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
