How to Calculate Scope 1, 2 and 3 Emissions Without a Carbon Accountant

At some point a customer questionnaire, a tender requirement or an investor asks for your carbon footprint, split into Scope 1, 2 and 3. Most small and mid-sized companies assume this requires a consultant. It usually doesn't. The calculation itself is one multiplication repeated many times — the real work is knowing where your numbers live and which conversion factors to use. Here's the whole method.

What are Scope 1, 2 and 3 emissions?

Scope 1 covers direct emissions from sources you own or control — fuel burned in company vehicles, gas boilers, refrigerant leaks. Scope 2 covers indirect emissions from the electricity, heat or steam you purchase. Scope 3 covers everything else in your value chain: purchased goods and services, freight, business travel, commuting, use of your products. The definitions come from the GHG Protocol and are summarised well by the US EPA and Climate Impact Partners.

Scope 3 is typically the biggest slice — Climate Impact Partners puts it at roughly 75% of the average company's footprint. It's also the hardest to measure, which is why the sensible order is: nail Scope 1 and 2 first, then approximate Scope 3.

What is the basic formula?

Every line of a carbon footprint is the same equation:

Activity data × emission factor = emissions (kg CO₂e)

Activity data is what your business actually did: litres of diesel, kWh of electricity, kilometres flown, pounds spent on a supplier category. The emission factor converts that activity into greenhouse gas. You don't invent factors — you take them from a published database and record which one you used.

Where does the activity data already live?

For Scope 1 and 2, you almost certainly have everything on file, as Seedling's small-business guide points out: fuel comes from fuel cards and expense records, gas and electricity from utility bills, refrigerants from your air-conditioning service invoices, company vehicle mileage from telematics or finance records.

For Scope 3, start with your accounting system. Spend by supplier category is the raw material for a spend-based estimate (more below). Travel data lives in expense reports; freight in carrier invoices.

Which emission factors should you use?

Use the official, free databases — not numbers from a blog:

• UK and many international categories: the UK government conversion factors for company reporting, published annually by DESNZ (commonly still called the DEFRA factors). The 2025 set was released in June 2025; notably, the UK grid electricity factor dropped about 14.5% to 0.128 kg CO₂e per kWh on the back of renewable generation, per Circular Ecology's summary.
• US operations: the EPA's Simplified GHG Emissions Calculator, a free tool aimed at small and mid-sized organisations doing their first inventory.

Record the factor source and year next to every calculation. When factors update annually, your numbers change even if your activity didn't — you want to be able to explain that.

How do you handle Scope 3 without data from suppliers?

You estimate, and you're transparent about it. Two accepted approaches:

Spend-based: multiply what you spent in each procurement category by an average emission factor per currency unit for that category. Crude, but it covers the whole value chain and highlights the categories worth investigating properly.

Activity-based: where you have real units — tonne-kilometres of freight, nights in hotels, flights taken — use those with the matching DEFRA factors. More accurate, more work.

Most first-year footprints mix the two: activity-based for travel and freight, spend-based for purchased goods. A common recommendation for first-timers, echoed by Scope Carbon Tracking's practical guide, is to cover all of Scope 1 and 2 plus only the Scope 3 categories likely to be material — then widen coverage each year.

How often should you update the numbers?

The annual-spreadsheet ritual is where most ESG reporting goes wrong: data gets rebuilt from scratch each year, methodology drifts, and nobody can answer a mid-year questionnaire. The alternative is to wire the recurring sources — utility bills, fuel cards, travel spend — into a live view, so the footprint is a running number instead of an annual archaeology project. We covered the dashboard side of this in How to Build an ESG Reporting Dashboard (Without a Sustainability Team).

FAQ

Do I legally have to report emissions? Depends on jurisdiction and size. In the UK, SECR applies to large companies; smaller firms mostly report because customers and tenders demand it. Check your local thresholds — and remember the commercial pressure usually arrives before the legal one.

What's the difference between CO₂ and CO₂e? CO₂e ("equivalent") bundles all greenhouse gases — methane, nitrous oxide, refrigerants — into one number weighted by warming potential. Published factors are in CO₂e, so your results are too.

Can I just use an online calculator? For a first rough cut, yes — the EPA's free calculator is built exactly for that. But a calculator output you can't decompose won't survive a customer audit. The factor-by-factor method above produces numbers you can defend line by line.

Is spend-based Scope 3 accepted? Yes, as a screening method. The GHG Protocol treats it as a legitimate starting point, with the expectation that material categories graduate to activity- or supplier-specific data over time.

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Sifra builds live ESG and impact dashboards — activity data in, factor library applied, one defensible footprint view that's current all year. See the Impact vertical, or get a free mock dashboard built on your own reporting categories.