Your gas meter measures gas usage in cubic meters, but in most states your usage is actually billed in ‘megajoules’ (MJ) on your bill. In WA only, usage is measured in 'units'. A unit of gas is equivalent to 1 kilowatt hour of energy. To determine units, the number of megajoules is divided by 3.6.
To calculate the number of megajoules you've used in a billing period, AGL first subtracts the start read on your meter from the end read, and then multiplies the result by two values - the heating value and pressure factor.
Both of these values are provided to us by your local gas Distributor - the company responsible for reading your meter and maintaining the pipes in your area. Heating value and pressure factor vary based on local conditions at different geographical locations, such as temperature, pressure and the appliance consuming the gas.
All of the values used in these calculations are shown on the last page of your gas bill. Here's an example of how they look:
And the calculations using these example numbers:
( End read - Start read ) x Heating value x Pressure factor = Usage MJ