You can either use the paper & calculator-based CRAGs Carbon Footprinting Worksheet, or the spreadsheet at CRAGs Carbon Footprinting Spreadsheet, or one of the web-based carbon accounting sites.
If you want to learn more about footprints, try looking through the carbon conversion factors and comparing the impacts of different fuels and activities. By totalling up your fuel use and travel during the year, and multiplying by the relevant conversion factors, it is relatively easy to calculate your total footprint. The worksheet and spreadsheet are designed to help you with this.
The carbon accounting websites simplify the process by doing the calculations for you, based on your meter readings and records of your trips. If you take regular meter and mileage readings, you can monitor your footprint and see how best to reduce it.
See also: Footprinting
alternatively try one of these
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The government’s new personal carbon footprinting tool:
http://actonco2.direct.gov.uk/carboncalc/html/index.aspx
Resurgence’s comprehensive carbon calculator:
http://www.resurgence.org/carboncalculator/