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Gas ventilation calculator
Work out the ventilation an installation needs in cm². Covers five situations, and which one you're in changes the answer completely.
Gas ventilation
Work out the permanent ventilation a domestic gas appliance needs — and what to fit.
Room volume 33.6 m³
Splits the requirement between them.
Required free area
25cm²
What to fit
9 × 3 airbrick is 229 × 76 mm. Free area is not the size of the opening — a 9 × 3 airbrick is 174 cm² of brickwork and roughly a quarter of that in free area. These are typical figures; use the free area printed on the product you actually fit.
Required free area
25cm²
What to fit
9 × 3 airbrick is 229 × 76 mm. Free area is not the size of the opening — a 9 × 3 airbrick is 174 cm² of brickwork and roughly a quarter of that in free area. These are typical figures; use the free area printed on the product you actually fit.
Guidance only. Follow Gas Safe procedures and the manufacturer's instructions. Gas work in the UK must be carried out by a Gas Safe registered engineer.
Five situations
Pick the wrong one and the figure is wrong, however carefully you enter everything else. Start here.
Open-flued, in a room
Air for combustion comes from the room itself.
Open-flued, in a compartment
High and low, and the two figures differ.
Room-sealed, in a compartment
Cooling air, not combustion air — still high and low.
Flueless cooker
Room volume and openings do the work.
Flueless water heater
Treated on its own terms, not as a cooker.
How it's worked out
The situation sets the method. Then the calculator applies the free area required for the appliance rating, adjusts for what you've told it about the room and the vents, and reports the total and the per-vent figure.
- Room volumeMatters for flueless appliances, and for whether the room can be counted on at all.
- Adventitious airWhether the incidental air leakage of the building can be counted. You decide; the calculator applies it consistently either way.
- Where the vent goesDirect to outside air, or into another internal space. The second case changes the requirement.
- How many ventsThe total free area is split across them, so you get the area each one needs rather than doing the division on a windowsill.
- Door to outside, openable windowBoth are asked about because both can bear on a flueless installation.
Free area is not the same as the size of the hole. A grille's free area is what's actually open once the louvres are accounted for — use the manufacturer's figure for the grille you're fitting.
Worked example
Open-flued in a compartment · to outside airAn open-flued boiler in an airing cupboard, vents straight through the external wall, one vent high and one low.
- Situationopen-flued, compartment
- Vents tooutside air
- Number of ventstwo — one high, one low
- Returnshigh-level cm², low-level cm², separately
- Plusa pass, warning or fail on what you described
Change one input — same boiler, but the compartment vents into a hallway rather than through the wall — and both figures change. That's why the situation and the destination are asked first rather than last.
What the result means
- Total free area requiredIn cm². Free area, not hole size.
- Area per ventThe total split across the number of vents you said you're fitting.
- High and low, separatelyWhere a compartment is involved. They are two different figures, not one figure halved.
- Pass, warning or failAgainst what you've described — a prompt to look again, not a determination that the installation complies.
A figure that looks unexpectedly large is usually the destination — venting into another internal space rather than to outside — or adventitious air switched off. A figure that looks small is worth checking against the situation you selected.
The calculator reports figures. Whether the installation is acceptable is your judgement, against the manufacturer's instructions and Gas Safe procedures.
The mistake people make
Working out one total for a compartment and splitting it in half
A compartment needs high and low ventilation, and they are not the same figure. Taking a single total and dividing it by two is a different calculation from the one that's needed, and it produces two vents that are both the wrong size.
Wrong
One total, halved. Two vents of equal area, neither of them derived from the requirement that applies to it.
Right
High level and low level worked out separately, each to its own figure. The calculator reports them as two numbers for exactly this reason.
The same trap catches people on a room-sealed appliance in a compartment — no combustion air is needed from the compartment, but cooling air still is, high and low.
Questions
How do you calculate ventilation for a gas appliance?
Start with the situation — open-flued in a room, open-flued in a compartment, room-sealed in a compartment, flueless cooker or flueless water heater. That sets the method. Then apply the free area required for the appliance rating, adjusted for room volume, whether adventitious air is counted, and whether the vent goes to outside air or another internal space.
Does a compartment need high and low ventilation?
Yes, and they're two different figures. The calculator reports the high-level and low-level areas separately rather than giving you one total to divide.
Does a room-sealed appliance in a cupboard need vents?
It takes its combustion air from outside, so it doesn't need combustion air from the compartment — but a compartment still needs cooling air, high and low. Select the room-sealed-in-a-compartment case and the calculator works to that.
What is free area?
The area actually open to airflow, which is less than the size of the hole once a grille's louvres are accounted for. Use the manufacturer's free area figure for the grille you're fitting rather than measuring the opening.
Can I count adventitious air?
The calculator asks, and applies your answer consistently. It's a judgement about the building in front of you — a modern airtight property is a different proposition from an older one — so it's an input rather than an assumption baked into the result.
Is this free?
Yes. No sign-up, no email, and the result isn't held back behind anything.
Guidance only. Follow Gas Safe procedures and the manufacturer's instructions. Gas work in the UK must be carried out by a Gas Safe registered engineer.
This calculator is part of OnPoint, which does certificates, scheduling and invoicing for UK trades. £19 for the first seat, £9 for each after.