The other thing I really like about this tool is that the first thing it asks is which country I live in. This is very relevant, particularly when it comes to calculating my impact from electricity usage.
If you live in a country like Iceland, where they generate 100% of their electricity with renewables (75% from large hydro, and 25% from geothermal) your carbon footprint from electricity consumption is zero.
By contrast, if you live in a country like Poland or China, where a high percentage of electricity is generated from burning coal, your carbon footprint from your usage of electricity will probably be huge.
What can you do about it?