What's your carbon footprint? Find out

• Climate Change in Minnesota: An MPR News special report

It can be hard to get a handle on carbon emissions. Virtually everything we do has some effect on the amount of carbon dioxide — the main greenhouse gas driving climate change — going into the atmosphere. But you can't see it, smell it or feel it.

So we built a simple calculator to convert your home energy use and your driving habits — big determinants of your overall carbon footprint — into the amount of carbon dioxide you generate.

A typical household fills 56 boxcars. How many would you fill?

And we're expressing it in the number of standard 50-foot railroad boxcars that your household would fill with carbon dioxide in a year.

For most of us, that's a lot of boxcars.

To use the calculator, you need:

Kilowatt hours. Add up a year's worth from your monthly electricity bills.

Therms. Add up a year's worth from your monthly natural gas bills. (A therm is the heat content of roughly 100 cubic feet of gas. The number of therms is what your bill is based on.)

Gallons of gasoline. Divide your total household miles driven in a year by the average miles per gallon your vehicles get.

This isn't all the carbon you generate. It doesn't take into account the food you eat, the waste you generate, the flights you take or what was involved in the manufacture of other goods you use. But for most of us, these three factors are the biggest.

Typical household

For example, a typical Xcel Energy residential household user consumes 7,802 kilowatt hours in a year. (The typical annual residential bill in 2014 was $1,021.) At 1.041 pounds per kilowatt hour, this translates into 8,122 pounds of carbon dioxide released when the electricity is generated. (That's an average; the actual number of pounds per kilowatt hour varies from day to day as the mix of wind, solar, nuclear, gas and coal sources of power shifts. It's also greater for other utilities that rely more on coal.)

The average residential natural gas customer for CenterPoint, the state's largest supplier, consumes 897 therms in natural gas in a year.

That costs the customer a little over $700 and, at 11.76 pounds per therm, translates into 10,548 pounds of carbon dioxide.

The average U.S. driver puts 13,476 miles a year on his or her car. So a household of two average drivers whose vehicles get 25 miles per gallon (roughly the average of new cars on the road) buys 1,078 gallons of gasoline in a year. At 19.6 pounds of carbon dioxide per gallon, that translates into 21,344 pounds.

So the total electricity, natural gas and gasoline carbon impact of that typical (and probably mythical) household is 40,014 pounds of carbon dioxide. At 8.741 cubic feet per pound, that's 349,762 cubic feet of gas.

Because a standard railroad boxcar has a volume of 6,235 cubic feet that household fills 56 boxcars a year with carbon dioxide.

Of course, this isn't a complete picture.

Air Travel

Estimates vary widely for air travel carbon generation, but it can be a huge factor. A round-trip flight from Minneapolis-St. Paul to Paris generates between 2,838 pounds (Carbon Neutral) and 7,744 pounds (Climate Friendly) of carbon dioxide per passenger. That's between 4 and 11 boxcars each.

A round-trip flight from Minneapolis-St. Paul to New York yields one to three boxcars full of carbon dioxide per passenger.

For comparison, driving from Minneapolis to New York and back at 25 miles per gallon generates about 2½ boxcars. (The driving-flying choice is a close call if you're alone; if there are more than one of you, driving is less carbon intensive.)

For a more detailed calculator for carbon footprints, see the U.S. Environmental Protection Agency calculator.

Getting your number down

Eight years ago, Minnesota established a goal for reducing overall carbon emissions by 2015. In a report in January, state officials forecast that to hit the goal, Minnesota needs to reduce carbon emissions from 2012 levels by another 9 percent or so.

If that burden fell to households, that's five boxcars for each typical household.

One thing going in some residents' favor: Xcel Energy announced a plan last month to reduce carbon emissions from the electricity it distributes in coming decades. As it shifts away from coal toward wind, solar and other sources, it will lower the average amount of carbon dioxide that each kilowatt hour generates. In the meantime, here are three ways to reduce your carbon dioxide generation by a boxcar:

1. Reduce your gasoline consumption by 36 gallons. That's about three tankfuls a year in a medium sized car or, at 25 miles per gallon, about 900 miles less driving.

2. Reduce your electricity consumption by 685 kilowatt hours. If you have a 100-watt light bulb burning day and night all year, turn it off.

3. Reduce your natural gas use by 61 therms. For the typical household that would be a reduction in gas use of about 7 percent.

Sources: Xcel Energy; CenterPoint; Federal Highway Administration; U.S. Environmental Protection Agency; Brian Davis; P.E.; Climate Friendly; Carbon Neutral; U.S. Energy Information Administration; Minnesota Department of Commerce; BNSF Railroad; Universal Industrial Gases.