Find your last gas bill. Look on the bill for the meter reading that was taken the last time British Gas calculated your bill.
Look at your gas meter. Find out if the meter is a "Digital Metric Meter" a "Digital Imperial Meter" or a "Dial Meter". Each meter will measure your gas consumption using different figures. However, every meter measures gas consumption in kilowatts (kWh).
Check the meter reading on your meter. Subtract the meter reading when your last bill was calculated from the meter reading now. This shows you the amount of gas in kilowatts you have used since your last bill.
Look at your bill
to find out which British Gas tariff charges apply to you. Find the section "How We Calculate Your Charges". Check what British Gas charge per kilowatt and calculate your usage from the last British Gas bill. Alternatively, go to the British Gas website (see resource below). Click on "Help and Advice". Scroll down to "Bill Queries" and move the cursor across to "Check Which Tariff I'm on". Click on which tariff applies to you. Scroll down and click on "Gas Unit Rates". Cross refer which company supplies your gas with your payment method. Read the terms and conditions at the bottom of the page especially the "Tier 1" and "Tier 2" calculation rates. Calculate your bill based on the kilowatt rates displayed.