Footprints offer clues about where we came from and where we're headed. Their impressions tell us something about the animals that leave them. But while actual footprints offer details on size, weight and speed, carbon footprints measure how much carbon dioxide (CO2) we produce just by going about our daily lives. A drive to school, a flip of a light switch and a flight to see your grandparents all rely on the combustion of fossil fuels like oil, coal and gas. When fossil fuels burn, they emit greenhouse gases like CO2 that contribute to global warming.

People concerned with the environment and global warming usually try to reduce their carbon output by increasing their home's energy efficiency and driving less. Some start by calculating their carbon footprint to set a benchmark -- like a weigh-in before a diet. A carbon footprint is simply a figure -- usually a monthly or annual total of CO2 output measured in tons.

One of the easiest ways to calculate your carbon footprint is by using an online carbon calculator. To remain as accessible as possible, most carbon calculators ask very simple questions about consumption. 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.

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Next, you’ll be asked about monthly power usage. To determine the amount of CO2 produced by home electricity, the calculator divides the estimated or exact usage by the price of power in your 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.

Ecological Footprints A carbon footprint is only one component of the broader ecological footprint. An ecological footprint compares the population's consumption of resources and land with the planet's ability to regenerate. According to the Global Footprint Network, the Earth's ecological footprint is currently 23 percent over capacity. It takes about one year and two months to regenerate what we consume in a year.

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?