To size an extractor fan you need two numbers: how much air the room needs moving (in m³/h or l/s) and how much resistance the ductwork will put in the fan's way. Get the first from the room volume and a sensible air change rate, check it against the Building Regulations minimum, then add a margin for the duct run before you pick a fan from its performance curve, not its headline figure.
Quick answer: Required extract (m³/h) = room volume (length × width × height, in m³) × air changes per hour. Divide m³/h by 3.6 to get l/s. Typical design rates are 4 to 6 air changes per hour for offices, 6 to 15 for toilets and washrooms, at least 30 for commercial kitchens and 3 to 6 for warehouses. Then add 20 to 30% for duct resistance, because a fan's catalogue airflow is measured in free air and drops as pressure rises. Example: a 60 m² office with a 2.7 m ceiling is 162 m³; at 4 air changes that is 648 m³/h (180 l/s), which suits a 200 mm in-line fan such as the S&P TD-800/200 Silent (up to 910 m³/h, £400.03) or, for longer duct runs, the Vent-Axia ACM200 (up to 1044 m³/h, £380.03). Always check the result against Approved Document F, which sets legal minimums such as 6 l/s per WC and 10 l/s per person in offices.
In this guide
The extractor fan sizing formula
The air change method is the quickest way to size general extract for a room. It works in three steps.
- Work out the room volume. Length (m) × width (m) × height (m) = volume in m³. Use the height to the ceiling (or to the underside of the roof for open warehouses), not to a suspended ceiling grid if the void is not being ventilated.
- Multiply by the air change rate. Volume (m³) × air changes per hour (ACH) = required airflow in m³/h.
- Add a margin for the system. Increase the figure by 20 to 30% for a typical ducted system so the fan still delivers the target once grilles, bends, ducting and the external louvre or cowl are added. A wall or roof fan discharging straight outside needs less margin.
The formula: Required airflow (m³/h) = L × W × H × ACH
Worked example: A meeting room 6 m × 5 m × 2.7 m = 81 m³. At 10 air changes per hour: 81 × 10 = 810 m³/h. In litres per second: 810 ÷ 3.6 = 225 l/s. With a 25% duct allowance, select a fan that delivers about 1,000 m³/h at the system's pressure.
The air change method gives you a practical design figure. It is not the only test. Building Regulations Approved Document F sets minimum rates per person, per WC or per appliance, and the higher of the two figures should win. For commercial kitchens and server rooms, the airflow is really set by heat and fumes from the equipment, so treat air changes as a sense check there (more on that below).
Converting m³/h, l/s and m³/s
UK regulations quote extract rates in litres per second (l/s), while most fan datasheets quote m³/h. Mixing them up is one of the most common sizing errors.
| To convert | Do this | Example |
|---|---|---|
| m³/h to l/s | Divide by 3.6 | 648 m³/h ÷ 3.6 = 180 l/s |
| l/s to m³/h | Multiply by 3.6 | 60 l/s × 3.6 = 216 m³/h |
| m³/s to m³/h | Multiply by 3,600 | 0.5 m³/s × 3,600 = 1,800 m³/h |
| l/s to m³/s | Divide by 1,000 | 180 l/s = 0.18 m³/s |
| CFM to m³/h | Multiply by 1.699 | 500 CFM × 1.699 = 850 m³/h |
Air change rates by commercial space
The figures below are typical design ranges widely used in UK practice, drawn from the Vent-Axia Ventilation Design Guidelines and the rule-of-thumb rates commonly quoted alongside CIBSE Guide B. Where sources differ we give a range. Use the lower end for lightly occupied spaces and the upper end where there is heat, moisture, odour or a high density of people. Approved Document F (2021) and, where it applies, HSE guidance set the legal and safety minimums.
| Space | Typical air changes per hour | Notes and governing guidance |
|---|---|---|
| Offices (open plan and cellular) | 4 to 6 (up to 10 for dense layouts) | Approved Document F Volume 2 minimum is 10 l/s per person or 1 l/s per m² floor area, whichever is higher. Add 20 l/s per printer or copier while in use. |
| Meeting and conference rooms | 8 to 12 | High occupancy for short periods. Check against 10 l/s per person. |
| Toilets and washrooms | 6 to 15 | Approved Document F Volume 2 minimum is 6 l/s per WC pan or urinal and 15 l/s per shower or bath. |
| Commercial kitchens | 30 minimum (often 40+) | Size on the canopy and appliances to BESA DW/172 and HSE Catering Information Sheet 10. Use air changes as a cross-check only. |
| Workshops and factories | 8 to 10 | General ventilation only. Dust, fume or solvent sources need local exhaust ventilation (LEV) designed to HSE HSG258. |
| Warehouses and stores | 3 to 6 | Lower end for ambient storage with few staff, higher for warm roofs, forklift charging or summer heat gain. |
| Plant and boiler rooms | 15 to 30 | Usually set by heat rejection. Combustion and ventilation air for gas plant must follow the appliance manufacturer and the relevant standard (for example BS 6644). |
| Changing rooms | 6 to 10 (15 to 20 in shower areas) | Check against 15 l/s per shower. |
| Classrooms | 5 to 7 | Schools in England are designed to DfE Building Bulletin 101 (BB101), which sets per-person rates and CO₂ limits. See our schools guide. |
| Retail, shops and supermarkets | 8 to 15 | Varies widely with footfall and lighting heat. Check against 10 l/s per person at peak occupancy. |
| Server and comms rooms | Not sized on air changes | Size on heat load: airflow (m³/s) ≈ heat load (kW) ÷ (1.2 × temperature rise in °C). A 2 kW rack with a 10 °C rise needs about 0.17 m³/s, or 600 m³/h. Larger loads usually need cooling, not just extract. |
Part F extract rates you must meet
Approved Document F (2021 edition) sets minimum extract rates. Volume 1 covers dwellings; Volume 2 covers buildings other than dwellings and applies the same figures to kitchenettes and sanitary areas in offices and similar buildings. These are minimums: your air change figure can be higher, but it should never be lower.
| Room | Minimum intermittent extract | Equivalent in m³/h |
|---|---|---|
| Kitchen with cooker hood extracting outside (Vol 1) or extract adjacent to hob (Vol 2) | 30 l/s | 108 m³/h |
| Kitchen with no external cooker hood (Vol 1) or extract remote from hob (Vol 2) | 60 l/s | 216 m³/h |
| Utility room | 30 l/s | 108 m³/h |
| Bathroom, or per shower or bath | 15 l/s | 54 m³/h |
| WC / sanitary accommodation, or per WC pan or urinal | 6 l/s | 21.6 m³/h |
| Microwave and drinks-only tea point (Vol 2) | 15 l/s | 54 m³/h |
For a full explanation of what Part F requires, including continuous extract and background ventilation, see our guide to Building Regulations Part F.
Four worked examples with fan picks
Each example follows the same steps: volume, air change rate, Part F check, duct margin, then a fan whose rated output comfortably covers the design figure. Prices are current at the time of writing.
1. A 60 m² open-plan office
- Volume: 10 m × 6 m × 2.7 m = 162 m³
- Air change rate: 4 ACH gives 162 × 4 = 648 m³/h (180 l/s)
- Part F check: 8 staff × 10 l/s = 80 l/s (288 m³/h), and 1 l/s per m² = 60 l/s. The air change figure of 180 l/s is higher, so it governs.
- Duct margin: ducted through a ceiling void to a wall grille, add 25%: about 810 m³/h target.
Best for quiet office extract: S&P TD-800/200 Silent £400.03
Sound-insulated 200 mm mixed-flow fan, up to 910 m³/h free air, 28 dB(A) at 3 m, 102 W, speed controllable. The low profile fits in a ceiling void. On a longer or more restrictive duct run, step up to the Vent-Axia ACM200 (up to 1044 m³/h, £380.03). View the S&P TD-800/200 Silent.
2. A 12-cover café kitchen
- Volume: 4 m × 3 m × 2.7 m = 32.4 m³
- Air change rate: 30 to 40 ACH gives 972 to 1,296 m³/h
- Canopy check: a commercial kitchen should be sized on the canopy and the cooking appliances under it, following DW/172. Use the larger of the canopy figure and the air change figure.
- Duct margin: grease filters, canopy and a duct run to atmosphere add significant resistance, so allow at least 25%: about 1,600 m³/h target.
Best for small commercial kitchens: Systemair MUB 315EV Multibox £791.99
Insulated box fan with a backward-curved centrifugal impeller, which holds its airflow far better against filter and duct pressure than an axial fan. Up to about 1,950 m³/h (0.54 m³/s per the Systemair datasheet), 132 W, single phase, air temperature up to 60 °C, speed controllable. Systemair recommends positioning it at least 3 m from the canopy. View the Systemair MUB 315EV.
For more on kitchen canopy extract, see our commercial kitchen extractor fan guide.
3. A 400 m² warehouse
- Volume: 20 m × 20 m × 8 m to the eaves = 3,200 m³
- Air change rate: 3 ACH gives 3,200 × 3 = 9,600 m³/h (2,667 l/s)
- Split the load: two fans of about 4,800 m³/h each, on opposite walls or high on the gable, gives better coverage than one large fan.
- Duct margin: wall fans discharging straight through a louvre have very little resistance, so a 10 to 15% margin is enough.
Best for warehouse wall extract: Systemair AW 450E4 Sileo (×2) £469.99 each
450 mm plate axial fan, up to 7,301 m³/h, 607 W, single phase 230 V, IP54, speed controllable by voltage reduction. Two units give roughly 14,600 m³/h maximum, so you can turn them down to the 9,600 m³/h design figure and boost in summer. If you need to extract through the roof, the Systemair DHS 560DS Sileo roof fan (up to 8,039 m³/h, 3 phase, £1,833.99) is the equivalent. View the Systemair AW 450E4.
Do not forget make-up air. A warehouse extracting 9,600 m³/h needs a matching inlet path, usually a weather louvre sized for low face velocity. See our weather louvre guide and where to use roof-mounted extractor fans.
4. A staff washroom block
- Room: 4 WCs and 3 urinals in a 6 m × 4 m × 2.7 m space = 64.8 m³
- Part F minimum: 7 pans or urinals × 6 l/s = 42 l/s (151 m³/h)
- Air change rate: 6 ACH gives 389 m³/h (108 l/s), which is higher than the Part F minimum, so it governs.
- Duct margin: ceiling grilles ducted to an external wall, add 25%: about 485 m³/h target.
Best for washroom extract: Vent-Axia ACM150 £238.99
150 mm in-line mixed-flow fan, up to 576 m³/h free air, around 2.5 times the pressure of a conventional axial fan, three-speed AC motor, IP44. Suits a short to medium duct run with two or three ceiling grilles. If the run is long or you want 8 to 10 air changes, use the ACM200 instead. View the Vent-Axia ACM150.
Let us match the fan for you. Our Fan Selector tool matches a fan to your airflow and pressure figures. Send us the room dimensions, use, duct size and length and the number of bends, and we will run the numbers and come back with a fan that meets the duty point, not just the headline figure. Get in touch or call 0161 399 3825.
Duct resistance and static pressure
Every fan datasheet quotes a maximum airflow, and that figure is measured in free air, with no ducting at all (0 Pa). As soon as you connect ductwork, the fan has to push against resistance, measured as static pressure in pascals (Pa), and its delivered airflow falls. The point where the fan's performance curve crosses the system's resistance curve is the duty point, and that is the airflow you will actually get.
Resistance comes from:
- Duct length and diameter. Long runs and undersized duct are the biggest cause of poor performance. Halving the duct area roughly quadruples the resistance at the same airflow.
- Bends and fittings. Each 90° bend adds resistance roughly equivalent to several metres of straight duct. Flexible duct adds far more than rigid duct, especially if it is compressed or kinked.
- Terminals. Grilles, valves, louvres, cowls and backdraught shutters all add pressure drop.
- Filters and canopies. Grease filters in kitchens add significant and rising resistance as they load up.
Rules of thumb: For a wall or roof fan discharging straight outside, add 10 to 15% to the design airflow. For a typical ducted system with a few bends and grilles, add 20 to 30%. For long runs, filters or kitchen canopies, do not guess: read the fan curve at the system pressure, or ask us to check it. Keep duct velocities moderate (roughly 3 to 5 m/s in branches near occupied spaces) to avoid noise.
For more on duct sizes and materials, see what is ducting. For noise control, see how contractors reduce noise in commercial ventilation systems.
When to move to mixed flow or centrifugal
The fan type determines how well the airflow holds up as resistance increases.
| Fan type | Pressure capability | Best for | Example |
|---|---|---|---|
| Plate axial | Low. Airflow drops sharply once ducted. | Through-wall or through-roof extract with little or no duct: warehouses, workshops, plant rooms. | Systemair AW 450E4 |
| In-line mixed flow | Medium. Around 2.5 times the pressure of an axial fan of the same size. | Ducted extract from offices, washrooms and meeting rooms with typical runs and bends. | Vent-Axia ACM200, Elta Jetflow SJMF 250 |
| Centrifugal (in-line, box or roof) | Medium to high. Holds airflow against long runs, filters and canopies. | Commercial kitchens, long duct runs, multiple branches, filtered systems. | Systemair MUB 315EV, Systemair DHS 560DS |
As a rule, if your axial fan selection only works at free air and you have more than a couple of metres of duct, move to mixed flow. If you have long runs, several branches, filters or a kitchen canopy, move to centrifugal. See what is a mixed flow extractor fan and plate vs cased axial fans for the detail, and our in-line duct fan rankings for current picks.
Common sizing mistakes
Choosing on the free-air figure. A fan rated at 600 m³/h may only deliver 400 m³/h on a real duct run. Always size to the duty point.
Mixing up l/s and m³/h. Part F quotes l/s. A 60 l/s requirement is 216 m³/h, not 60 m³/h.
Ignoring make-up air. Air you extract has to be replaced. Without an inlet path (door undercuts, transfer grilles, louvres or a supply fan), the extract fan will under-perform and doors may whistle or slam.
Oversizing massively. Too much extract wastes heat, increases noise and running costs, and can draw in fumes from elsewhere. Size to the need and use speed control for headroom. Our commercial fan running costs guide shows the cost of getting this wrong.
Using general extract for hazardous fume or dust. Welding fume, wood dust and solvents need LEV designed to HSE guidance, not a wall fan.
Related guides
- Best commercial extractor fans UK: buyer's guide 2026
- Building Regulations Part F ventilation explained
- Best in-line duct fans ranked by our experts
- What is a mixed flow extractor fan?
- Best kitchen extractor fans for commercial projects
- Top ventilation design tips for professionals
- Types of industrial ventilation
FAQs
How do I calculate what size extractor fan I need?
Multiply the room's length, width and height in metres to get its volume in m³, then multiply by the recommended air changes per hour for that type of room. The result is the airflow you need in m³/h. Check it against the Approved Document F minimum (for example 6 l/s per WC or 10 l/s per person in offices), use whichever is higher, then add 20 to 30% for a ducted system.
How many air changes per hour does a commercial kitchen need?
At least 30 air changes per hour is the commonly used minimum, and many busy kitchens run at 40 or more. In practice, commercial kitchen extract should be designed around the canopy and cooking appliances to BESA DW/172 and HSE Catering Information Sheet 10, with air changes used as a cross-check.
How do I convert m³/h to l/s?
Divide by 3.6. For example, 360 m³/h ÷ 3.6 = 100 l/s. To go the other way, multiply l/s by 3.6.
What extract rate does Part F require for a toilet or bathroom?
Approved Document F (2021) requires a minimum of 6 l/s (21.6 m³/h) per WC, or per WC pan or urinal in non-domestic buildings, and 15 l/s (54 m³/h) per bathroom or per shower or bath. These are minimums: commercial washrooms are usually designed at 6 to 15 air changes per hour, which often gives a higher figure.
Why does my extractor fan not move as much air as the box says?
The box figure is measured in free air, with no ducting. Duct length, bends, flexible duct, grilles and louvres all add resistance, and the fan's real airflow drops as pressure rises. Shorten and straighten the duct, increase the duct size, or choose a mixed-flow or centrifugal fan that holds its airflow better under pressure.
Is there an extractor fan calculator I can use?
The formula in this guide (volume × air changes per hour, plus a duct margin) is what most online calculators do. Our Fan Selector tool goes a step further and matches a fan to your airflow and pressure figures. Send us your room details or call 0161 399 3825 and we will size it for you.
Last updated: October 2026. Specifications and prices are indicative and can change; confirm current availability, lead time and pricing before ordering. For a sizing recommendation, get in touch.