The boundary is the whole argument
Personal carbon calculators disagree with each other constantly, and the reason is almost never arithmetic. It is scope. One counts household energy and driving. Another adds flights. Another adds an estimate of everything you bought. Another divides national emissions by population and calls the result your footprint. These produce numbers that differ by a factor of three or more for the same person, and every one of them can be defended by its author.
This page draws the line at direct energy and travel, plus a diet estimate, and prints the line in the results. What that buys you is a number you can act on: every input is something you could measure from a bill or an odometer, and every output responds to a decision you could actually make. What it costs you is comparability. If someone tells you their footprint is 16 tons and yours came out at 9, you have learned nothing until you know which boundary they used.
Why the grid factor is a field and not a constant
Electricity has no inherent emissions. It has the emissions of whatever made it, and in the United States that varies enormously by region. Grid subregion emission rates published by the EPA span roughly an order of magnitude between the cleanest and the most coal-dependent, and within a region they shift year to year as plants retire and renewables come online.
A calculator that hard-codes a national average is therefore wrong for nearly everyone who uses it, and wrong in a way that changes the ranking of the choices. On a low-carbon grid, home electricity becomes a minor line and an electric car becomes almost free of operating emissions; on a coal-heavy grid, electricity dominates the total and switching a car to electric produces a much smaller reduction than the marketing implies. The same appliance, the same car, the same behaviour, different answers.
Find yours before trusting the total. EPA publishes eGRID subregion rates, many utilities publish their own generation mix and emission rate, and some publish it on the bill. If you are on a specific renewable tariff, that changes the number again, though whether it changes actual emissions depends on how the tariff is structured.
The diet figure is the shakiest number here
The four diet options come from published work on the greenhouse gas footprint of self-reported diets, which measures food production, processing and distribution. They are useful for showing the rough size of food relative to energy and travel, which is: comparable, and larger than most people expect. They are not useful as a precise statement about your food.
| Problem | What it does to the number |
|---|---|
| Diet categories are broad | Two people in the same bracket can differ by a factor of two depending on how much of the meat is beef |
| Studies are usually of one country's food system | Production methods, transport distances and waste rates differ elsewhere |
| Food waste is counted inconsistently | Some studies count what was produced, some what was eaten |
| Non-CO2 gases dominate agriculture | Methane and nitrous oxide are converted to a CO2 equivalent using a time horizon that is itself a choice |
Treat the diet line as an order of magnitude. Its main job on this page is to stop the total from implying that food is free, which it is not.
Reading the result without deceiving yourself
The useful output of this calculator is the ranking, not the total. Which of your lines is largest, and by how much, is robust to most of the uncertainty in the factors, because the factors that are uncertain are uncertain by tens of percent while the lines usually differ by multiples. If flying is four times your household gas, that conclusion survives any plausible revision to either factor.
The total, by contrast, is soft in both directions at once. It is too low because it excludes goods, manufacturing and infrastructure. It may be too high or too low on any individual line depending on how well the average factors fit your particular bus route, aircraft, food or grid. So it is a reasonable basis for deciding where your own effort would do the most, and a poor basis for a claim about how you compare to anyone else.
Questions people ask
Why does this not tell me how I compare to the average person?
Because there is no average that this number can be honestly compared against. National per-capita figures divide total national emissions, including steel mills, freight, agriculture and government, by population, and most of that has nothing to do with any household decision. Other personal calculators use wider or narrower boundaries than this one. Publishing a comparison would give a false sense of precision to the least reliable thing on the page. The comparison that does work is against yourself: run it again after a change and see how far the total moves.
How accurate is the flight figure?
It is a reasonable order of magnitude and no better. The 0.15 kg per passenger-mile used here sits in the range that airline and government calculators produce for economy travel, but published tools differ by a factor of two on the same flight depending on assumed load factor, aircraft type, seating class and how much of the take-off and climb fuel is attributed to each mile. Short flights are worse per mile than long ones because take-off and climb dominate. Premium cabins take several times the floor area per passenger and are counted accordingly in tools that model class. And none of the CO2-only figures here include the warming effects of contrails and high-altitude nitrogen oxides, which published assessments treat as adding substantially to aviation impact but which are not carbon dioxide and are not on this page.
Should I count the electricity and gas for the whole household or just my share?
Enter the whole household figure from the bill and set the number of people; the calculator divides. That is the convention most personal footprint tools use and it makes household sizes comparable. Bear in mind it flatters large households on paper, since a two-bedroom apartment with four people in it genuinely does use less energy per person than the same apartment with one. Driving and flying are counted as entirely yours, since you enter your own miles.
What is missing that would matter most if I added it?
Goods and services, by a distance. Clothing, electronics, appliances, furniture, restaurant meals, deliveries and the manufacturing of the car you drive are all outside this boundary, and consumption-based accounting studies typically find that category rivals household energy and transport put together. After that: the construction and maintenance of the house you live in, the roads you drive on and the grid you draw from; water supply and waste treatment; and your share of public services. Adding them would roughly double a typical total and would also make almost every line an estimate rather than a measurement, which is the trade-off this page declines to make.