What this checker works out
Extractor fans are the part of a damp problem people are most willing to spend money on and least likely to get right. A fan that meets the regulations on paper can move a small fraction of its rated air once it is on the end of five metres of concertina duct, and the household concludes that ventilation does not work.
This checker gives you two figures. The first is the extract rate the room needs, taken from Approved Document F, which is the guidance supporting Part F of the Building Regulations in England. The second is the rate you should actually shop for, allowing for what the ducting will take out.
Fans are rated on free air — running with nothing attached to them. That is a fair way to compare fans against each other and a poor guide to what will happen in a ceiling void. The gap between those two numbers explains most of the disappointing extractor fans in Britain.
No prior knowledge is assumed. The sections below explain where the rates come from, why the position of a kitchen fan changes the requirement, and the several ways a correctly sized fan can still fail to clear a room.
The room and the installation
The rates, and where they come from
These are the minimum extract rates in Approved Document F, the guidance supporting Part F of the Building Regulations. Part F covers ventilation, and Volume 1 covers dwellings.
| Room | Intermittent fan | Continuous system, high rate |
|---|---|---|
| Kitchen, fan or hood above the hob | 30 l/s | 13 l/s |
| Kitchen, fan elsewhere in the room | 60 l/s | 13 l/s |
| Utility room | 30 l/s | 8 l/s |
| Bathroom or shower room | 15 l/s | 8 l/s |
| Separate WC | 6 l/s | 6 l/s |
The kitchen figure doubles when the fan is not above the hob for a simple reason: steam that is caught at the moment it leaves the pan is caught easily, and steam that has already spread across the room has to be chased. Position is worth more than power.
Approved Document F applies in England. Wales has its own Approved Document F, Scotland works to the Technical Handbooks under the Building (Scotland) Regulations, and Northern Ireland to Technical Booklet K. The figures are similar but not identical, and the compliance routes differ. If the work is anywhere other than England, check the document for that nation before ordering anything, and speak to building control if the job is notifiable.
How the calculation works
Two separate numbers come out of this page, and confusing them is the most common way a compliant-looking fan ends up underperforming.
The required rate
This is a lookup, not a calculation. Approved Document F sets a figure per room type, and the figure does not depend on the size of the room. That surprises people who expect a bigger bathroom to need a bigger fan, but the rate is set by what happens in the room — a shower produces much the same quantity of steam whatever the room is like — rather than by its volume.
The rate to shop for
Fan performance is quoted on free air: the fan running with nothing attached to it. Real installations attach a duct, and a duct resists airflow. The longer the run and the more bends in it, the less air actually leaves the building.
The calculation converts the installation into an equivalent length of straight duct, then applies a loss per equivalent metre.
Equivalent length = duct length + (number of 90° bends × 1.5 m)
Rate to look for = required rate ÷ (1 − loss)Treating each 90° bend as 1.5 m of extra straight duct is the usual trade rule of thumb. The loss per equivalent metre depends on what the duct is made of, and this is where the largest avoidable error in domestic ventilation lives.
The corrugations that make flexible duct easy to fit are ridges sticking into the airflow, and every one of them takes something out. Worse, flexible duct is usually fitted at whatever length came in the packet, so a 1.5 m run gets 3 m of duct with the slack looped behind the fascia, and the sag between the loops adds bends nobody counted. Rigid duct, pulled straight, in as close to a direct line as the building allows, is the single cheapest improvement to any extract installation.
Why the loss estimate is a guide and not a specification
Real duct resistance depends on the diameter as well as the length, on the fan's own pressure curve, on the terminal fitted at the outside end, and on whether the grille has a backdraught flap. A fan chosen properly is matched against its performance curve at the calculated system pressure. This page gives a sensible allowance to shop with, not a system design.
Worked example: replacing a bathroom fan
A bathroom fan has stopped clearing the mirror. The room has a window. The existing fan sits on an internal wall, with flexible duct running about 2.5 metres through the ceiling void and out through the soffit, with two bends in it.
| Input | Value |
|---|---|
| Room | Bathroom or shower room |
| System | Intermittent |
| Ducting | Flexible duct, concertina type |
| Duct run | 2.5 m |
| Bends | 2 |
The required rate is 15 l/s, which is 54 m³/h. The equivalent length is 2.5 m plus two bends at 1.5 m each, giving 5.5 m. At the flexible-duct figure that is a loss of about 77%, which the calculator caps at 60% and flags — the run is long enough and lossy enough that a simple percentage stops being a fair description.
That flag is the actual answer here. The usual 100 mm axial fan sold for bathrooms is designed to push air a short distance through a wall. On five and a half equivalent metres of concertina duct it will move a fraction of its rated output, which is exactly why the mirror is not clearing. Fitting a more powerful axial fan of the same type changes very little.
What to do instead
- Replace the flexible duct with rigid. Same route, same fan: the equivalent length is unchanged but the loss per metre roughly halves, to about 33%. A fan rated at 22 l/s or better would then meet the requirement.
- Take out a bend if the route allows. Each one is worth 1.5 m of duct.
- Change the type of fan. Where the run genuinely has to be long, an inline or centrifugal fan is built to work against duct resistance in a way an axial fan is not. This is a specification job rather than a shopping one.
Note what is not on that list: buying the most powerful axial fan available and hoping. It is the most common response and the least effective.
Controls matter as much as the rate
A fan that moves the right quantity of air at the wrong time achieves very little. Most of the moisture produced by a shower is still in the room after the person has left it, so a fan wired to the light switch stops roughly when it is needed most.
- Run-on timers. Keep the fan going for a set period after the light goes off, typically 15 to 20 minutes. Approved Document F requires controls of at least 15 minutes' overrun where a wet room has no openable window.
- Humidity sensing. Runs the fan until the air is actually dry rather than for a fixed period, which handles a long bath and a quick visit differently. Generally the better choice for a bathroom or kitchen. Approved Document F advises against humidity control for a separate WC, where odour rather than moisture is the pollutant being removed.
- Pull cords and switches alone. Rely on somebody choosing to run the fan for long enough, which in practice means the fan runs for the length of the shower and no longer.
Extraction only works if replacement air can get into the room. A fan behind a closed door with a well-sealed floor and no trickle vents is trying to pull air out of a sealed box, and it will move far less than its rating however good the ducting is. An undercut door of about 10 mm, or a transfer grille, gives the fan something to work with. This is a genuinely common cause of a correctly specified fan that does nothing.
Things that affect the result
- Duct diameter. Not asked for here, and it matters. A 100 mm duct on a fan sized for 150 mm throttles it severely. Never reduce the diameter from what the fan is designed for, and never fit a 100 mm fan where the kitchen rate is 60 l/s.
- The outside terminal. A grille with a fine mesh, a stiff backdraught flap or a partly blocked cowl can cost more than several metres of duct. It is also the part nobody inspects.
- Vertical runs. A duct rising into a loft collects condensate, which pools in the low spots of flexible duct and blocks it. Ducts should be insulated where they cross cold spaces and should fall slightly towards the outside.
- Recirculating cooker hoods. A hood that filters the air and returns it to the kitchen removes grease and smell but not water vapour. For condensation purposes it is not ventilation at all, and it does not satisfy the extract requirement.
- Fan age and cleanliness. Dust on the impeller and grille measurably reduces output. Cleaning a five-year-old fan sometimes recovers more than replacing it would.
- Whole-dwelling ventilation. Extract from wet rooms is only one part of Approved Document F. Background ventilators — trickle vents — and purge ventilation are separate requirements, and a house that meets the extract rates and has every trickle vent taped shut is still under-ventilated.
Common mistakes
- Comparing a fan's free-air rating with the required rate. They are not the same number. The rating is what the fan does with nothing attached.
- Mixing up l/s and m³/h. Fans are sold in both. One litre per second is 3.6 cubic metres per hour, so 15 l/s and 54 m³/h are the same fan. A figure that looks generous may be the same figure in different clothes.
- Fitting the kitchen fan away from the hob and using the 30 l/s figure. Away from the hob the requirement is 60 l/s, which is a different class of fan.
- Using surplus flexible duct rather than cutting it to length. The extra is not free. It sags, it adds bends, and it collects condensate.
- Ducting into the loft rather than outside. This moves the moisture from a room where you would notice it to a cold void where you would not, and it causes condensation and rot in the roof structure. It is not permitted, and it is common.
- Forgetting the make-up air. No gap under the door, no trickle vents, no airflow.
- Assuming a window is enough. A window is purge ventilation. It does not substitute for extract in a kitchen or bathroom, and it tends to be shut in exactly the weather when condensation is worst.
When not to use this checker
Not as a ventilation system design. A designed system is calculated against the fan's pressure curve at the system resistance, not by adding a percentage. For anything beyond replacing a fan like for like, use a competent installer.
Not for building control sign-off. Since June 2022, mechanical ventilation in new dwellings in England must be commissioned and its airflow measured with a calibrated instrument, with the results given to building control and the owner. A calculation is not a measurement, and no calculation satisfies that requirement.
Not for outside England without checking. Wales, Scotland and Northern Ireland have their own documents.
Not for anything other than a dwelling. Volume 2 of Approved Document F covers other buildings, with different rates and a different approach.
Not for rooms with fuel-burning appliances without further advice. An extract fan in a room with an open-flued boiler, stove or gas fire can pull combustion products back down the flue into the room. This is a carbon monoxide risk and it is taken seriously in the regulations for good reason. Where an open-flued appliance and an extract fan share a space, the installation needs checking by a suitably registered engineer — Gas Safe for gas, HETAS or equivalent for solid fuel.
Frequently asked questions
Does a bigger bathroom need a bigger fan?
Not under Approved Document F. The intermittent rate for a bathroom is 15 l/s regardless of room size, because the requirement is set by what happens in the room rather than by its volume. Room size does affect how quickly you notice the difference, and a very large bathroom may benefit from more, but the compliance figure does not change.
What is the difference between l/s and m³/h?
They measure the same thing in different units, and fans are sold in both. Multiply litres per second by 3.6 to get cubic metres per hour. A 15 l/s bathroom fan is a 54 m³/h fan. Checking which unit a product is quoted in before comparing prices avoids buying something four times weaker than intended.
Is flexible duct really that much worse than rigid?
Yes, by roughly a factor of two per metre, and in practice worse than that because flexible duct is usually fitted longer than it needs to be and sags between fixings. If you change one thing about an underperforming installation, change this.
Do I need building regulations approval to replace an extractor fan?
Replacing an existing fan like for like is generally maintenance. Installing a fan where there was not one, or altering the ventilation provision as part of other work such as a new bathroom or an extension, is likely to be notifiable. Electrical work in a bathroom brings Part P into it as well. Ask your local authority building control if you are unsure — the question is free.
My fan is rated well above the requirement but the mirror still steams up. Why?
Three usual causes, in order of likelihood: the ducting is long, flexible or full of bends, so very little of the rated output reaches outside; there is no path for replacement air, so the fan is pulling against a sealed room; or the fan stops when the light goes off, before the moisture has cleared. All three are cheaper to fix than a new fan.
Can I duct the fan into the loft?
No. It moves the moisture into a cold, unventilated space where it condenses on the roof structure, and it causes rot and mould in a place nobody looks. Ducts must discharge outside the building. This is one of the most common faults found in domestic ventilation and one of the most damaging.
Is a recirculating cooker hood enough for a kitchen?
Not for ventilation purposes. It filters grease and smell and returns the air, water vapour included, to the room. It does not meet the extract requirement and it does not help with condensation. A kitchen with only a recirculating hood needs a separate extract route.
What about continuous mechanical ventilation?
Continuous systems run all the time at a low rate and boost when needed, which suits airtight houses where intermittent extraction leaves moisture sitting in the house between uses. The minimum high rates are lower than the intermittent figures because the system is always running. Select the continuous option above to see them.
Does an extract fan interfere with a gas boiler or a wood burner?
It can, and this is a safety matter rather than an efficiency one. A fan can lower the pressure in a room enough to pull combustion gases back down an open flue. Where an extract fan shares a space with an open-flued appliance, have it checked by a Gas Safe registered engineer for gas appliances, or an equivalently qualified person for solid fuel.
Related tools
References
These organisations publish the guidance behind this page.
- Approved Documents, GOV.UK — Approved Document F Volume 1, the source of the extract rates and control requirements quoted here
- Building Standards Technical Handbook, Scottish Government — the equivalent ventilation standards for work in Scotland
- Building regulations guidance, Welsh Government — the Approved Documents that apply to work in Wales
- Building Research Establishment — research on ventilation performance in dwellings and the gap between rated and installed airflow
- Gas Safe Register — checking an extract fan against an open-flued gas appliance in the same space
- NHS — the health effects of damp and mould in the home
External guidance changes. Check the current position at the source before relying on it for a decision that matters.