Frequent Questions: EPA’s Greenhouse Gas Equivalencies Calculator
This page answers some of the most commonly asked questions about the Greenhouse Gas Equivalencies Calculator. Click on the questions below to view the answers.
- What conversion factors are used to generate the calculator’s outputs?
- To what timeframes do the equivalencies refer? For example, does the the “miles driven by an average passenger vehicle” refer to miles driven over one year?
- Which Global Warming Potentials (GWPs) are you using for greenhouse gases in the calculator?
- How can burning 2,000 pounds of coal yield more than 4,000 pounds of carbon dioxide?
- How can it be that the equivalent number of “homes’ energy use for one year” is lower than the equivalent number of “homes’ electricity use for one year?” Home energy use includes electricity, natural gas, LPG, and fuel oil, so shouldn’t the number of homes be higher?
- Why don’t you provide equivalencies in metric units as well as pounds, gallons, miles, etc.?
- How do I calculate the carbon footprint of different food types?
- How do I calculate an ecological footprint?
- How do I calculate average household emissions?
- Can I build my own version of the calculator and post it on my organization’s website?
Assumptions and Sources
All of the calculations, data sources, and assumptions for EPA’s Greenhouse Gas Equivalencies Calculator are provided on the Calculations and References page.
Most of the equivalencies refer only to quantities, rather than quantities over a period of time. The exceptions are easy to identify, since they all include a time frame: for example, “passenger vehicles driven for one year,” “homes’ energy use for one year,” or “coal-fired power plants in one year.”
The calculator uses GWPs from the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (2007). A list of the GWP values is available on the calculator’s Revision History page.
When coal is burned, the carbon in the coal combines with oxygen to form carbon dioxide. One molecule of carbon dioxide is 3.67 times heavier than a molecule of carbon, due to the additional weight of the two oxygen atoms. Coal is not 100 percent carbon: burning a pound of coal emits 2.07 pounds of CO2. The calculations and sources for this information can be found on the Equivalencies Calculator’s Calculations and References page.
How can it be that the equivalent number of “homes’ energy use for one year” is lower than the equivalent number of “homes’ electricity use for one year?” Home energy use includes electricity, natural gas, LPG, and fuel oil, so shouldn’t the number of homes be higher?
According to the Calculations and References page, each home emits 10.97 metric tons of CO2 per year from all energy use combined, versus 7.27 metric tons of CO2 per year from electricity alone. When an emissions value is entered into the calculator, it calculates the number of households it would take to emit the same amount of CO2 through their overall energy use per year, as well as the number of households it would take to emit the same amount if we consider their electricity use alone. Because the emissions per home are higher when we consider total energy use, it would take fewer homes to emit a given number of metric tons through their total energy use than through their electricity use alone. That’s why the equivalency for the number of homes is smaller for total energy use than for electricity use.
The calculator is designed for use in the United States, and metric units may mislead users into thinking that the equivalencies provided by the calculator are relevant to other countries. As indicated in the Calculations and References page, which presents all the sources, assumptions, and calculations used in the calculator, half of the equivalencies in the calculator are based on US-specific data and would not be accurate outside of the United States.
Extending the Calculator
EPA has not developed a methodology for estimating the life-cycle carbon footprints of food types. The EPA Waste Reduction Model (WARM) includes estimates for emissions associated with food waste, but not for food production and distribution. Documentation for Greenhouse Gas Emission and Energy Factors Used in the Waste Reduction Model (WARM) provides links to studies on life-cycle emissions from a variety of food sources.
Ecological footprints typically include a carbon footprint that represents the area of forest land that would be required to sequester CO2 emissions from human activities. According to the Calculations and References page of EPA's Greenhouse Gas Equivalencies Calculator, one acre of average U.S. forest sequesters 1.06 metric tons of CO2 annually. Using that figure and assuming you have calculated your CO2 emissions, you can estimate the number of acres of forest that would be required to sequester those emissions.
EPA does not provide an official estimate for average household emissions, but there are two ways you could derive this information from EPA sources:
- EPA’s Inventory of Greenhouse Gas Emissions and Sinks provides a breakdown of emissions by sector, including details for the residential sector. If you divide the total by the number of households in the United States, you will obtain an estimate of residential emissions per household. Note, however, that this does not include transportation. Transportation-related emissions are in Table 2-13. U.S. households own an average of 1.9 vehicles per household.
- EPA’s Household Carbon Footprint Calculator provides values for average emissions from households for specific areas such as vehicles and specific fuels. These values could be built up into an estimate, although note that this does not include sources such as air travel or food. The Household Calculator was designed primarily as an educational tool and does not produce precise estimates.
You may use the calculations to build your own calculator, but please note that EPA updates the calculator periodically (typically annually), so you would need to check back for updates to the conversion factors. The Revision History page documents the changes made in each update.