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Run the appliance at full rate, time how long the meter takes to pass a known volume of gas, then: gas rate (m³/h) = 3600 × gas used (m³) ÷ test time (seconds). Multiply by the calorific value and divide by 3.6 for gross kW, then divide by 1.11 for net. Compare with the data plate.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed July 2026
Gas rating confirms an appliance is burning gas at the input the manufacturer designed, no more, no less. You time the meter over a known volume of gas, convert that to cubic metres per hour, then into kilowatts, and compare the result against the appliance data plate. It is done at commissioning and service, and it catches problems that combustion readings alone can miss.
Every appliance is designed to burn at a stated heat input, and rating it proves it really is. Undergassing points to supply problems: low working pressure, a part-blocked filter or injector, or pipework that can't carry the load. Overgassing pushes the appliance outside its design and risks incomplete combustion. That is why confirming heat input, whether by gas rate or burner pressure, is part of commissioning and servicing, and why burner pressure or heat input appears on the CP12 alongside the other safety checks.
Run the appliance at full rate and turn off other gas appliances, so the meter is measuring this one alone. On a modulating boiler, use the commissioning or maximum-rate mode in the manufacturer's instructions, or it will turn itself down mid-test and skew the result.
Note the meter reading, time a test period (say 120 seconds), and note the reading again. The difference between the two readings is the gas used in cubic metres. A longer test smooths out reading error.
Gas rate (m³/h) = 3600 × gas used (m³) ÷ test time (seconds). Using 0.073 m³ over 120 seconds: 3600 × 0.073 ÷ 120 = 2.19 m³/h.
Multiply the rate by the calorific value and divide by 3.6 for the gross heat input in kW: 2.19 × 39.5 ÷ 3.6 = 24.0 kW gross. Divide by 1.11 for net: 21.6 kW net.
UK data plates are usually quoted net, so compare the net figure. A result well outside the manufacturer's stated input needs investigating, not recording and moving on.
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The meter measures the gas supplied, which gives you a gross figure: it includes the latent heat in the water vapour that combustion produces. Appliance data plates are usually quoted net, which leaves that latent heat out, so divide the gross result by 1.11 to compare like with like. Mixing the two up is the classic way to convince yourself a healthy appliance is overgassed.
The calorific value is on the gas bill for the property, quoted as an average of the gas actually supplied. UK natural gas is around 39.5 MJ/m³, and gas transporters are required to keep it between 38 and 41 MJ/m³, so 39.5 is a reasonable default when the exact figure isn't to hand.
Older imperial meters have a test dial, typically registering one cubic foot per revolution. The principle is exactly the same: run the appliance at full rate and time one complete revolution of the test dial. The slower the dial, the lower the rate. Convert the dial volume to cubic metres (one cubic foot is about 0.0283 m³) and the metric formula above takes over, or use the conversion tables most engineers keep in the van. Either way, the answer still ends up as a net kW figure checked against the data plate.
First, don't condemn on one number. Re-check your timing and readings, confirm other appliances really were off, and make sure a modulating appliance was actually held at maximum for the whole test. Then check the basics: standing and working pressure, the gas filter, the injectors, and whether the pipework is up to the load when other appliances run.
The pass mark is the manufacturer's, not a universal percentage. The data plate and installation instructions state the design input and any tolerance on it, and those figures are what you work to. If the appliance can't be brought onto its designed input, follow the manufacturer's instructions and the current industry unsafe situations procedure: investigate, correct what you can, and classify what you can't. Walking away with the appliance still firing wrong isn't an option, and neither is guessing.
Whatever the result, record it against the appliance. A gas rate on its own is a number; a gas rate next to last year's is a trend. Manifold keeps each appliance's readings, photos and history joined up, so next year's figure lands beside this year's instead of in a separate PDF nobody opens.
Gas rating is gas work, and gas work must only be carried out by a Gas Safe registered engineer qualified for the appliance type. This guide and the calculator are working aids, not a pass or fail verdict; the manufacturer's commissioning data and current procedures always take precedence.
Last reviewed July 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.