# Calculator methodology

## Driving

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

We start by assuming the gas mileage of your car based on the year, make, and model. Our data is taken directly from the U.S. Environmental Protection Agency (EPA)'s ratings, which you can explore more fully on their website at fueleconomy.gov. These ratings are expressed as miles per gallon, or mpg. The EPA provides one fuel economy rating for highway driving, a second for city driving, and a third which combines the two at a ratio of 55% city - 45% highway. We use this third figure in the calculator.

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. Multiplying the number of miles you specify by the fuel economy rating gives us the total quantity of fuel you will use in a year, in gallons.

Finally, we multiply the fuel quantity by a greenhouse gas emission factor. A greenhouse gas emission factor is the number of pounds of carbon dioxide which will be emitted when your car combusts a gallon of fuel. These factors are widely published, but aren't always exactly the same from one publication to the next due to minor differences in assumptions such as the carbon content of the fuel, or numerical rounding. For regular gasoline, we use 19.42 pounds of carbon dioxide per gallon. The resulting number might seem high - a typical car emits 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 from estimates you find on other websites. Some websites include other greenhouse gases that your car emits, in addition to carbon dioxide. Other websites factor in the emissions from "upstream" processing of your gasoline. For example, EPA's fueleconomy.gov website assigns a greenhouse gas rating which includes the full life-cycle of the fuel, including its extraction, transportation, and dispensing. Our calculation is comparatively simple, using only carbon dioxide emissions, and only from fuel combustion inside your car.

Factors used in the driving calculator:

• Gasoline: 19.42 lbs/gallon
• Diesel: 22.38 lbs/gallon
• Biodiesel (B20): 17.90 lbs/gallon
• Compressed Natural Gas (CNG): 14.46 lbs/gallon equivalent
• Ethanol (E85): 2.91 lbs/gallon

Note: Blended biofuels only account for emissions from the fossil portion of the fuel

Special Cases: Electric vehicles

All-electric vehicles use electricity-charged batteries as fuel. Since electricity isn't measured in gallons, and since the greenhouse gas emissions from electricity use are not as simple as those for liquid fuels, our calculator treats these cars differently.

First, electric cars are not available in our calculator's vehicle drop-down menu, because the underlying calculations are so different. Instead, you use the alternate page (click on "Click here to enter your mpg directly").

Next, the car's greenhouse gas emissions will occur where the electricity is generated, not at the car's tailpipe. The emissions from electricity generation vary widely, depending on the type of generating plant and its fuel source. We make assumptions to simplify the calculation, as noted below.

For all-electric vehicles, the calculator uses the following equation:

Annual miles driven divided by Miles per gallon-equivalent
multiplied by
Kilowatt-hours per gallon-equivalent
multiplied by
Greenhouse gas emission factor for kilowatt-hours, in pounds carbon dioxide-equivalent emitted per kwh generated

We use the following factors and data sources:

• Annual miles driven: You enter this yourself
• Miles per gallon-equivalent: You enter this yourself; figures are available at fueleconomy.gov for the Nissan Leaf only.
• Kilowatt-hours per gallon-equivalent: 33.7; US EPA, based on the energy content of a gallon of gasoline (equal to 34.12 kwh) discounted for losses in the battery
• Greenhouse gas emission factor for kwh: 1.37 pounds CO2e per kwh; US EPA's eGRID 2010, Annual Output Emission Rates for Calendar Year 2007, US Nationwide average, including carbon dioxide, methane, and nitrous oxide.

Similar to our calculations for internal combustion engines, we do not account for the transmission and distribution losses associated with the electricity, nor any of the upstream emissions associated with the fuels used to produce the electricity.

## 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