# How Carbon Footprints Work

Calculating Carbon Footprints
Carbon calculators convert your annual electric bill into a quantifiable amount of CO2.
© Photographer: Aberenyi | Agency: Dreamstime

To remain as accessible as possible, most carbon calculators ask very simple questions about consumption. They accept estimates of annual electricity usage or mileage instead of exact and difficult-to-provide totals.

Carbon calculators usually start by asking for your location. Regardless of your personal consumption, your carbon footprint is partly determined by the state you live in. Some states rely more on dirtier sources of power like coal; others use larger percentages of renewable sources that produce less CO2.

Most calculators also ask for the size of your household. The calculator can then differentiate between your personal carbon footprint and that of the house as a whole.

To determine the amount of CO2 produced by home electricity, the calculator divides the estimated or exact usage by the price of power in the area. The calculator then multiplies this number by the state's emissions factor, a figure that relates to the type of energy the state uses. Calculators also factor in natural gas, heating oil and propane use.

Most calculators account for the lower emissions of green power subscribers. Some power companies allow customers the option to pay a bit more on their monthly bill to receive a certain percentage of power from renewable sources. This reduces the state's production of fossil-fueled power and helps develop a green market.

Carbon footprints also include the CO2 produced by transportation. Most people don't know their annual fuel usage, so calculators usually ask for an estimated annual mileage and the car's make, model and year. The calculator divides mileage by the car's fuel efficiency to determine annual fuel usage. This figure is then multiplied by the emissions factor of gasoline or diesel fuel, which converts it to pounds of CO2.

For air travel, some carbon calculators ask for an annual estimate of mileage. Other calculators account for increased emissions during takeoff and ask for the number of short, medium, long or extended flights.

After compiling all the figures, the calculator produces a total CO2 output in tons -- a carbon footprint. Some calculators even put footprints in context by comparing them with the national or global averages. If you have the data ready, the whole process takes the calculator about 30 seconds.

But what do you do after figuring out how much CO2­ you produce? In the next section, we'll learn how to knock down your footprint a few sizes.­