000 mm², and a single-storey home 10,000 mm² for habitable rooms.
How much window opening do I need for purge ventilation?
One twentieth of the floor area if the window opens 30 degrees or more, and one tenth if it opens 15 to 30 degrees. A 14.0 m² room needs 0.70 m² or 1.40 m².
What is purge ventilation?
The extra ventilation for quickly removing steam, fumes or heat, enough for four air changes an hour. It can come from an openable window or a mechanical extract system.
What does equivalent area mean on a vent?
It is how much air the vent passes, measured to a standard, in mm². It is smaller than the physical slot. The number is printed on the vent or its data sheet.
Do I need trickle vents in a utility room or WC?
Approved Document F sets no minimum background vent area for a utility room or a separate WC (Table 1.7), but they still need extract ventilation.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.