This will convert your gas usage according to your meter readings into kilowatt hours (kWh).
Simply enter the units used in hundreds of cubic feet or cubic metres, depending on your meter, and click on calculate.
If you know the Calorific Value for your billing area, you can enter that as well. If you don't the default value will give a close approximation, which is adequate for most purposes. The actual value will be somewhere between 37.5 and 43.0 for the UK, with the exception of Stornoway, which uses LPG and has a much higher calorific value of around 93.