Frequently Asked Questions

What do my calculator results mean?

Your personal carbon footprint is the sum of the information you input into the calculator, resulting in an estimate of the carbon dioxide that is produced by home energy use, transportation, and household waste. The result is compared against an estimated annual per capita U.S. average of 16,008 pounds resulting from home energy, transportation and household waste emissions.

Here are the comparative results:

Pounds of CO2 emissions per year	Result
Below 6000 lbs/yr	Much less than average
Between 6000 and 11,010 lbs/yr	Less than average
Between 11,010 and 21,010 lbs/yr	Average
Between 21,010 and 26,005 lbs/yr	More than average
Above 26,005 lbs/yr	Much more than average

How did we do the math on the calculator?

The calculator for your personal footprint was based on several areas of energy use that have the greatest impact on carbon dioxide emissions for most individuals: home energy, personal tranportation, and waste.

Home Energy Use

Based on the type of heating source you input, we used the following formulas:

Electricity Emissions = (average monthly electric bill / price per kWh) * electricity emission factor * months in a year
We assumed a national average US price per kWh = $0.1
We used an electrical emission factor (electricity/kWh) = 1.37 [Source: Energy Information Administration (EIA). Electric Power Annual 2005, Table 5.1 (October 2006)]

Natural Gas Emissions = (average monthly gas bill / price per thousand cubic feet) natural gas emission factor * months in a year
We assumed a national average US price per thousand cubic feet = $13.83 [Source: Energy Information Administration (EIA): Natural Gas Annual]
An emission factor (natural gas/thousand cubic feet) = 120.61 was used. [Source: U.S. EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004, Annex 2,Table A30]

Fuel Oil Emissions = (average monthly fuel oil bill / price per gallon) fuel oil emission factor * months in a year
A national average of $2.37 per gallon was used. [Source: Energy Information Administration: US No. 2 Distillate Prices by Sales Type, Table 15.]
Our emission factor used was (fuel oil/gallon) = 22.28 [Source: U.S. EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006, Annex 2, Table A30]

Transportation

Vehicle Emissions = (number of miles driven per week * weeks in a year) / average vehicle fuel efficiency * pounds of CO2 emitted per gallon
We used an emissions factor of 19.564 per gallon of gasoline [Source: Energy Information Administration Fuel and Energy Source Codes and Emissions Coefficients]

Air travel emissions per passenger mile are significantly affected by the length of the flight because a high percentage of fuel use and emissions occur on take-off. Therefore we ask for number of short, medium, and long flights.

The calculator input is the number of each type of flight, defined as each leg of a flight. For example: a round trip flight with one stop each way has four legs. We began with values from the GHG Protocol Initiative, a widely recognized international accounting tool for government and business leaders to understand, quantify, and manage greenhouse gas emissions.

GHG Flight definitions:
short haul: less than 452km*1.6094 = 727.45 miles (1.6094 is the conversion factor for kilometers to miles)
med haul: less than 1600km*1.6094 = 2575.04 miles
long haul: more than 1600km*1.6094 = 2575.04 miles

We then created "averages" for each flight definition. For short haul, rather than average between 0 and 727; we assumed that few would fly if the distance they need to travel can be driven in 4 hours or less. So, 4 hrs. @ 55 mi/hr = 220 (low value) + 727.45 (high value) = 947.45/2 = 473.7 for an average short haul flight.

For medium haul, we went with the assumptions from GHG Protocol Initiative - less than 2575.04 miles, and averaged that with the 727.45 value of the upper limit for short haul. 1651.25 miles is the result.

For long haul, we researched the 10 longest distance non-stops and averaged that number to get the higher value for the range: 8,793 miles. We then averaged again between 8,793 and the 2575.04 miles that represents the low value for the long haul category to achieve an average for the long haul of 5684.02 miles.

Different per passenger emissions factors are used for each flight length, as follows:

Short flight: .64 lbs/mile = 0.2897 kg CO2 per mile*2.2046 (factor for conversion from kilograms to pounds)
Medium flight: .44 lbs/mile = 0.2028 kg CO2 per mile*2.2046 (factor for conversion from kilograms to pounds)
Long flight: .39 lbs/mile = 0.1770 kg CO2 per mile*2.2046 (factor for conversion from kilograms to pounds)

The total number of miles for each type of flight is multiplied by the emissions factor for that type of flight to get pounds of carbon dioxide, which are then added together to get the total carbon footprint from air travel.

Since we created average flight distances to simplify our calculator tool, your actual flight emissions may be higher or lower. For business reporting and emissions management we suggest tracking actual fuel use and type of fuel use for flights - or exact mileage, if fuel use is not available. GHG Protocol provides spreadsheets and instructions for more precise reporting of mobile emissions.

Air travel definitions and per passenger emissions factors for each are from the GHG Protocol Mobile Combustion Tool.
Emissions factors for fuel used in the formula are from the EIA Coefficients Table: http://www.eia.doe.gov/oiaf/1605/coefficients.html

Waste

We based our estimate of the environmental impact of household waste on data from the EPA, with 1010lbs. per person annually being the average. This average was obtained by dividing the tons of waste deposited annually at US landfills by US population.
Emissions = number of people in household * average lb CO2 equivalent. generated from waste per person per year [Source: Based on EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2004, Chapter 8, Table 8-3]

How Was the Reduction Calculated?

Home

Replacing incandescent light bulbs with compact fluorescents: assumes that lights are on for 4 hours per day.

Replacing old refrigerator with an ENERGY STAR® model: assumes old model uses 820 kWh per year; ENERGY STAR model uses 440 kWh per year.

Turning up thermostat for central air conditioner: assumes average household electricity use of approximately 900 kWh per month, and that air conditioners account for 16 percent of residential electricity consumption. [Source: EIA 2001. End-use Consumption of Electricity]

Turning down thermostat in winter: assumes 1 percent savings in energy use for a 1 degree decrease. assumes thermostat is turned down for 8 hours each night November through March.

Replacing single-glazed windows with ENERGY STAR windows: Assumes 2000 square-foot house, 300 square feet of glass, gas heat, and electric air conditioning.

Replacing an old boiler or furnace with an ENERGY STAR model: Assumes a 20 percent savings on heating fuel costs.
Except where noted above, all home emissions reductions were based on data from: ENERGY STAR.

Transportation

Emissions reductions were calculated using the same formulas shown above, and the resulting amount was subtracted from your previous total

Waste

For our estimated reduction in greenhouse gas emissions that you can achieve by recycling, we used emissions data for each source: newspaper, glass, plastic, and metal. Our reduction calculations assume that households recycle 100 percent of all recyclable materials generated as waste. The plastic material type used in the calculator includes PET and HDPE, and the metal material type includes aluminum and steel cans.
[Emissions Factors Source: U.S. Environmental Protection Agency, 2006. Solid Waste Management and Greenhouse Gases: A Life-Cycle Assessment of Emissions and Sinks, EPA530-R-06-004.]

FInd energy data from the US Energy Information Administration online at: http://www.eia.doe.gov/
Environmental information can be found online at: http://www.epa.gov/
Find energy data for home and transportation fuel use online at: http://www.eia.doe.gov/oiaf/1605/coefficients.html
Want to know more about carbon footprints? Visit the Wikipedia page: http://en.wikipedia.org/wiki/Carbon_footprint

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