How is your carbon footprint calculated?

What goes into the carbon calculator black box?

My household, of two adults, a toddler and two cats, holds a confusing position in the world: we are either climate villains, or conscientious and sustainability-focused citizens.

On average we create an annual 22 tonnes of carbon dioxide equivalent (t/CO₂e, a figure which incorporates a range of greenhouse gases from carbon to methane and nitrous oxide), according to 10 different carbon calculators. That’s a very respectable 7 t/CO₂e a year each, considering the national per capita average is 19 t/CO₂e (a problematic figure, this; but more on that later).

We got there by punching in a range of data including total annual electricity spend or kilowatt hours used ($1323 or 4121 KWh), our estimated car use (11,569 km in 2021), an estimate about how often we eat meat (three, four times a week? Do we include daycare lunches?), number of flights and various other movements.

We are either climate villains, or conscientious and sustainability-focused citizens.

Each took anywhere between five and 20 minutes. Some calculators were extremely vague on detail, like the Global Footprint calculator, which involved guessing where we sat on a line from ‘never’ to ‘very often’, whereas others demanded a forensic analysis of a year’s worth of power and water bills and some mental arithmetic – such as Carbon Neutral’s request for bin size, collection frequency, and fullness at collection.

My household average combines a wide range of readings from a minimal 8.6 t/CO₂e to 42 t/CO₂e per year.

So are we as much part of the problem as our dual-car, foreign-travel-loving, mansion-owning Boomer parents? Or can we remain in safe complacency because we’re actually pretty awesome?

The answer is, of course, somewhere in between. Welcome to the world of carbon calculators: an important but opaque set of websites that rely on a variety of methods and data sources used to reach your personal footprint – and, for some calculator creators, an incentive to make that number as big and scary-looking as possible.

How simple is too simple?

Carbon calculators spit out an individual’s or a business’s carbon footprint, which is an estimate of the total carbon emissions generated by that person or entity over a specific period of time.

Cosmos measured nine carbon calculators designed for individuals only that were either Australian-made or produced an Australia-specific number.

CalculatorAnnual Williamson household emissions, (t/CO₂e and t/CO₂*)Options offered by the calculator owner to reduce carbon footprints
Carbon Positive Australia22 t/CO₂eCarbon offsets
Carbon Neutral41.72 t/CO₂eCarbon offsets
Global Footprint Network10.2 t/CO₂ 
Active Super40.3 t/CO₂eOffer to switch to its superannuation fund
Tree Project16.6 t/CO₂e 
Trace15.6 t/CO₂eCarbon offsets
Carbon Click23.7 t/CO₂eCarbon offsets
WWF16.9 t/CO₂ 
Climate Hero8.6 t/CO₂eCarbon offsets

* Most calculators use CO₂e, which covers a specific range of gases, but some focus only on carbon dioxide (CO₂).

Carbon calculators are simple by necessity.

Most people won’t know exactly how many kilometres they drive or fly in a year, nor will they care to tot up their precise electricity usage by the kilowatt hour. But with simplicity comes inaccuracy, as the level of detail fails to capture the full scope of a person’s lifestyle – and thus their carbon cost.  

The Australian Greenhouse Calculator (AGC) was, when it still worked, the most comprehensive piece of personal carbon accounting software. It was backed by a 99-page outline of how each part of the calculator worked and, according to its creator, and asked for extensive and detailed information.

Carbon calculators are simple by necessity.

“That’s the problem – most of them are very simple,” says Alan Pears, senior industry fellow at RMIT and AGC creator.

“Say I’m about to renovate my house. What do I do? Or how should I revisit the kinds of foods I should eat? Most of them are too simplistic.

“If you look at the wine industry, the emissions per bottle of wine can vary by a factor of 10, because the large volume winemakers are very efficient compared to a boutique one.”

Wine making is just one industry where bottle weight and electric tractors are part of a push to go green, and it means a range of questions are becoming increasingly difficult to measure.

One example is whether one eats meat, because Australian sheepmeat was carbon neutral according to one measure in 2020 and the broader livestock industry aims to hit net zero by 2030. 

Inside the black box

Different calculators use different data and make different assumptions, based on nationwide averages that are unlikely to reflect any one individual’s lifestyle.

The average emissions attributable to each individual Australian in 2020 worked out at 19.4 t/CO₂e, as total national emissions hit 499 million tonnes CO₂e for a population of 25.69 million. The World Bank’s estimate was 15.5 t/CO₂e in 2018.

All carbon calculator comparisons sit between these two figures. But this figure isn’t an accurate benchmark to measure your personal emissions against, because it includes ‘country emissions’ you didn’t have a part in creating.

The average emissions attributable to each individual Australian in 2020 worked out at 19.4 t/CO₂e

Carbon Click is one calculator that incorporates these emissions into personal footprints, including emissions from sources such as public services, infrastructure and exports.

“These ‘Country emissions’ are included in your results, and may make your footprint appear higher than you would expect from your own activities,” it says. “However, it is imperative to include these to get a true measure of a citizen’s footprint.”

Another problem is calculators that don’t open the black box at all. Carbon Positive has a detailed list of where it takes the data from, while Carbon Neutral asks its users to look through the National Greenhouse Accounts Factors to figure out what assumptions it uses. Global Calculator Trace only states that it uses “GHG protocol guidelines” and “emissions factors and benchmark data from public sources”. 

The data problem

In a world awash with information, finding data that is granular enough to paint an accurate picture is becoming increasingly difficult.

“That’s the biggest struggle with carbon calculators, to be honest – finding reputable sources of data,” says Jess Fitzgerald, the carbon project coordinator at Carbon Positive and architect of its calculator.

Fitzgerald used government data such as National Greenhouse Accounts Factors in order to rely on a verified Australian-specific number.

Much household-level data is no longer available, because major surveys like the Census or others run by the Australian Bureau of Statistics (ABS) no longer look at the fine print of how households run. The ABS category 4602 covers environmental issues and used to include a survey of how many fridges and washing machines Australians owned and how often they were used weekly – today that data is collected in a more limited form by Monash University researcher Yolande Strengers and Energy Consumers Australia.

“That’s the biggest struggle with carbon calculators, to be honest – finding reputable sources of data.”

Jess Fitzgerald, Carbon Positive

“Grocery spend and spend on clothing and consumables would be really useful, and it seems like that data used to exist, especially around grocery spend, but I couldn’t find it,” Fitzgerald says.

“Going forward I’m not sure how we will manage [reduced public household data].”

There are two approaches to collecting the kind of data required for an accurate-enough carbon footprint calculator: the vast majority of number crunchers use top-down data like Carbon Positive, which spits out an emissions number, but not which activities contribute to the outcome.

Pears’ calculator used a bottom-up model, which was possible due to his existing knowledge of the micro-power use of household appliances like fridges and televisions. 

“No other calculator allows you to look at cycles on a washing machine or how often I use it, or essentially a lot of the choices or factors that I can change,” he says.

“Most methodologies are bottom-up models. So for example, if you put a fridge in a small space, the model will calculate the extra heat generated in that space, and [how much power is lost by] opening the door. Things people are actually interested in.”

Conflicts of interest

Once you’ve got your carbon footprint calculation, many carbon calculators tested by Cosmos will give you the option to offset that figure – or, in the case of Active Super, to switch to its superannuation product.

This is both useful – in that you don’t need to scramble through the internet to find a way to mitigate the damage you’re allegedly doing to the Earth – and a major conflict of interest: those companies have an incentive to make your footprint seem more alarming, if not higher as a total, than it might otherwise be.

“That’s why it’s important for more calculators to be very transparent in how they calculate,” Fitzgerald says.

Without delving into where your undies were made and what emissions are attributable to tonight’s roast chicken, a rough number is the best and most useful piece of data most people need in order to make changes to their lifestyles.

But short of opening the black box and working out for yourself why your travel emissions are so much higher than expected (it was those flights to the Sunshine Coast you didn’t offset), a rough number is the best you can expect.

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