How to calculate a gas bill from meter readings
Your meter measures volume. Your bill charges for energy. The conversion between them is a formula printed on every UK gas bill.
The formula
kWh = volume in m³ × 1.02264 × calorific value ÷ 3.6
It comes from the Gas (Calculation of Thermal Energy) Regulations, and it is the same one your supplier uses. Every figure in it is on your bill.
Step one: get the volume in cubic metres
Subtract the previous reading from the current one. What you do next depends on your meter:
- Metric meter. Reads directly in cubic metres, and usually says m³ on the face. Nothing more to do.
- Imperial meter. Reads in hundreds of cubic feet. Multiply the difference by 2.83.
Forgetting that multiplication is the single most common error in a gas calculation. It makes the answer roughly a third of what it should be, so if your figure comes out far below your bill, check the meter type first. There is more on telling them apart in how to read a gas meter.
Step two: the volume correction factor
A fixed 1.02264 for domestic meters. A cubic metre at your meter is not a cubic metre at standard temperature and pressure, and this reconciles the two. Leave it alone unless your bill states something different.
Step three: the calorific value
This is the figure that matters, and the one people skip. Gas is not a constant product — its energy content depends on what is in the network, and it changes daily. National Gas may deliver anywhere between 37.5 and 43.0 MJ/m³.
Your bill states the average for your billing period, usually printed near the meter readings. Using a generic mid-range figure instead changes the answer by up to 14%, which is the difference between a calculation you can check against your bill and one you cannot.
Step four: the cost
Once you have kWh, gas bills exactly like electricity: usage times the unit rate, plus the standing charge times the number of days.
A worked example
A metric meter goes from 4,000 to 4,380 — 380 m³. At a calorific value of 39.5: 380 × 1.02264 × 39.5 ÷ 3.6 = 4,264 kWh. At 6.2p per kWh that is £264.37, plus 91 days of standing charge at 31.6p, or £28.76. Total £293.13.
Expect seasonality
Gas use is dominated by heating, so a winter quarter can be four or five times a summer one. Multiplying a January bill by four to get an annual figure will overstate it substantially.
The gas cost calculator does all four steps and tells you whether the result should match your bill, based on whether you supplied your own calorific value.