The conversion formula
Gas is sold by energy, but most meters measure volume. Suppliers convert one into the other using two numbers that appear on your bill.
Calorific value in kWh per m³ = calorific value in MJ per m³ ÷ 3.6
The volume conversion factor corrects for the pressure and temperature of the gas at your meter. The calorific value says how much energy each cubic metre holds. It varies slightly with the gas mix, so suppliers publish an average for your area and billing period.
Worked example in cubic metres
A household used 100 m³ between two readings. The bill states a volume conversion factor of 1.02264 and a calorific value of 39.5 MJ per m³.
39.5 ÷ 3.6 gives 10.972 kWh per m³. Multiply by 1.02264 and each cubic metre on the meter delivers 11.221 kWh. So 100 m³ becomes 1,122 kWh. At 0.08 per kWh that costs 89.77.
Worked example in hundreds of cubic feet
Some older meters in the UK count in hundreds of cubic feet. One hundred cubic feet equals 2.83168 m³. A reading difference of 100 units is therefore 10,000 ft³, which is 283.17 m³, and the same factors give 3,177 kWh. Pick the unit in the form so the calculator does this step for you.
Convention in Germany and neighbouring countries
German bills use the same idea with different names. Volume in m³ is multiplied by the Zustandszahl, the correction factor, which is often a little below 1, and by the Brennwert, the gross calorific value, which commonly lies between 10 and 12 kWh per m³. As an illustration, 1,000 m³ with a Zustandszahl of 0.95 and a Brennwert of 11.5 kWh per m³ gives 10,925 kWh. Choose kWh per m³ as the calorific value unit and type the correction factor into the volume conversion field. The figures to use are always the ones printed on your own bill.
Why your bill may differ slightly
Calorific values change month to month and region to region, so a fixed number from last year will not match this year's bill exactly. Gross and net calorific values also differ by about 10 percent. Bills use the gross value. If you take a net value from a boiler specification and apply it to a bill, your conversion will come out low.
When you need this conversion
You need kWh when you compare gas with electricity, work out how much heat your home uses, or size a heat pump. The heat pump vs gas boiler calculator asks for heat demand in kWh, and this conversion is the first step. It also helps when a supplier quotes a price per kWh and your meter shows cubic metres.
Frequently asked questions
What is the standard conversion factor?
There is no single universal figure. The common UK volume correction is 1.02264, and the average calorific value is around 39 to 40 MJ per m³, which equals roughly 10.8 to 11.1 kWh per m³ before correction. Always check the numbers on your bill.
Do smart meters give kWh directly?
Many do, which removes the conversion. If your meter display or app shows kWh, you can skip this step and use those figures.
Is 1 m³ of gas always 10 kWh?
No. Ten is a handy rule of thumb, but the real figure is closer to 11 kWh in many places once you apply the correction factor.
How do I read the meter?
Subtract the earlier reading from the later one, and make sure both readings use the same digits. Your supplier's reading guide shows which digits count on your meter type.