Check out the EPA's Household Emissions Calculator...
1. To get the most accurate results, gather your recent electric, gas, and/or oil bills so you can use real numbers for your household’s energy consumption. Remember that your energy bills vary by season, so use an average of winter and summer values if you can.
2. Allow yourself 10-15 minutes to enter the data.
3. After entering data, use the TAB key to continue moving through each section of the calculator. When you get to the end of a section, click “Next Section” to move on.
4. Visit the What You Can Do section of the climate change website to learn about other actions you can take to reduce your greenhouse gas emissions.
(via Real Simple)
(photo via here)