Free tool
Gas rate calculator
Time the meter, enter what it burned, and get the flow rate and heat input in kW. For engineers gas rating an appliance on site. If you are trying to work out a gas bill, this is the wrong calculator.
Read the meter, run the appliance at maximum rate with everything else off, and time it. Put the seconds in, not the minutes.
How to gas rate an appliance
- Turn off every other gas appliance in the property, including pilots where practical.
- Run the appliance at maximum rate until it settles. On most boilers that means putting it into its maximum-rate service mode rather than just calling for heat.
- Note the meter reading, start timing, and take a second reading after a known number of seconds.
- Subtract to get the volume burned, and enter the volume and the seconds above.
- Compare the result against the heat input on the data badge, and check whether the badge quotes gross or net.
The formula
For a metric meter:
flow (m³/h) = volume × 3600 ÷ seconds
gross kW = flow × CV ÷ 3.6
net kW = gross × 0.901
On an imperial meter the volume is in cubic feet and the calorific value in BTU/ft³, so the gross figure is flow × CV × 0.000293071.
Worked example
An appliance burns 0.040 m³ in 120 seconds on natural gas at 39.3 MJ/m³.
| Step | Working | Result |
|---|---|---|
| Flow rate | 0.040 × 3600 ÷ 120 | 1.20 m³/h |
| Gross input | 1.20 × 39.3 ÷ 3.6 | 13.10 kW |
| Net input | 13.10 × 0.901 | 11.80 kW |
Common reasons a gas rate looks wrong
- Another appliance is burning. A hob pilot or a second boiler will inflate the reading.
- The appliance is not at maximum rate. Modulating boilers will drop back long before the meter test is finished unless they are held in service mode.
- Gross compared against a net badge figure. The two differ by about 10%, which is roughly the margin that makes a healthy appliance look overgassed.
- The wrong calorific value. 39.3 MJ/m³ is a sensible default, not a fixed constant. Your supplier publishes the actual figure.
The other calculators
The app
Every service figure for the boiler you are stood at
Flint holds the manufacturer manual, the fault codes and the structured service figures for 1,100+ gas boilers across 48 makers, with every figure cited to its page in the manual. These four calculators are in there too, and Ask Flint answers off the real manual rather than a rule of thumb.
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Common questions
What is the formula for gas rating a boiler?
Gas rate in m³/h is the volume burned multiplied by 3600 and divided by the number of seconds you timed. Gross heat input in kW is that flow rate multiplied by the calorific value and divided by 3.6. Net heat input is the gross figure multiplied by 0.901.
What calorific value should I use?
39.3 MJ/m³ is the usual figure for UK natural gas and it is what this calculator starts with. The exact value changes slightly by region and by day, and your gas supplier publishes it. Use 1040 BTU/ft³ on an imperial meter.
How long should I time the meter for?
Two minutes is the common choice because it gives a readable volume on a U6 meter without keeping the appliance at full rate for longer than needed. One minute is enough on a high input appliance. Whatever you time, put the seconds into the calculator rather than rounding to a minute.
Should I compare gross or net to the data badge?
Check which one the manufacturer quotes on the badge or in the instructions, because both appear. UK manufacturers commonly quote net for condensing appliances. Comparing a gross figure against a net badge value is the usual reason a correct gas rate looks wrong.
Is this gas rate calculator free?
Yes, it is free to use here with no sign-up. The same calculator is in the Flint app along with the CO/CO₂ ratio, temperature rise and expansion vessel calculators, plus the manuals and service figures for 1,100+ gas boilers.
Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.