The Maths Behind a Carbon Number You Can Defend

When someone asks me for our carbon footprint, I don't reach for a headline number. I reach for the workings. A footprint is not a feeling or a marketing claim — it is a calculation, and like any calculation it can be checked, traced, and challenged. If you can't show your arithmetic, you don't have a defensible figure. You have an estimate wearing a suit.

Start with the boundary, not the spreadsheet

Before you quantify anything, decide what you are quantifying. The GHG Protocol Corporate Standard asks you to set an inventory boundary first. A complete corporate inventory covers all three scopes, even though their operational boundaries and data requirements differ. Scope 1 is direct emissions from sources you own or control — the boiler in your building, the van in your fleet. Scope 2 is indirect: the purchased electricity, steam, heating, and cooling you consume. Scope 3 is everything else in your value chain, upstream and downstream. Skipping Scope 3 because it is hard doesn't make it disappear; it just makes your number incomplete.

The one equation that does most of the work

Most of carbon accounting reduces to a single multiplication: activity data × emission factor. Litres of fuel consumed, multiplied by a factor per litre. Kilowatt-hours purchased, multiplied by a grid factor. That's it. The sophistication lives in the choices around it.

Which brings me to the factor itself. Document its source, its unit, its boundary, and where it applies. A combustion factor covers only what comes out of the tailpipe or chimney. A life-cycle factor also includes extraction, processing, and transport. Both are legitimate — but they answer different questions. Use the factor suited to the scope and the activity, and write down why you chose it. Future-you, and your auditor, will thank present-you.

Turning many gases into one number

Different gases warm the planet differently. So we convert each one to carbon-dioxide equivalent using its global-warming-potential value, then aggregate. That aggregation matters because IFRS S2 requires disclosure of absolute gross GHG emissions in metric tonnes of CO₂e. One unit, one scale, comparable across entities.

On which GWP values to use: IFRS S2 specifies values from the latest IPCC assessment, while permitting regulator- or exchange-required values for the portion covered by those requirements. Either way, disclose your material measurement approaches, inputs, and assumptions. The number without the method is just trivia.

Scope 2 has two answers, and you may need both

Scope 2 accounting distinguishes location-based from market-based methods. Location-based uses grid-average factors. Market-based uses contractual instruments or supplier-specific information, where applicable. If you buy renewable energy certificates, your market-based figure will differ from your location-based one. Report both where relevant, and be clear about which is which. Readers deserve to know whether you are describing the grid you sit on or the contracts you signed.

Data quality is a choice you make repeatedly

Prefer the most appropriate and accurate data available. For transport, GHG Protocol guidance describes fuel-use data as an optimal basis for calculations; distance- or spend-based estimates are fallbacks for when fuel data aren't available. Spend-based is fast and rough. Fuel-based is slower and truer. Know which one you're using, and say so.

Where the standards stand, and where Hong Kong fits

For IFRS S2, the GHG Protocol Corporate Standard is the default emissions-measurement method. The December 2025 amendments provide relief where a jurisdictional authority or listing exchange requires another method — but only for the part of the entity subject to that requirement. The rest still uses the GHG Protocol. No cherry-picking your way out of consistency.

In Hong Kong, HKICPA guidance in its March 2025 update listed HKFRS S1 and S2 as effective for annual reporting periods beginning on or after 1 August 2025, while noting the standards are voluntary unless authorities or regulators mandate them. Later secondary sources describe 2026 climate-disclosure requirements and subsequent HKFRS developments — check those against current primary Hong Kong regulatory and HKICPA publications before treating them as compliance guidance. Standards move; your source of truth should be the primary text.

If you want to go deeper into the mechanics — the factor selection, the unit conversions, the audit trail — I recommend Carbon Accounting in Practice: A Mathematics Guide for Sustainability Professionals. It treats the maths as the point, not an afterthought. For the standards themselves, start with the GHG Protocol Corporate Standard FAQ and the IFRS S2 amendments.

Do the arithmetic. Show the arithmetic. That's what makes a footprint defensible.