How is a carbon footprint calculated? Step by step

The arithmetic of a carbon footprint is simple: multiply activity data by an emission factor and sum. The difficulty is not the arithmetic but three decisions: what to include, where the data comes from, and how you will defend the result.

Step 1 — decide what you are calculating

The first question is product or organisation, because they are different methods. A product calculation follows ISO 14067 or EN 15804 and is expressed per product. A corporate calculation follows the GHG Protocol or ISO 14064-1 and is expressed per year.

For a product calculation you define the functional unit here — one tonne of cement, one m² of tile, one m² of surface protected for ten years. For a corporate calculation you choose the organisational boundary: which subsidiaries and which sites are included.

Step 2 — draw the system boundary

Decide where it starts and stops. On the product side that is cradle-to-gate, cradle-to-grave, or a module selection (A1-A3, A4-A5, B, C, D). On the corporate side it is Scope 1, Scope 2 and which Scope 3 categories are included.

Excluding Scope 3 is tempting because it is hard; but in manufacturing 70 to 90 per cent of the total sits there. A calculation limited to Scope 1 and 2 measures a small slice of your footprint and misdirects abatement decisions.

Step 3 — collect the activity data

• Fuel — natural gas, coal, diesel, LPG consumption; from invoices and stock records
• Electricity — meter readings; any renewable supply contracts recorded separately
• Process emissions — chemical reactions such as calcination; calculated from production volume
• Fugitive emissions — refrigerant top-ups, SF₆ replenishment; from maintenance records
• Raw materials and packaging — by mass, with supplier and origin
• Transport — distance, vehicle type, load; both inbound and outbound
• Waste — type, quantity and treatment destination

Step 4 — pick factors and multiply

Multiply each activity line by the appropriate emission factor. The critical point is that the factor matches your activity data in geography, year, unit and gases covered. Record the source and version of every factor you use — reproducibility depends on it.

Step 5 — find the hotspots

When the first result arrives, the thing to do is not report the number but look at the distribution. Almost always, ten per cent of the lines account for eighty per cent of the total. That distribution tells you which data is worth improving and which intervention will move the needle.

The purpose of a first calculation is not to find the right number but to learn where to look. A rough model does that; waiting for a perfect one costs months.

Step 6 — make it verifiable

You will hand the number to a customer, a tender or a report, which means you have to be able to defend it. A verifier asks for source documents, calculation steps, factor choices and how data gaps were filled. A calculation living in formulas buried in spreadsheets struggles with that audit; the path from source data to result has to be traceable.