How do we work it all out?

Ask our resident genius, Gav, how he fit all the pieces of our carbon puzzle together and he’ll calmly tell you ‘it wasn't as complicated as we first thought’. Taking a leaf out of The Ultimate Chiller's book, this page endeavours to demonstrate the depth and accuracy of our methodology without boring the pants off of you. This is a strictly interested-enough-for-pants zone. Do read on.

The 30 second summary

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    Our carbon data is supplied by Prof. Mike Berners-Lee’s Small World Consulting. 
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    Our calculator uses a top down model. This means we take the total emissions of a country, allocated by industries, and break it down for the average resident. In the UK, for example, this gives us a figure of 13.1 tonnes CO2e per person. We then modify the starting score according to survey answers and ONS averages. 
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    Actions & Habits uses a bottom-up model. This means we look at the individual aspects of a behaviour, assign a carbon number to each and then add it all up for an estimated footprint.  

A bit about our data...

Small World’s emissions factors are drawn from one of two core methodologies or a blend of the two. Each has its own strengths and weaknesses, so combining the two into a hybrid approach often gives us the best of both worlds.

Bottoms up!

The bottom-up model (or process based life cycle analysis/PBLCA) involves building a footprint with building blocks—like constructing a lego house. First, you gather information about every single piece (bricks, tiles, windows, doors, etc.). Then, you calculate the emissions of each of them, before adding it all up for an estimate carbon footprint.


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    Most used and widely understood
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    Can be quite accurate, on the parts you manage to capture


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    Resource/time intensive and requires many questions to gather enough data 
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    Works well for a product but becomes complex if it’s a service
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    Complexity demands truncation error, damaging accuracy


Originally created to estimate the impact of economic activity, the top-down (or environmentally extended input output) model can be understood in the context of financial transactions. It essentially helps us recognise the impact of a pound spent in different industries, for example £1 spent in oil vs insurance.


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    Holistic view of impact of goods and services
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    Useful assessing complex products or services
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    Easy to derive emissions factors from financial transactions


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    Results are more general than PBLCA

Emissions factor sources


Ten years of carbon analysis

Over the last decade, Small World’s carbon analysis has spanned everything from supermarkets to clothing brands; construction companies to tech giants; national parks to local governments, and many more.


A macro-economic carbon model

Small World has refined and tested this model over a decade and used it with a range of clients, from some of the largest US tech giants to local micro businesses.


Publically available datasets

Of course, only the most credible sources are used.


Analysis of product life cycles

Taken from academic papers and (less frequently) company reports.


Our own tools

Small World has developed its own tools for combining and enhancing all of the data into a methodologically coherent dataset.

How our calculator works

A user’s footprint is split into four main buckets: Home, Travel, Diet, Other/Spending. These buckets are then subdivided into numerous individual data points. The data points form a tree as deep as needed to represent the data. 

Each data point in the hierarchy holds an average carbon emission score which can be summed to the data point above. The overall breakdown is displayed to the user one level deep.

Each user is allocated the default scores for each data point. Each survey question then modifies the user’s score away from that average. These are applied as multipliers to the average score to indicate using more or less carbon than the average resident. 

Answers to questions can affect multiple data points and data points can be affected by different questions.

As you can see, we rely on the information you give us to ensure you receive the most accurate figures possible. Your survey answers are as important as our methods. 

A bit about Mike...

Mike is a professor, researcher and author of “There is no Planet B”, “How Bad are bananas, the Carbon footprint of Everything” and “The Burning Question” . He’s Pawprint’s Scientific Advisor and helps us ensure our numbers are as accurate as possible.

Dive deeper

This page has covered the important stuff, but there’s a lot more to our machine than that. If you’d like to learn more (or if you’re a carbon-calculator thief… niche) then go ahead and download our full methodology document below.
Download PDF

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