# Calculator methodology

## Driving

Our carbon calculator estimates the amount of carbon dioxide your car puts in the air every year, based on your mileage and the type of vehicle your drive. Several assumptions underlie this calculation, so the final number is meant to be an estimate only.

We start by assuming the gas mileage of your car based on the year, make, and model. Our data is derived from the Environmental Protection Agency (EPA)'s ratings, which you can explore more fully on their web site at fueleconomy.gov.

The EPA provides an average fuel economy for highway driving and another for city driving. It also recommends these be combined to an overall average of 55% city and 45% highway driving.

We then plug in the number of miles you drive each year (as specified by you). The average American drives 12,000 miles per year, but you can adjust this figure yourself. Using all of these numbers, we can figure out how many gallons of gas you burn every year.

Finally, we multiply the number of gallons of gas by 19.564 (for regular gasoline – diesel engines use a different factor of 22.91) to figure out how many pounds of carbon dioxide your car puts out. The resulting number might seem high – a typical car puts out many times its own weight in carbon dioxide every year. But remember that when fuel burns, it combines with oxygen in the air and becomes much heavier.

Our estimate may differ slightly from the estimates you find on other web sites. One reason for the discrepancy may be that some web sites include other greenhouse gases that your car creates in addition to carbon dioxide. Other reasons could be that we use slightly different assumptions about your driving habits, or slightly different fuel ratings for your car.

Factors used in the driving calculator:

• Gasoline: 19.564 lbs/gallon
• Diesel: 22.91 lbs/gallon
• Biodiesel: 4.582 lbs/gallon
• Compressed Natural Gas (CNG): 16.09 lbs/gallon
• Ethanol: 16.16 lbs/gallon

## Air travel

Our air travel carbon calculator offers three different methodologies for calculating your carbon emissions from flying. At the present time, none of these methodologies involves the use of a Radiative Forcing Index (RFI).

1. Simple short/medium/long

These calculations are based on average flights from multiple sources.

The calculator uses a number of assumptions to come up with this estimate, so the final amount should be regarded as a general guide rather than a precise figure. These assumptions are based on the greenhouse gas emissions protocols developed by the World Resource Institute (WRI).

The carbon index is just an approximation based on industry averages. The actual amount of fuel burned per passenger per mile depends on the type of plane, the number of people flying, the weight of the cargo, and other factors.

The carbon amounts for each are as follows:

• Short: 500 lbs
• Medium: 1,250 lbs
• Long: 2,000 lbs

Because take-off and landing use more fuel than flying at the same altitude, we add an additional per-stopover emission calculation of 225 lbs CO2.

2. WRI short/medium/long flight data

This method calculates the carbon dioxide emissions from your flying based on the number and distance of the trips you take.

The calculator uses a number of assumptions to come up with this estimate, so the final amount should be regarded as a general guide rather than a precise figure. These assumptions are based on the greenhouse gas emissions protocols developed by the World Resource Institute.

We start by calculating the distance between your origin and destination city, based on their latitude and longitude. This is a standard trigonometric calculation, and we ignore any stopovers you might make along the way. If you wish to calculate the effect of stopovers, simply enter the legs of the flight as separate trips.

Next we categorize your flight as a short-, medium-, or long-haul trip. Because planes burn more fuel at takeoff and landing than at cruising altitude, short-haul trips are less fuel-efficient per mile flown.

For each of the three types of trips, we use a different carbon index that indicates the amount of fuel burned, on average, per mile of the journey. By multiplying this index by the distance of your trip, we determine how much fuel was burned per passenger for that particular flight.

The carbon index is just an approximation based on industry averages. The actual amount of fuel burned per passenger per mile depends on the type of plane, the number of people flying, the weight of the cargo, and other factors.

But on average, the resulting flight emissions profile provides a good approximation of the global warming impact of your flying. Chances are, it's bigger than you thought.

Emissions factors:

• Short (less than 281 miles): 0.64 lbs/mile
• Medium (281 to 994 miles): 0.45 lbs/mile
• Long (over 994 miles): 0.39 lbs/mile

3. Detailed calculation using data from TRX Travel Analytics
The TRX emissions data is pulled from one of the most comprehensive flight databases.

It uses emissions factors by route, airplane and capacity to produce an accurate assessment of the likely amount of jet fuel used.

## Home energy

The TerraPass home emissions calculator uses a combination of consumption, price and emissions statistics that are made publicly available by various government agencies.

Consumption and price information for electricity, natural gas, heating oil and propane all come from the Energy Information Administration at the Department of Energy.

Electricity emissions data comes from the EPA's eGRID program. Emissions rates for electricity varies by area due to regional differences in fuel sources for power plants. You can check your electricity's emissions rate using the EPA's Power Profiler.

Other emissions factors:

• Natural Gas: 12.0593 lbs/CCF (100 cubic feet)
• Propane: 12.669 lbs/gallon
• Heating Oil: 22.384 lbs/gallon