How to convert a gas meter reading to kWh

Your gas meter counts the volume of gas passing through it, in cubic metres or cubic feet. Your bill is charged in kilowatt hours (kWh), which measure energy. Getting from one to the other needs two extra numbers: a volume correction factor and the calorific value of the gas that day.

This guide sets out the formula National Gas publishes for working this out yourself, with a labelled example for both meter types. Our gas meter reading to kWh tool does the calculation for you if you would rather skip the maths.

How do you convert a gas meter reading to kWh?

If you would rather not do the arithmetic, the gas meter reading to kWh calculator does all four steps for you, including the imperial conversion.

Multiply the volume of gas you have used by the volume correction factor, then by the calorific value, then divide by 3.6. National Gas sets out the formula as:

(The number of units used) x (the volume correction factor) x (the calorific value), divided by 3.6 = kWh

For a metric meter, “units used” is the number of cubic metres between two readings. For an imperial meter reading in cubic feet, there is an extra first step: multiply the cubic feet figure by 2.83 to convert it to cubic metres, then run the same formula.

You need three things to do this calculation:

1. Your gas used, in cubic metres or cubic feet (this month’s reading minus last month’s).
2. The volume correction factor, usually 1.02264.
3. The calorific value (CV) that applied to your supply over that period, in megajoules per cubic metre (MJ/m³).

The 3.6 divides through because one kWh equals 3.6 megajoules.

Is my meter imperial or metric?

Check the face of the meter before you start. National Gas describes the two types like this:

• Imperial meter. It will probably show the words “cubic feet”, or the letters “Ft”, somewhere on the front, and it measures in hundreds of cubic feet.
• Metric meter. It will probably say it measures gas in “cubic metres”, or show the letter “M” on the dial.

Most older business premises still on their original supply may have an imperial meter, but every new meter fitted today, including smart and advanced meters, is metric. If your premises has had a smart meter fitted, see our guide to smart meters for business.

What is the volume correction factor?

The volume correction factor adjusts the raw volume reading for the temperature and pressure of the gas at your meter, standardising every reading to the same reference conditions so that a cubic metre of gas means the same thing everywhere on the network. National Gas says the figure that applies to your supply is shown in the small print of your bill, and for the great majority of UK supplies it is 1.02264.

You do not need to work this figure out. It is set for your meter and appears on your bill or in your supplier’s usage breakdown.

What is the calorific value and where do I find it?

The calorific value (CV) tells you how much energy is in each cubic metre of gas, in megajoules per cubic metre (MJ/m³). Natural gas is not a fixed, uniform product. Its energy content varies slightly depending on where it entered the network and blends as it moves through the system, so the CV is measured rather than assumed.

Most non-domestic customers are billed using a daily average CV for their local charging zone. National Gas explains that this is called the Flow Weighted Average Calorific Value (FWACV), which it calculates and publishes on behalf of the gas distribution networks that own the local pipes. Your charging zone depends on where you are connected into the network, not simply your postcode area.

Gas entering the network has to meet quality standards set out in the Gas Safety (Management) Regulations 1996, which is the legislation that underpins the consistency of the CV figure suppliers use.

Where is the calorific value shown on my bill?

National Gas confirms that both the volume correction factor and the calorific value are shown in the small print of a gas bill, though suppliers vary in exactly where they place this. Look near the meter reading and usage breakdown, or in a section headed “conversion” or “calculation of charges”. If you cannot find it, ask your supplier for the CV it applied for the billing period, and see our guide to reading a business energy bill for what else should be there.

Can I look up the calorific value myself?

Yes. National Gas publishes CV information by charging zone on its Gas Data Portal, which anyone can view or download. This is useful if you want to check your supplier used a reasonable figure, or if you are trying to reconcile several months of bills against your own meter readings.

Why is gas billed in kWh rather than cubic metres?

Gas meters measure volume because that is what physically passes through them. But the energy content of a cubic metre of gas is not fixed, it depends on what is in the gas. Billing in kWh means you are charged for the energy you actually received, not for a volume of gas that might carry more or less energy depending on the day. It also lets your gas and electricity charges sit on a like-for-like basis, and it is what the unit rate on your contract is quoted against. Our guide to kW vs kWh explains the difference between the two units if you want the basics first.

This is also why the same physical amount of gas can convert to a slightly different number of kWh from one bill to the next, even if your usage pattern has not changed. The volume correction factor stays effectively constant, but the calorific value moves with the gas actually flowing through the network.

Quick-reference table

These figures use the same illustrative correction factor and calorific value as the worked examples above, so you can sanity-check a bill quickly. Always replace the calorific value with the one shown on your own bill for an exact figure.

Metric reading (m³)	Approximate kWh (at CV 39.5 MJ/m³)
10	112.2
50	561.0
100	1,122.1
500	5,610.3
1,000	11,220.6

Worked example: converting a metric reading to kWh

This is an illustrative example using a round number and a typical calorific value, to show how the formula works. Always use your own reading and the CV shown on your bill for a real calculation.

Inputs:

• Gas used: 1,000 m³ (this reading minus the last one)
• Volume correction factor: 1.02264
• Calorific value: 39.5 MJ/m³ (an illustrative figure; yours will be shown on your bill or the Gas Data Portal)

Calculation:

1. 1,000 x 1.02264 = 1,022.64
2. 1,022.64 x 39.5 = 40,394.28
3. 40,394.28 ÷ 3.6 = 11,220.6 kWh

So in this example, 1,000 m³ of gas converts to roughly 11,220.6 kWh. As a rough rule of thumb, one cubic metre of gas is typically a little over 11 kWh, though the exact figure moves with the calorific value on the day.

Worked example: converting an imperial reading to kWh

Imperial meters need one extra step to convert cubic feet into cubic metres before the main formula applies.

Inputs:

• Gas used: 200 units on an imperial meter reading in hundreds of cubic feet (hcf)
• Conversion to cubic metres: 200 x 2.83 = 566 m³
• Volume correction factor: 1.02264
• Calorific value: 39.5 MJ/m³ (illustrative, as above)

Calculation:

1. 566 x 1.02264 = 578.8
2. 578.8 x 39.5 = 22,862.6
3. 22,862.6 ÷ 3.6 = 6,350.7 kWh

Why does my kWh usage look different from what I expected?

Two things routinely trip people up when they check their own bill against the meter.

• The calorific value changes. It is a daily figure for your charging zone, not a constant. A bill covering a period with a higher average CV will convert the same cubic metres into more kWh, and vice versa.
• AQ and SOQ are different figures. Your Annual Quantity (AQ) is an industry estimate of your yearly use used to allocate network charges, not the actual reading conversion. See our guide to your MPRN number for what AQ means and how it is set.

If your converted kWh figure looks a long way out from what the formula gives you, check you have used the CV that actually applied to your billing period rather than a current or average figure, and check whether your bill is based on an actual or an estimated meter reading.

Does the calorific value affect my Climate Change Levy or VAT?

Once your usage is converted into kWh, that figure is what everything else on your bill is calculated from. The Climate Change Levy is charged per kWh, so a higher calorific value in a given period feeds through into a slightly higher CCL charge on the same volume of gas, and the same applies to your unit rate charge. The reduced 5% VAT rate for small quantities of gas, covered in our guide to VAT on business energy, is also assessed against your kWh consumption, not the raw meter reading. This is one more reason to make sure the conversion your supplier has used lines up with what the formula gives you.

What if my meter shows kWh directly?

Some modern commercial meters, and most smart and advanced meters, can display or transmit consumption already converted into kWh, using the correction factor and calorific value applied automatically. If that is the case for your premises, you do not need to do this calculation yourself, though the same formula is still what sits behind the figure your supplier bills you for.

Checking your supplier used the right figures

Ofgem’s rules require suppliers to take all reasonable steps to reflect an accurate meter reading in your bill, and to explain in plain language how a bill was worked out if you ask. If you think a bill has used the wrong calorific value or correction factor, ask your supplier to show its working, and compare it against the published figures on National Gas’s Gas Data Portal for your charging zone and billing dates.

For everyday use, our gas meter reading to kWh tool applies this formula automatically once you enter your readings and meter type. And when you are ready to see how your usage compares, you can compare business energy quotes with us.

Frequently asked questions