WinACC recommends the carbon calculator REAP-Petite because it gives the most complete estimate of your carbon footprint of all the calculators we've seen (August 2013). This is because it estimates indirect emissions from things like shopping, food and leisure activities, as well as direct emissions from electricity, gas and travel. But if you’ve used another calculator before, such as Act on CO2, and are happy with it, we suggest you stick with it. What's more important is to see whether your footprint is going up or down, and to do that you need to compare estimates using the same calculator each time. Similarly, if you want to aim at a percentage reduction in emissions, you should stick with the same calculator. (Of course, if you’re willing to dig out your energy bills from previous years you can re-calculate your emissions using REAP-Petite.) The choice is yours.