Gas rate calculator
Work out an appliance's gas rate and heat input from the meter, then check it against the data plate. Natural gas or LPG, metric or imperial meters, and timed tests or one turn of the test dial.
- Works out
- Gas rate, and gross and net heat input
- Meters
- Natural gas or LPG, metric or imperial
- Tests
- Timed, or one turn of the test dial
- Checks
- Against the data plate, to ±5%
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.
Ready to test
- Guidance only. Follow Gas Safe procedures and the manufacturer's instructions.
- Your readings stay on this device. They're kept as you type, and Save keeps a test by name.
- The same calculator engineers use in OnPoint's Tools.
01What it works out
How much gas it's really burning.
A gas rate test measures what an appliance actually uses. You run it at full rate, read the gas meter over a set time, and turn that volume into a heat input in kilowatts. Then you compare it with the heat input on the data plate.
The calculator gives you the gas rate in m³/h (or ft³/h), the gross and net heat input in kW, BTU per hour, and how far the appliance is from its plate figure.
When you'd use it
- ServicingCheck a boiler, fire or water heater is burning gas at the rate the manufacturer intended.
- CommissioningConfirm a new appliance is set up right before you fill in the commissioning record.
- Gas safety recordsThe landlord gas safety record asks for the operating pressure or the heat input of each appliance.
- Fault findingAn appliance that won't get up to temperature, or keeps locking out, may be under or over gassed.
02The formula
Four lines of arithmetic.
These are the exact steps and figures the calculator uses, so you can check any answer by hand.
01Gas rate
gas rate (m³/h) = 3600 × volume used ÷ seconds
Volume used is the final reading minus the first. On a one-revolution test it's the volume the test dial passes in one turn.
02Gross heat input
gross kW = gas rate × 39.2 × 1.02264 ÷ 3.6
39.2 MJ/m³ is the calorific value of UK natural gas, and 1.02264 corrects the volume for pressure and temperature. Dividing by 3.6 turns megajoules an hour into kilowatts.
03Net heat input
net kW = gross kW ÷ 1.11
Net leaves out the heat that goes up the flue in water vapour. Most modern data plates quote net, marked Qn.
04Against the plate
difference = (measured − plate) ÷ plate × 100
You tell it whether the plate quotes net or gross, and it compares like with like. Within ±5% reads as expected.
The figures it uses
- Calorific value, natural gas39.2 MJ/m³Varies by region and supplier. You can change it.
- Correction factor1.02264For pressure and temperature. You can change it.
- Gross to net1.11You can change it.
- Calorific value, LPG50 MJ/kgLPG readings are in kg, or gallons on imperial at 0.54 kg a litre.
- Imperial meters35.3147 ft³ to 1 m³Readings in ft³ are turned into m³ first.
- Quick method10.76 kW per m³/hSkips the calorific value and correction. 13.8 kW per kg/h for LPG, 1,040 BTU per ft³ on imperial.
- Test length10 s to 10 minA one-revolution test can be as short as 5 s.
- Plate tolerance±5%Outside it, the result says to check the appliance.
03A worked example
A 30 kW combi, on hot water.
Run the combi at full rate on hot water, read the meter, and read it again 2 minutes later. The plate says Qn 30.0 kW, which is net.
- One turn of the dial gives the same answerTime one revolution of a 0.01 m³ test dial instead: it comes round in 12 seconds. 3600 × 0.01 ÷ 12 = 3 m³/h, so 30.1 kW net again.
- The mistake that sends you huntingCompare the gross figure, 33.41 kW, with that net plate and it reads +11.4%. That's outside ±5%, and nothing is wrong with the boiler. It's why the calculator asks which figure the plate quotes.
Gas rate test
Combi boiler, kitchen
Natural gas · metric meter · timed test · plate Qn 30.0 kW net
First reading
After 2 minutes at full rate
- 1Volume used0.100 m³4821.405 − 4821.305
- 2Gas rate3 m³/h3600 × 0.100 ÷ 120
- 3Gross heat input33.41 kW3 × 39.2 × 1.02264 ÷ 3.6
- 4Net heat input30.1 kW33.41 ÷ 1.11
Against the plate, net with net
+0.3%
30.1 kW measured against 30.0 kW on the plate. That's what you'd expect.
04Reading the result
When the answer looks wrong.
Compare the figure that's on the same basis as the plate. If it still surprises you, it usually points to one of these.
Guidance only. The calculator gives you a figure. Whether the appliance is working as it should is your judgement: follow Gas Safe procedures and the manufacturer's instructions. In the UK, gas work must be done by a Gas Safe registered engineer.
- Within 5% of the plateWhat you'd expect. Note it and move on.
- About 11% highAlmost always a gross figure compared with a net plate. Check which figure the plate quotes before you look at the appliance.
- Well over the plateSomething else was using gas through the meter during the test, or the appliance was firing at a different rate from the one you meant to measure.
- Well under the plateLook at the supply: the pipe size, the working pressure at the appliance, or an appliance that never reached full rate during the test.
- Out by about 35 timesCheck the units. There are 35.3 cubic feet in a cubic metre, so an imperial meter read as metric, or the other way round, is out by that much.
More free calculators.
All free, with no account, and the same ones engineers use inside OnPoint.
OnPoint for your trade
- Gas pipe sizing calculatorSize a run section by section against one pressure drop limit, in copper or steel, with the fittings counted.
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- Every calculator in OnPointThese four, plus the tightness test, IGEM/UP/1 testing and unsafe situations walk-throughs in the app.
Questions people ask.
Spotted something wrong in a sum? Ring +44 7884 697405 or email us.
How do you work out gas rate from a gas meter?
Run the appliance at full rate and time how much gas passes through the meter. Gas rate in m³/h is 3600 × the volume used ÷ the seconds it took. Multiply that by the calorific value (39.2 MJ/m³) and the correction factor (1.02264), then divide by 3.6, for the gross heat input in kW. Divide by 1.11 for net.
What's the difference between gross and net heat input?
Gross includes the heat carried away in water vapour in the flue gases, and net leaves it out, so net is lower. On natural gas the calculator divides gross by 1.11. Modern data plates usually quote net, marked Qn, and older ones quote gross. Compare the measured figure on the same basis as the plate, or it reads about 11% out.
What calorific value does the calculator use?
39.2 MJ/m³ for UK natural gas, with a correction factor of 1.02264 and a gross to net factor of 1.11. For LPG it uses 50 MJ/kg. You can change all of them under Calculation values if your supplier or the meter paperwork gives different figures.
How long should a timed gas rate test be?
The calculator takes anything from 10 seconds to 10 minutes, with 1, 2 and 3 minutes one tap away. A longer test is easier to read accurately. Its timer keeps time even if the phone screen goes to sleep, and beeps when the time is up.
How does a one-revolution test work?
Pick the volume printed on the meter's test dial, such as 0.01 m³ or 1 ft³. Tap when the dial passes a mark and tap again when it comes back round, and the calculator uses that time. A one-revolution test can be as short as 5 seconds, because a small dial on a big appliance turns fast.
Does it work with an imperial gas meter?
Yes. Choose imperial and enter the readings in cubic feet. The calculator turns them into cubic metres (35.3147 cubic feet to a cubic metre) before working out the heat input.
What if my figure doesn't match the data plate?
Check three things before the appliance: that you're comparing net with net or gross with gross, that the units match the meter, and that nothing else used gas through the meter during the test. After that, a real difference is for you to investigate, following the manufacturer's instructions and Gas Safe procedures.
Is the gas rate calculator free?
Yes. There's no sign-up and no limit. Your readings are kept on your device as you type, and you can save each test with a name, such as the appliance and the room. Nothing you enter is sent to OnPoint.
Put the figure on the record.
OnPoint's gas safety and service records work out the gas rate on the record itself, and Use this figure puts it in the heat input box. The certificate is checked as you fill it in, signed on the phone and sent from the job.
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