What extraction rate do I need for my kitchen?

Every hood is advertised with an extraction figure measured at free air, meaning with nothing attached to the outlet. Once there is a duct, some bends and a grille, real delivery is a fraction of it. Sizing a hood is therefore two calculations: what the kitchen needs, and how much bigger a number you must buy to actually deliver it.

What you'll need

• A tape measure
• The kitchen's floor dimensions and ceiling height
• The intended duct route, measured with its bends counted

Step by step

1. Measure the kitchen and work out its volume.Length times width times ceiling height, in metres, giving cubic metres. A 4.0 by 3.0 metre kitchen with a 2.4 metre ceiling is 28.8 cubic metres. In an open-plan room, measure the kitchen area rather than the whole space.
2. Multiply by ten air changes an hour.Ten changes an hour is the working rule for a hood over a hob. 28.8 times 10 is 288 cubic metres an hour, and that is the air you actually need to move at the hood, not the number on the box.
3. Now derate for the ducting.A short straight rigid run loses roughly a third; a long run with several bends, flexible duct or a reducer loses well over half. Divide the target by about 0.6 for the first case and 0.4 for the second: 288 divided by 0.6 is 480, divided by 0.4 is 720.
4. Shop against the derated figure.For this kitchen that is a hood rated 480 to 720 cubic metres an hour. A hood claiming 300 will not clear it whatever the marketing says, because 300 is its free-air ceiling and not its delivery.
5. Check the answer against the regulatory floor.Approved Document F for England and Wales requires mechanical extraction in a kitchen of 30 litres per second where it is over the hob, or 60 litres per second elsewhere in the room. Those convert to 108 and 216 cubic metres an hour. They are legal minimums for a new installation, not comfort targets, and the volume calculation is what actually clears steam.
6. Add for a gas hob or a wide range.Gas combustion adds its own moisture and a great deal of heat, so a gas hob needs more extraction than the same-size induction one. A wide range cooker also needs a wide hood, because the hood must be at least as wide as the hob to catch the outer burners.
7. Look at the noise figure at the setting you will use.Extraction figures and noise figures are both quoted at maximum. A hood you have to run flat out is loud enough that people stop using it. Buying above the requirement means running it on a quieter middle setting, which is the real argument for sizing up.
8. Check the manufacturer's own duct guidance before finalising.Good manufacturers state a maximum equivalent duct length and a minimum duct diameter. If your route exceeds it, either change the route or specify an in-line or external motor, which handles long runs far better.

Tips

• If the hood is over an island rather than against a wall, size up again. There are no walls to help contain the plume and cross-draughts pull it away.
• Never reduce the hood's outlet diameter. Going from 150mm to 120mm roughly halves the free area and costs more extraction than several metres of duct.
• An in-line or external motor moves the noise out of the kitchen and copes with long runs. It costs more and it is the right answer for an island or a difficult route.
• Run the hood on a low setting from before you start until ten to fifteen minutes after you finish. Continuous low extraction beats a burst on boost at the end.

Common mistakes

• Sizing to the Building Regulations minimum — Those figures are the legal floor for compliance, not a figure that clears a kitchen of steam. Meeting them exactly typically produces a hood that visibly fails to cope.
• Taking the box figure as delivery — It is measured with nothing attached to the outlet. Every real installation delivers less, often much less, and no hood is ever tested with your duct on it.
• Fitting a hood narrower than the hob — The plume from the outer rings rises outside the canopy and goes straight into the room. A hood should be at least as wide as the hob and ideally one size wider.

Questions people ask