What is the activity-based method in carbon accounting?

Quick Answer: The activity-based method is a carbon accounting approach that calculates emissions using actual operational data, such as litres of fuel consumed or kilowatt-hours of electricity used, rather than financial spend. It produces more accurate emissions figures than spend-based alternatives and is the preferred approach under the GHG Protocol where data is available.

What is the Activity-Based Method?

The activity-based method is a way of calculating greenhouse gas emissions by measuring the physical quantity of an activity and multiplying it by a corresponding emissions factor. For example: kilometres driven multiplied by a vehicle emissions factor, or kilowatt-hours of electricity consumed multiplied by a grid emissions factor.

This contrasts with the spend-based method, which estimates emissions by multiplying financial expenditure by an industry-average emissions factor. Spend-based data is easier to collect but less precise, because it cannot account for differences in how a supplier produces goods or delivers a service.

The activity-based method sits at the more accurate end of the data quality spectrum. The GHG Protocol, the internationally recognised standard for corporate carbon accounting, recommends using activity-based data wherever it is practical to do so.

Why Does the Activity-Based Method Produce Better Data?

Spend-based emissions factors are calculated from sector-wide averages. They reflect what a typical company in a given industry emits per pound spent, not what your specific suppliers actually emit. As a result, spend-based figures can be significantly over or under-stated depending on your supply chain.

Activity-based data removes that uncertainty by grounding the calculation in a physical quantity. A litre of diesel has a known carbon content. A kWh of electricity from a specific grid has a published emissions factor. The calculation is direct, traceable, and reproducible.

This matters for a few reasons:

• Accuracy: Figures are tied to what actually happened, not an industry proxy
• Credibility: Stakeholders, auditors, and frameworks like SBTi and SECR expect activity-based data for material emission sources
• Progress tracking: Year-on-year comparisons are more meaningful when the underlying data reflects real operational changes

For companies working towards science-based targets or producing compliance-ready outputs, activity-based data is not optional for key emission sources. It is the baseline expectation.

Where is the Activity-Based Method Used in Practice?

Activity-based data is used across all three scopes of a GHG inventory, though its availability varies by category.

Scope 1 emissions (direct emissions from owned or controlled sources) almost always use activity-based data. Fuel consumption records, refrigerant top-up logs, and natural gas meter readings are all activity data.

Scope 2 emissions from purchased electricity also use activity-based data by default. The units consumed (kWh) come directly from energy bills.

Scope 3 is where it gets more complex. Some categories, such as business travel, can be calculated using activity data (flight distances, hotel nights). Others, particularly further upstream in the supply chain, often default to spend-based data because suppliers do not share granular operational figures.

The practical goal for most companies is to use activity-based data for their highest-emission categories and spend-based data as a fallback where activity data is not available or not worth the effort to collect. This is sometimes called a hybrid approach.

What Does This Mean for How You Collect Data?

Shifting from spend-based to activity-based data requires collecting different inputs. Instead of pulling a supplier invoice total, you need the physical quantity: tonnes of goods shipped, nights in a hotel, cubic metres of waste sent to landfill.

This is one of the reasons carbon accounting can feel time-consuming. Gathering activity data often means going back to suppliers, reviewing operational records, or running internal surveys. The payoff is a more defensible footprint that holds up to scrutiny.

Seedling is built to reduce that collection burden where possible. Accounting integrations with tools like Xero, Sage, and QuickBooks pull financial data automatically, and built-in supplier and employee surveys help gather activity data without manual back-and-forth. For categories where activity data is available, the platform supports its use directly, so you are not forced into spend-based estimates when better data exists.

As data quality improves year on year, a larger proportion of your footprint can shift from estimated to activity-based, which strengthens both the accuracy of your figures and the credibility of any reduction claims you make.