Ventilation Rate Calculator UK
Check Approved Document F compliance instantly. Calculate bathroom extract fan size, kitchen ventilation airflow, whole house ventilation rates, trickle vent requirements and ACH – all built around UK Building Regulations Part F.
UK Ventilation Rate Calculator
💡 Select a room type – required extract rate per Approved Document F is shown instantly.
🏡 Continuous whole‑dwelling ventilation rate per Approved Document F (0.3 l/s per m²).
🔄 Enter room volume and airflow rate to get Air Changes Per Hour.
📘 Ventilation Rate Formula (ACH)
The standard engineering formula for Air Changes Per Hour used in UK ventilation design is:
Where:
- Q = airflow rate in litres per second (l/s)
- V = room volume in cubic metres (m³)
- 3600 = seconds per hour conversion
For UK compliance, Approved Document F specifies minimum extract rates (l/s) rather than ACH, but both metrics help assess ventilation adequacy. Our calculator applies these rates automatically.
📜 Approved Document F – UK Building Regulations Part F
Approved Document F (Part F of the Building Regulations) sets the legal minimum ventilation standards for new dwellings and major refurbishments in England. It covers whole dwelling ventilation, intermittent extract, background ventilation (trickle vents), and purge ventilation to safeguard indoor air quality and prevent condensation.
Key Requirements at a Glance
| Ventilation Type | Requirement | Notes |
|---|---|---|
| Whole dwelling (continuous MEV/MVHR) | 0.3 l/s per m² floor area | Operates continuously; boost in wet rooms |
| Intermittent extract – kitchen | 60 l/s (or 30 l/s if cooker hood) | Adjacent to hob |
| Intermittent extract – bathroom | 15 l/s | With bath and/or shower |
| Intermittent extract – utility room | 30 l/s | Where moisture/chemicals are produced |
| Intermittent extract – WC | 6 l/s | Sanitary accommodation only |
| Background ventilators (trickle vents) | Size based on floor area & room type | See AD F Table 1.2 for equivalent area |
| Purge ventilation | 4 ACH per habitable room or mechanical equivalent | Openable windows or boosted mechanical rate |
🏡 Whole House Ventilation Calculation
For continuously operated mechanical ventilation systems (MEV, MVHR, dMEV), the minimum whole‑dwelling airflow is given by:
For a typical 3‑bedroom house of 120 m², this gives 36 l/s (129.6 m³/h). In addition, intermittent boost rates for kitchens (60 l/s) and bathrooms (15 l/s) must be available when needed. Our whole house ventilation calculator at the top of this page provides instant results.
Worked Example – New Build Semi‑Detached
A 4‑bedroom house with 140 m² floor area requires a continuous ventilation rate of 0.3 × 140 = 42 l/s. If using an MVHR system, it should be sized to deliver at least this normal continuous rate, with boost capability for simultaneous kitchen and bathroom extract.
🚿 Bathroom & Kitchen Extract Fan Requirements UK
UK Building Regulations set precise intermittent extract rates to remove moisture and odours quickly.
Bathroom Extractor Fan Sizing
An intermittent bathroom fan must extract 15 l/s (54 m³/h). If the bathroom is large, a boost timer and adequate background ventilation (trickle vents) must be provided. For en‑suites and shower rooms, the same 15 l/s applies.
Kitchen Extractor Fan / Cooker Hood
- Kitchen without cooker hood: 60 l/s intermittent extract (wall/ceiling fan)
- Kitchen with ducted cooker hood: 30 l/s when hood is operating at normal speed
- Purge ventilation (boost) must provide at least 4 ACH in the kitchen
🪟 Trickle Vents & Background Ventilation
Background ventilation ensures a continuous supply of fresh air without draughts. Trickle vents in windows or walls are sized according to Approved Document F Table 1.2. For a habitable room with a floor area up to 10 m², the equivalent area should be at least 8000 mm². For larger rooms, the area increases proportionally. In whole‑house mechanical systems, background ventilators may be omitted if the system provides adequate continuous airflow.
⚙️ MVHR & MEV Systems – UK Guidance
Mechanical Ventilation with Heat Recovery (MVHR) and Mechanical Extract Ventilation (MEV) are the two main continuous systems used in UK low‑energy homes.
- MVHR: Supplies fresh filtered air to living rooms and bedrooms while extracting from wet rooms. Up to 90% heat recovery. Requires airtight building envelope.
- MEV: Central extract fan ducted from kitchens/bathrooms; background ventilators supply make‑up air. Simpler to install, lower capital cost.
Both must meet the whole‑dwelling rate of 0.3 l/s per m² continuously, with boost on demand. Our calculator helps size the base rate.
📋 UK Domestic Ventilation Worked Examples
1. Bathroom Extractor Fan – 3m × 2m × 2.4m
Volume = 14.4 m³. Required extract = 15 l/s. ACH = (15 × 3.6) / 14.4 = 3.75 ACH. (Still meets moisture removal when operated for adequate time.)
2. Kitchen with Cooker Hood – 4m × 3m × 2.4m
Volume = 28.8 m³. Hood rate = 30 l/s. ACH = (30 × 3.6) / 28.8 = 3.75 ACH. Purge boost to 4 ACH requires 32 l/s.
3. Whole House MVHR for 110 m² – 3 bedrooms
Continuous rate = 0.3 × 110 = 33 l/s. Select MVHR unit with at least 33 l/s normal capacity, boost to 60 l/s for kitchen + 15 l/s for bathroom simultaneously.