How do you calculate the gas rate of an appliance?

Isolate the appliance so it's the only gas load, run it at full rate, and time how long the meter takes to use a set volume of gas. Gas rate in m³/hr = volume used × 3600 ÷ time in seconds. Multiply by the calorific value (39.5 MJ/m³ for UK natural gas) and divide by 3.6 to get the gross heat input in kW.

What calorific value should I use?

UK natural gas is regulated between 38.0 and 41.0 MJ/m³, and 39.5 MJ/m³ is the mid-band value engineers assume when the exact local figure isn't available. The precise daily value for a postcode is published by the gas network; using it instead of 39.5 changes the result by only a percent or two.

What's the difference between gross and net heat input?

Gross includes the energy that would be recovered by condensing the water vapour in the flue gases; net excludes it. Gas rating is done on gross calorific value, but appliance data plates quote the net input, so divide your gross result by 1.11 before comparing to the plate.

Can I use this for LPG appliances?

No. LPG (propane) has a much higher calorific value and is metered and rated differently, so this natural-gas calculator will give the wrong answer. Use the LPG-specific method and calorific value for propane appliances.

My gas rate doesn't match the data plate. What does that mean?

A rate well below the plate can indicate low gas pressure, an undersized or partially blocked supply, or a meter issue; well above can indicate an incorrectly set appliance. Check the standing and working pressure at the appliance and the pipe sizing before adjusting anything, and only if you're Gas Safe registered.

Do I need to be Gas Safe registered to work on gas?

Yes. By law, anyone carrying out gas work in the UK on appliances or pipework must be on the Gas Safe Register. This calculator is a check aid for qualified engineers, not a licence to work on gas.

Free Gas Rate Calculator

Work out an appliance's gross and net heat input in kW from a timed gas-meter test: the standard gas-rating check for natural gas.

Gas rating is how a Gas Safe engineer checks an appliance is burning at its rated input: time how much gas the meter uses over a set period, and convert to kilowatts. Enter the gas volume used and the test time and this calculator returns the gas rate (m³/hr or ft³/hr) and the gross and net heat input in kW, using a natural-gas calorific value of 39.5 MJ/m³.

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Gas Rate Calculator results

Fill in the details and select Calculate to see your figures.

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How to use the Gas Rate Calculator

1Turn off every other gas appliance and pilot so only the appliance you're testing is drawing gas.
2Run the appliance at maximum rate and time how long the meter takes to move through a set volume (e.g. 0.05 m³, or one full test-dial revolution).
3Choose your meter type, then enter the volume used and the time in seconds.
4Read the gross heat input in kW. Divide by 1.11 (done for you) to get the net figure, which is what the appliance data plate quotes.
5Compare the net figure to the data plate. A big shortfall or excess points to a gas-supply, meter, or appliance fault to investigate.

How the Gas Rate Calculator works

Gas rating converts a timed volume of gas into a heat input. It's a gross-CV calculation; appliance plates are net, so convert at the end.

Gas rate (m³/hr) = volume (m³) × 3600 ÷ time (s) → Gross kW = gas rate × 39.5 ÷ 3.6 → Net kW = gross ÷ 1.11

0.05 m³ used in 120 s → 0.05 × 3600 ÷ 120 = 1.5 m³/hr.
Gross kW = 1.5 × 39.5 ÷ 3.6 = 16.5 kW.
Net kW = 16.5 ÷ 1.11 = 14.8 kW.
Imperial: gas rate (ft³/hr) = volume × 3600 ÷ time, then gross kW = rate × 1040 ÷ 3412.

Uses natural gas at CV 39.5 MJ/m³. LPG has a different calorific value and rating method. Don't use this for LPG. Gas rating is a Gas Safe competency; this tool is a field check aid, not a substitute for correct commissioning.

Typical UK benchmarks

UK natural gas calorific value	38.0–41.0 MJ/m³ regulated band; 39.5 mid-band assumed (Gas Safety (Management) Regulations 1996, Schedule 3 (GS(M)R), as amended; Gas Safe / NICEIC Pocket Guide Gas 3)
1 m³/hr of natural gas	≈ 10.97 kW gross at CV 39.5 MJ/m³ (39.5 ÷ 3.6)
Gross → net conversion	Net = gross ÷ 1.11 for natural gas; appliance data plates quote net

Disclaimer

This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.

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