COTAP’s Carbon Calculator Methodology

General Notes

  • Cotap’s CO2 calculator focuses on U.S. Residents and the major sources of their annual emissions – car travel, air travel, and home energy use.  You should be able to complete it in 10 minutes or less.
  • You are welcome to calculate part or all of your footprint elsewhere, and to override one or more categories in the calculator’s ‘Total’ section by selecting ‘I want to enter my own value’  before matching your tonnes with one or more of our projects.
  • Results are in metric tons, also referred to as tonnes, which are 2,205 pounds.

Car Travel

  • Burning one gallon of gasoline emits 19.4 pounds of CO2 into the atmosphere, and one gallon of diesel emits 22.2 pounds (U.S. Environmental Protection Agency).
  • Example (gasoline):  12,000 miles per year ÷ 30 miles per gallon average = 400 gallons.  400 gallons x 19.4 pounds per gallon = 7,760 pounds ÷ 2,205 pounds per tonne = 3.52 tonnes.
  • Currently, our calculator only allows for two cars.  If you have more cars, you’ll need to combine the miles and average the mileage figures for one of the cars.

Air Travel

  • Formula for a given round trip:  (Round Trip Miles x Emissions Factor x Radiative Forcing Index Factor) ÷ 2,205 lbs per tonne = tonnes per round trip.
  • Example formula for a medium round trip flight: (2,000 miles x .371 lbs CO2 per mile x 1.9 RFI) ÷ 2,205 lbs per ton = .639 tonnes per flight.
  • Round trip mileage amounts used.  Short: 1,000 miles.  Medium:  2,000 miles.  Long:  5,500 miles.  International:  10,000 miles.
  • Emissions factors used.  Short: .631 lbs CO2 per mile.  Medium:  .371 lbs CO2 per mile.  Long/International:  .427 lbs CO2 per mile.
  • Emissions factors (CO2 emitted per mile flown) and RFI of 1.9 are derived from the UK Department for Environment, Food, and Rural Affairs’ (DEFRA) July 2011 Greenhouse Gas Conversion Factors.
  • Radiative Forcing Index (RFI).  CO2 emissions from air travel have a greater effect on climate change because they are released directly into the upper atmosphere, instead of at lower elevations.
  • If you like, you can fine-tune your air travel footprint by using decimals.  For example, for a 3,000-mile roundtrip flight you can input 1.5 in the “Medium” flights field.

Home Energy

  • Technical note: For home energy calculations, our calculator can only handle one method at a time.  If you start using the Average method and decide to switch to the Actual method, you’ll need to reset the Averages’ State/Heating Type inputs to ‘Select,’ and vice versa.

Average Method

  • Electricity formula (general):  Your state’s CO2 lbs per kilowatt-hour (kWh) emissions rate x your state’s average residential kwh’s.  The U.S. EPA publishes state electricity CO2 lbs per kWh, which is important to use because state emissions per kWh vary greatly due to how the electricity is generated (nuclear, coal, renewables, etc.).  We also use the U.S. Energy Information Administration’s (EIA) data on average electricity consumption by state, because your region can heavily affect how much electricity you use for heating and/or air conditioning.
  • For more information on why CO2 emissions per kWh vary by state, see the EPA’s Power Profiler.
  • Electricity as heating type.  If you select ‘electricity’ as your heating type, we assume your electricity consumption for heating is already reflected in the residential kWh average for your state.
  • Non-electricity heating types.  If you choose natural gas, heating oil, or propane as your heating type, we reduce your electricity-related tonnage by 9.1% (the 2009 U.S. household average amount of electricity used for space heating) before adding tonnage for those respective heating types.
  • Natural gas:  (Your state’s residential cubic feet ÷ Your state’s residential customers) x .12 lbs CO2 per cubic foot ÷ 2,205 lbs per tonne.  Based on year 2009 data.
  • Heating Oil.  Household average for U.S. using heating oil as the primary heating source.  Based on 2009 U.S. residential consumption of 4.02 billion gallons for home heating oil (Distillate No.2), divided by 8 million households, equals 503 gallons/household per year, which is then multiplied by 22.33 lbs CO2 per gallon, then divided by 2,205 lbs per tonne to get 5.09 tonnes of CO2 per year emitted by the average U.S. heating oil household.
  • Propane.  Household average was 552 gallons of LPG per year in 2005.  Burning a gallon of propane results in 12.65 lbs of CO2 emissions.  So, (552 x 12.65) / 2,205 lbs per tonne = 3.17 tonnes produced by the average propane household.

Average Method Accuracy

  • Cotap’s average method for your home energy footprint generates accurate results in most situations.  However, if you have an uncommon region/heating type combination, an inaccurate total will likely result because there is no available data on residential heating oil and propane consumption by state.
  • As an example, if you live in Hawaii and happen to heat your home with heating oil (which is unlikely), then the Cotap calculator will still add 5.09 tonnes to your household total, but that 5.09 tonnes will be inaccurate because most heating oil customers are located in the Northeast region of the U.S., where the winters are much colder than in Hawaii.
  • If you disagree with your home energy footprint calculation using this method, please know that you are able to override it (with a number you prefer and/or calculator elsewhere) on the totals page prior to matching your CO2 footprint with one of our projects.

Actual Method

  • You’ll need access to your utility bills and/or your online utility account for this method.
  • For the most part, the above methods and logic are applied (i.e. CO2 lbs per kWh for your state, CO2 lbs per cubic foot of natural gas, and CO2 lbs per gallon of heating oil or natural gas.
  • The main difference is that you enter your actual kWh consumption and heating fuel usage (if not electricity) for the past 12 months.
  • For natural gas, the EIA reports consumption in cubic feet.  Most people are billed in therms or “CCF”, which means hundred cubic feet.  Cotap’s calculator is set up for therms.  One therm = 1.03 CCF.  If you are billed in CCF’s, multiply your annual total by 1.03 to get therms and thus ensure the highest level of accuracy.

Other categories

  • Cotap focuses on the major sources of your carbon footprint: car travel, air travel, and home energy use.
  • If you would like to refine your CO2 footprint by accounting for categories like diet, pets, buying local, recycling, etc. you may adjust your total before checking out with us.  If you feel strongly about us making this a formal part of our calculator, please send us feedback (instructions on that below).

Non-U.S. residents and using CO2 footprints calculated elsewhere

  • If you prefer a calculator from another website, you are more than welcome to generate your CO2 footprint figure there and then overwrite your total tonnes on Cotap before checking out through us.
  • Figures generated by Cotap’s calculator are metric tons, also referred to as ‘tonnes” (2,205 pounds), not short tons (2,000 pounds).  If you calculate your footprint elsewhere in short tons, convert it by dividing by 1.1 (which is 2,205 ÷ 2,000) before checking out through us.  Similarly, if you calculate it in pounds, divide by 2,205.