Ventilation Rate Calculator
Enter a room's dimensions and its extract flow to work out the air changes per hour, and check it against the Building Regulations Part F minimums. Free, no sign-up.
Air changes per hour tells you how often the room's air is fully replaced. Higher ventilation carries away the moisture that drives condensation and mould, so under-ventilated rooms are where problems start.
Building Regs (Part F) intermittent extract minimums
- Kitchen (intermittent extract)30 l/s adjacent to hob, or 60 l/s elsewhere
- Utility room30 l/s
- Bathroom15 l/s
- WC6 l/s
Diagnose ventilation on site.
SurveyMate captures ventilation provision alongside temperature and humidity readings, so the condensation risk is evidenced in the report.
See the inspection appGuidance only. Reference rates are the Approved Document F intermittent extract minimums for England; check the current regulations and your local requirements for a formal assessment.
Ventilation is half the battle against mould
Cooking, washing, drying clothes and simply breathing add litres of water to indoor air every day. If that moisture is not ventilated out, humidity rises until it condenses on the coldest surfaces, and mould takes hold. Adequate ventilation, especially extract at source in kitchens and bathrooms, is one of the two levers, alongside surface temperature, that control condensation.
Use this with the condensation risk calculator and the U-value calculator to assess both sides of the problem, and see the damp and mould guide for the fundamentals.
Ventilation calculator FAQs
What is a good number of air changes per hour?
It depends on the room and its use. Wet rooms such as kitchens and bathrooms need enough extract to remove moisture at source, which is why Building Regulations Part F sets minimum extract rates for them. For damp and mould control, the goal is enough ventilation to keep humidity down without over-cooling the room.
How is air changes per hour calculated?
Multiply the extract flow in litres per second by 3.6 to get cubic metres per hour, then divide by the room volume (length times width times height). The result is how many times the room's air is replaced each hour.
How does ventilation prevent mould?
Everyday activities add a lot of moisture to indoor air. Ventilation carries that moisture outside before it condenses on cold surfaces. Under-ventilated rooms trap humidity, which is why condensation and mould so often appear in sealed-up bathrooms and bedrooms.