It is easy to produce a good-looking carbon dashboard. Producing a number you could defend to a sceptical customer is much harder. Most of the difficulty in Scope 3 is not in the sums, which are simple multiplications. It lies in knowing whether the numbers going into them mean anything.
The short version
- Scope 3 calculations come down to activity or spend, multiplied by an emission factor.
- Activity-based methods use real quantities. Spend-based methods use money as a stand-in.1
- UK government guidance says you should report which methods you used.1
- Improve the data for your largest categories first, and be open about what is estimated.
Two ways to calculate
The UK government’s conversion factors let users calculate and report emissions based on activity data, such as the volume of fuel used, kWh of electricity purchased or distance travelled, for Scope 1, 2 and 3 separately.1
The government also explains how these differ from the spend-based emissions multipliers published by the Department for Environment, Food and Rural Affairs. Those can give an initial assessment of a user’s full supply-chain emissions based on spending across different goods and services. It says users may wish to use spend-based methods where they do not have sufficient data for activity-based methods, and that they should report the methods used.1
| Spend-based | Activity-based | |
|---|---|---|
| Data you need | What you spent, by category | Real quantities, such as tonnes, kWh or km |
| Good for | A first look across everything | Categories that matter most |
| Weakness | A price rise looks like more emissions | More work to gather the data |
| Effort | Low | Higher |
That weakness in the spend-based method is the crucial one for your reporting, and the example is our own. If a supplier raises prices by 10% and you buy exactly the same goods, a spend-based estimate says your emissions rose by 10%. Equally, switching to a cheaper but more carbon-intensive product could appear to reduce them.
Where to start
Assessing the whole value chain can be complex, and the GHG Protocol’s Scope 3 Calculation Guidance was written to reduce the barriers for companies that are just beginning. It covers the 15 categories, how to select an appropriate calculation method, and worked examples.2
Our suggested order of work:
- Screen everything using spend data. It is quick, and gives you a rough picture across all categories.
- Find the biggest categories. Effort should go where the emissions are.
- Improve those with better data. Replace estimates with quantities, and then with supplier-specific data where you can get it.
- Leave small categories as estimates, and say so.
- Repeat every year, and show what improved.
What good data looks like
For each number, you should be able to answer five questions. This is our own checklist.
- Where did it come from? An invoice, a meter, a supplier or an estimate?
- What period does it cover? Does it match your reporting year?
- Which factor did you use? Which edition and which publisher? The UK conversion factors are updated annually, and the 2026 set was published on 11 June 2026, with a later update on 31 July 2026.3
- Who checked it? And could someone else repeat it?
- How confident are you? High, medium or low is enough to start with.
Common mistakes
These are the errors to look for:
- Mixing units. Kilograms and tonnes, or miles and kilometres, can introduce huge errors silently.
- Counting things twice. The same delivery can appear as both a supplier’s freight and your own purchase.
- Using the wrong year’s factor, or a factor from a different country.
- Hiding assumptions. Saying “estimated using spend” is fine. Presenting it as a measured figure is not.
- Changing method silently. If you change method, your numbers can jump. Explain it, and consider restating last year.
- Trusting a tool you cannot inspect. If you cannot see how a number was worked out, you cannot defend it.
Why dashboards come second
A dashboard is useful once you trust the data beneath it. Before that, it can actually make things worse, because a clean chart makes a weak number look solid. Data problems are similar in any field, as our article on spreadsheet errors shows.
It is better to have a plain table with clear sources than a beautiful chart that nobody can explain. Treat your carbon data with the same care as financial data, with a clear trail back to the source.
Being honest is the safe option
You may not have perfect data, and nobody expects it in your first year. Customers and auditors can work with uncertainty, as long as you show it. What causes trouble is a precise-looking number with nothing behind it.
For more on who is asking for this and why, see who has to report Scope 3 in the UK. If you would like help turning supplier invoices and activity data into figures you can explain, that is what our Scope 3 reporting service does.
Sources
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Department for Energy Security and Net Zero, UK government conversion factors for company reporting of greenhouse gas emissions, accessed 7 November 2026. ↩ ↩2 ↩3 ↩4
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GHG Protocol, Scope 3 Calculation Guidance, accessed 7 November 2026. ↩
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Department for Energy Security and Net Zero, Greenhouse gas reporting: conversion factors 2026, accessed 7 November 2026. ↩
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