How to calculate a gas bill – a step by step guide

Nobody looks forward to receiving household bills. But when it comes to knowing how to calculate a gas bill, it might seem like you need a high level of mental gymnastics! Gas bills are thick with jargon, strange terms and units of consumption which you’re unlikely to find in any other household bill.

Laden with terms like KwH, joules and calorific values, learning how to read your gas bill can be challenging. Doing so will help you understand and manage your utility bills and household expenses better.

‘It’s important to understand your gas bill,’ says Bronwyn Huband from British Gas, ‘to ensure that you only pay for gas that you have used, which will ensure you don’t underpay or overpay for your energy.’ Gas suppliers charge us for the gas we use in a quite complicated way. But we’ll explain how those calculations are made and how that relates to what you are charged.

In understanding how to calculate a gas bill, you may discover you’re using more gas than a house of your size should when you’re trying to keep your house warm in winter. Becoming fluent with the terms and calculations can help change your habits and routine, meaning more efficient gas usage and lower monthly bills.

Why is it important to understand your gas bill?

‘Energy bills and statements contain useful information and it’s really important to understand them’ explains Jean Fiddes. ‘For example, it’s perfectly normal for your gas use, and therefore spend, to go up and down throughout the year, even if you’re on a fixed tariff. Because most UK homes use gas central heating, we expect to see people use more energy when it’s colder and less when it’s warm.’

‘To make it easier to budget across the year, you can pay the same amount every month by direct debit, so the credit built up over summer offsets your increased usage during the winter months. Your bill will clearly set this information out,’ she adds.

Understanding the terms used to calculate a gas bill

Before embarking on the sums and figures contained in your gas bill, get to know some of the more unusual terminology you might come across.

What does Kwh mean?

‘The amount of energy you’ve used is shown in kilowatt hours (kWh), explains Bronwyn Huband from British Gas. ‘To give you a rough idea, one kilowatt hour is about the amount of energy it would take to boil ten kettles, run a full cycle of your washing machine, or keep your laptop powered for two days.’

What is Correction Factor?

‘Temperature and pressure cause gas to expand and contract ‘ says Christopher Dalley from EDF. ‘To account for this we use a correction factor in our calculation. This is fixed and standardised across the industry.’

What is calorific value?

Calorific values, or how much heat is given off when gas is burned, vary. ‘The figure on a customer’s bill is the average calorific value for the gas supplied to your property,’ explains Christopher. ‘Calorific value is measured across the gas pipeline by National Grid, in accordance with Ofgem. Industry standard values are then sent onto the relevant suppliers.’ The exact figure will be quoted on your bill as megajoules per cubic metre (MJ/m³). The figure will be somewhere between 38 MJ/m³ to 41 MJ/m³ - gas transporters are regulated to maintain this figure to prevent problems with gas burning appliances. The typical value is 39.5 MJ/m3.

What you need to calculate a gas bill

In order to save energy at home, when working out your gas bill you’ll need some information to hand. Follow these simple steps:

1. Have two separate meter readings from two different days

You should have the meter reading which you last provided your energy supplier with.

2. Know your meter type

‘In the UK,’ explains Christopher Dalley from EDF, ‘we have gas smart meters and traditional meters which were installed before smart meters. Customers who have a gas smart meter that’s connected to the Data Communications Centre don’t need to submit regular meter readings, as this is automatically done by the meter. Customers who have a traditional meter are still required to submit regular meter readings to ensure they receive an accurate bill.’

‘If you have a prepayment meter or Pay As You Go meter, you will pay for your energy as you use it by topping up a card or key,’ says Bronwyn Huband from British Gas. ‘You will pay a standing charge. If you owe any arrears from a previous bill and this has been loaded onto your prepayment meter, this will also be deducted at an agreed amount when you top up.’

3. Find out the amount you pay for each kWh of gas you use

This can be found in the information about your gas tariff. It’s usually quoted in pence per kilowatt hour - for example 2.5905p/kWh.

How to calculate a gas bill from meter readings

There are quite a few calculations required to get to your bill’s bottom line. Have a calculator and notepad at the ready. The figures below are firmly grounded by rules set out by government legislation.

Once you have your meter reading, your meter type and the unit price you’re charged (as outlined above) you’re ready to calculate your bill. ‘It’s the UK standard to calculate energy in kWh.’ says Jean Fiddes from E-ON. ‘To work out your gas bill, convert the usage from your meter into kWh.'

• If your meter measures in hundreds of cubic feet (hcf), take the amount of gas used, shown as 'hcf' on your bill, and multiply by 2.83 to convert into cubic metres (m³).
•  If your meter measures in cubic metres (m3), take the amount of gas used, shown with 'm³' on the front of your bill.
•  Multiply the m3 figure by a conversion factor of 1.02264, then by the calorific value. Calorific values vary; you should find this on your bill.
•  Divide this figure by 3.6 to show your usage in kWh.
•  Multiply your usage in kWh by your unit prices to work out your gas charge.

Things to look out for on a gas bill

Jean Fiddes from E.ON Next suggests watching out for the following:

• Previous balance: the balance carried over from your last bill.
• Charges: the cost of electricity and/or gas used plus any standing charges.
• Payments: how much you've paid since your last bill.
• Balance: the amount now due for payment.

If you pay by Direct Debit, you'll see a list of monthly payments and the balance shows whether you're in credit or debit. ‘Look out for the meter readings, dates they were taken and the price they pay per kWh as these make up the main part of a bill’ says Christopher Dalley from EDF. ‘For customers who pay by Direct Debit they should also check that any payments made have been listed correctly. Customers can also check that the meter number on the bill matches the one in their home.’

Bronwyn from British Gas adds, ‘It’s good practice to review that the energy tariff on your bill matches what you believe you have signed up to.’ It's also worth keeping an eye on water bills.

What uses the most gas in a home?

‘It’s difficult to predict what uses the most gas in a home as there are a lot of variable factors with gas,’ says Christopher Dalley from EDF. ‘The age and efficiency of an appliance will have an effect on the cost to run it. An older appliance which isn’t as energy efficient. will cost more to run.’ Add to that the fact that every home differs, both in size and in usage patterns.

‘Most of us rely on our gas boilers to ensure we have constant heating and hot water which account for the most gas usage in our homes,’ says Jean Fiddes from E.ON Next. 'If you heat your home using a gas boiler, it’s really important that you check the age of your boiler.’ You can then decide when is the best time to replace a boiler. ‘If it’s older than, say, 10 to 15 years, look to upgrade to a more energy efficient one,’ adds Jean. ‘This could help to save you money on your gas bills, as well as reducing your impact on the environment.’