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Gas rate calculator
Work out an appliance's actual gas rate and heat input from meter readings, so you can check it against the data plate. Natural gas or LPG, metric or imperial, timed test or one revolution.
Gas rate
Time the meter, get the heat input, check it against the data plate.
Note the initial reading, run the appliance at maximum, then start.
Leave blank to skip the comparison.
Not enough to calculate
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.
How it's worked out
Three steps. Volume per hour from the meter, gross kilowatts from the volume, then net if you need it.
- 01
Volume per hour
volume per hour = 3600 × volume used ÷ time in seconds
- 02
Gross kilowatts
gross kW = (volume per hour × calorific value × correction factor) ÷ 3.6
For UK natural gas the calculator uses a calorific value of 39.2 MJ/m³ and a correction factor of 1.02264. LPG uses its own values, and the calculator switches them with the fuel.
- 03
Net kilowatts
net kW = gross kW ÷ the gross-to-net factor
Only needed when the plate quotes net. Which it usually does on anything modern — see below.
Fuel
Natural gas or LPG
Meter
Metric m³ or imperial ft³
Test
Timed, or one revolution
Worked example
Metric meter · natural gas · timed testA combi at full rate. Dial read 0.120 m³ over a timed 120 seconds. The plate says Qn 24.0 kW net.
- Volume per hour3600 × 0.120 ÷ 120 = 3.60 m³/h
- Gross kilowatts(3.60 × 39.2 × 1.02264) ÷ 3.6 = 40.1 kW
- Net kilowatts40.1 ÷ 1.11 = 36.1 kW
- Compare against the plate36.1 kW net vs 24.0 kW net
That's well over the plate, which on a combi usually means the appliance was firing on hot water rather than the rate you thought you were measuring — or another appliance was drawing gas through the same meter during the test.
What the result means
The number to compare is the one on the same basis as the plate. Once you've got that, a surprising answer usually points at one of these.
- Close to the plateWhat you'd expect. Note it and move on.
- About 11% highAlmost always a gross figure being compared against a net plate. Check the basis before you check the appliance.
- Well overAnother appliance drawing through the meter during the test, or the appliance firing at a different rate than intended.
- Well underWorth looking at supply — pipe run, working pressure at the appliance, or the appliance not reaching full rate during the test.
- Wildly out either wayCheck the units. An imperial dial read as metric is a factor-of-35 error, and it looks like a broken appliance.
The calculator reports a figure. Whether the installation is acceptable is your judgement, against the manufacturer's instructions and Gas Safe procedures.
The mistake people make
Comparing a gross rate against a net plate figure
Modern condensing appliances state Qn — net — on the data plate. Older plates state gross. Compare a measured gross rate against a net plate figure and it reads about 11% high, which sends you hunting a fault that isn't there.
The calculator asks which basis the plate quotes, and you have to answer it honestly for the comparison to mean anything. If you're not sure, read the plate again before you read the meter again.
And on the ten-second rule
A one-revolution test is bounded by the dial, not by the engineer. A 0.01 m³ test dial on a 40 kW appliance comes round in about nine seconds, so a ten-second minimum would make the test impossible. The calculator handles a one-revolution test on its own terms rather than rejecting it.
Questions
How do you calculate gas rate from a meter?
Take the volume used over a measured time, convert it to volume per hour with 3600 × volume ÷ seconds, then multiply by the calorific value and the correction factor and divide by 3.6 for gross kilowatts. Divide by the gross-to-net factor if the plate quotes net.
What's the difference between gross and net?
Gross includes the energy in the water vapour in the flue gases; net doesn't. The same appliance quotes a lower figure net than gross — around 11% lower on natural gas. Modern condensing plates quote net (Qn); older plates quote gross. Compare like with like.
What calorific value should I use?
This calculator uses 39.2 MJ/m³ for UK natural gas with a correction factor of 1.02264. LPG has its own values, which the calculator applies when you change the fuel.
How long should the test run for?
Long enough that the reading is precise relative to the dial you're watching, with the appliance settled at the rate you mean to measure. On a one-revolution test the dial sets the duration — a 0.01 m³ test dial on a 40 kW appliance comes round in about nine seconds — so the calculator treats that as its own mode rather than applying a minimum.
What if my reading doesn't match the data plate?
Check three things before the appliance: that you're comparing gross with gross or net with net, that the units match the meter, and that nothing else was drawing gas through the meter during the test. About 11% high is the classic gross-versus-net mismatch. After that, a genuine discrepancy is a matter for your investigation and the manufacturer's instructions.
Is this free?
Yes. No sign-up, no email, no limit on how many times you use it, 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.