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After taking a few different "tests," I have realized that results may vary quite significantly.

Is there an online, comprehensive carbon footprint "test" that is both precise and accurate?

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  • A good test must certainly include transportation, housing, diet, purchase habbits, and your local energy system. I noticed many tests do not allow one to choose things like what their car runs on (I have an electric car).
    – user2433
    Nov 19, 2015 at 21:32
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    One would first need to spend hours putting accurate numbers into such a test. If you want an accurate answer you need accurate inputs, so it's not enough just to enter your daily food intake as total kilojoules consumed, one needs to specify the supplier of ones organic whole milk as well as the means by which it is transported.
    – Ⴖuі
    Nov 21, 2015 at 20:06
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    It seems to me that it might be more useful to have a spreadsheet (much as I distrust them) that one could download, with a lot of pre-loaded factors for uncertain factors (such as type of energy generation), which one can override by entering more specific data in other cells. Ideally the inputs should be on one sheet and the calculations on another, so one could download updated calculations without losing one’s inputs.
    – PJTraill
    Dec 3, 2015 at 15:46

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It's not possible to accurately estimate it.

There are only approximations. There are so many uncertainties. For example:

  • When you use electricity, should you use average carbon intensity, or marginal carbon intensity? Each tends to vary constantly (unless you're on one of the handful of already-zero-carbon electricity grids)

  • What adjustment do you make for the change in congestion if you drive?

  • What's the CO2e multiplier for flights? (big uncertainties on that one)

  • What's the embedded carbon in the objects you've consumed?

  • Which methane-multiplier should you use - 5-year, 30-year, 100-year?

  • How much methane leakage should you include? (It's surprising how little we know about that)

That's why you'll get such wide variations: there are big uncertainties attached to most of the calculations.

So no, one cannot accurately estimate one's own carbon footprint. The best you can do is get a wide range of possible values, and some clues as to where it's easiest to start reducing it.

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I agree with the answer given by EnergyNumbers. At best you can get a rough estimate of your carbon footprint. If you want an accurate result you would need to do a carbon footprint analysis of all products you buy, all services you use, and all activities you do, and then aggregate the results. And even then the accuracy of the end result is questionable. As EnergyNumbers also mentioned, we have to make a lot of assumptions because there are so many unknowns.

Nevertheless, there are some things you can do to get better results, or have to keep in mind when interpreting the results.

  1. Consult a carbon footprint calculator that has been made specifically for your country of residence. This is because many calculators rely on national (average) data.
  2. If possible, try to consult multiple calculators. The ones with the most questions and parameters are more likely to give a better, more accurate result for your specific situation.
  3. Be careful when comparing the results of different calculators because calculators often do not have the same scope. For example, not all calculators include indirect CO2 emissions (that were emitted to manufacture a product or service before you started using it). Often comparison between calculators is impossible, because many calculators don't make their exact scope clear.

One of my favourite carbon footprint calculators is the CoolCalifornia calculator because it includes indirect emissions, shows the results for individual categories and has quite a bit of parameters. It does have it's downsides though; default settings (and probably underlying data model) are for US residents, there is no option for electric cars, and personally I do think it is a bit negative when it comes to emissions for transportation.

Alternatively, US residents can use the EPA carbon footprint calculator but it lacks (indirect) emissions from things like food, clothing and services. The nice thing however is that the EPA also provides a spreadsheet, which allows you to see all the used calculations and parameters. So if you really want to make an effort and know what you are doing, you could tweak those parameters to make results more specific for your situation (perhaps even for your country of residence if you are not a US resident).

I've read that the Carbon Story calculator is quite good as it was created to meet the principles identified in this research, but I haven't tried that one myself yet.

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