What the calculator works out
The cooker hood extraction calculator takes the numbers you can actually measure in your own kitchen, room dimensions, hob type, and the length and number of bends in the ducting, and turns them into the figures a kitchen fitter or showroom would otherwise quote at you without explanation. It returns the minimum extraction rate set by Approved Document F, a separate sizing figure worked out from the volume of air in the room, an estimate of how much that figure drops once the air has travelled through your ducting, and the hood speeds that still clear the Approved Document F minimum after that loss. It cannot see your kitchen, so the result is only as good as the measurements you put into it, and it does not replace the manufacturer's own installation instructions or a building control sign-off.
Why the extraction rate matters
A hob throws off steam, grease and, if it burns gas, the by-products of combustion. A hood that moves too little air leaves that moisture to settle on cold surfaces around the kitchen, which is how condensation and mould take hold on ceilings and in corners a long way from the cooker. Extraction is also one of the few parts of a kitchen refit that building regulations set a minimum for, rather than leaving entirely to taste, so getting the figure right is not just a comfort question.
The Approved Document F minimum
Approved Document F sets the height a hood must be fixed above the hob when the appliance instructions do not give their own figure: 650mm above an electric hob and 750mm above a gas hob, at paragraph 1.21. Gas hobs are treated more cautiously because they add combustion products to the steam and grease that any hob produces, so the extraction and clearance both need to work harder. This guidance applies in England; the position can differ in Scotland, Wales and Northern Ireland, and where your own kitchen sits close to a boundary case, that difference is worth checking against the guide to gas, electrics and building control in a kitchen refit.
Sizing by air changes
Alongside the Approved Document F minimum, the calculator works out a second figure based on how many times the air in the kitchen needs to be replaced each hour, using the room's volume. A large open-plan kitchen and a small galley kitchen with the same hob can call for quite different extraction rates once the room volume is taken into account, which is why the two figures sit side by side in the result. Where they disagree, the higher of the two is the one to size the hood against.
What the ducting costs you
The extraction rate printed on a hood's box is measured at the outlet, before the ducting has taken anything off it. Every bend in the run and every metre of straight duct adds resistance, so a hood that clears the required rate on a short, straight run to an outside wall can fall short of it once the same hood is fitted with a long run and two or three bends to reach the outside. This is the estimate the calculator returns as the loss from ducting, and it is the figure most easily missed when a hood is chosen from a showroom display.
Common mistakes when sizing a hood
- Choosing the hood before planning the duct run: the extraction rate on the box assumes a duct route that your kitchen's layout may not give you.
- Treating gas and electric hobs the same: the fixing height and the case for a stronger extraction rate both change with the hob type.
- Ignoring bends: a run with several bends can lose a significant share of the rated extraction rate before the air reaches outside.
- Sizing to the hob width alone: a wide hob under a low ceiling, or in a large open-plan room, can need more extraction than the hob width alone would suggest.
- Relying on recirculating filters where a duct run to outside is possible: a recirculated hood cleans and returns the air rather than removing the moisture and grease from the room, so it is worth checking whether ducting outside is achievable before ruling it out.
Using the figures in practice
Once you have a duct route in mind, measuring its length and counting its bends before you choose a hood puts you in a position to check a specific model's rated extraction rate against what it will actually deliver. Use the cooker hood extraction calculator with your kitchen's own dimensions and duct route to see the Approved Document F minimum, the air-change figure and the estimated loss from ducting side by side, and treat the hood speeds it returns as a starting point for checking against the manufacturer's own figures.
Where to go next
For the fuller picture of extraction rates, noise levels at each hood speed and what Approved Document F requires, see cooker hoods: extraction rates, noise at every speed and what Part F requires. Where the installation involves gas work, that work has to be carried out by a Gas Safe registered engineer, and where you are unsure whether your kitchen's ventilation needs building control involvement, the guide to gas, electrics and building control in a kitchen refit sets out who to ask.