Your gas meter shows gas consumption in cubic meters whereas usage on your bill is shown in Usage Units.
To determine your gas usage in Units, the following calculation is performed:
Usage units = ((End read - Start read) x Pressure Factor x Heating value)/3.6 = Usage (MJ)
While this is a common formula used to calculate your gas consumption in megajoules, it can vary slightly depending on the type of gas meter you have at your property.
The Pressure Factor and Heating Value are used to convert the metered gas volume you have consumed into an amount of energy calculated in megajoules.
A unit of gas is 3.6 megajoules and is equivalent to 1 kilowatt hour.