There are four main carbon accounting methods. Spend-based uses money spent and average factors. Average-data (activity-based) uses physical quantities such as kWh or tonnes. Supplier-specific uses your suppliers' own emissions data. Hybrid combines them. Most large companies use a hybrid: precise data for their biggest sources and spend data for the rest.
The main methods are spend-based, average-data (often called activity-based), supplier-specific and hybrid. They differ in what data they use and how accurate the result is.
All four follow the same basic sum: activity data multiplied by an emission factor. What changes is the type of activity data and how specific the factor is. The GHG Protocol's Scope 3 Calculation Guidance describes these methods for purchased goods and services, the largest Scope 3 category for many companies. The same logic applies across most of the inventory.
For the basics of the full process, start with our carbon accounting overview.
Spend-based is fastest and least precise. Supplier-specific is most precise and hardest to get. The table shows the trade-offs.
| Method | Activity data | Emission factor | Accuracy | Effort | Best for |
| Spend-based | Money spent, by category | Average kg CO2e per dollar spent, from environmentally extended input-output (EEIO) models | Low | Low | First screening; long tail of small purchases |
| Average-data (activity-based) | Physical units: kWh, litres, tonnes, km | Average kg CO2e per unit, from databases or government factors | Medium | Medium | Energy, fuel, transport, key materials |
| Supplier-specific | Supplier's own product or allocated emissions | Supplier's own data, ideally verified | High | High | Top suppliers and materials that drive your footprint |
| Hybrid | A mix of the above | Supplier data where you have it, averages elsewhere | Medium to high | Medium to high | Most large companies with complex supply chains |
The spend-based method estimates emissions from how much you spend on each type of goods or service. You multiply spend by an average emission factor per dollar for that category.
How it works. Take spend data from your ERP or accounts payable, map each line to a category, such as "chemicals" or "air transport", and apply an EEIO factor.
Pros. It is quick, cheap and covers your whole supply chain using data you already have.
Cons. It uses averages. It cannot tell a low-carbon supplier from a high-carbon one, and emissions move with prices and exchange rates, not with real changes. That makes reductions hard to show.
The activity-based method uses physical quantities instead of spend. You multiply the amount of an activity, such as kWh of electricity or tonnes of steel, by an average factor for that activity.
How it works. Collect quantities from meters, invoices, bills of materials or logistics systems, and match each to a factor from a recognised database or government source.
Pros. It is more accurate than spend, and emissions change when your real activity changes, for example if you use less steel or switch fuels.
Cons. It takes more work to collect. The factors are still averages, so supplier differences stay hidden.
The supplier-specific method uses emissions data from your suppliers themselves, such as a product carbon footprint for a material you buy or a supplier's allocated emissions.
How it works. Ask key suppliers for product-level footprints, or for their Scope 1 and 2 emissions plus an allocation to what they sell you.
Pros. It is the most accurate method and it rewards suppliers who cut emissions.
Cons. Many suppliers cannot provide it yet, data quality varies, and it takes ongoing engagement. Our guide to engaging suppliers on Scope 3 covers how to get it.
The hybrid method combines methods: supplier-specific or activity data where you have it, and spend-based estimates to fill the gaps.
How it works. Start with a full spend-based screen to find hotspots. Then replace spend estimates with better data for the categories and suppliers that make up most of your footprint.
Pros. You get full coverage plus precision where it counts, and you can improve year by year.
Cons. It needs clear records of which method applies to which line, so that changes in method are not mistaken for real reductions. Recalculate your base year if a method change shifts results significantly.
Choose by materiality and data availability, not by one method for everything. Spend more effort on the sources that drive most of your emissions.
Different methods apply in different scopes. Scope 1 and 2 are almost always activity-based, using fuel and energy data. Scope 3 is where the choice matters most. See methods of calculating Scope 3 emissions for category-by-category guidance.
Whatever method you use, the GHG Protocol Corporate Standard sets five accounting principles: relevance, completeness, consistency, transparency and accuracy.
| Principle | What it means in practice |
| Relevance | The inventory reflects your real emissions and serves the decisions users need to make |
| Completeness | All sources inside the boundary are counted; any exclusion is disclosed and justified |
| Consistency | Methods stay the same over time, so years can be compared; changes are documented |
| Transparency | Assumptions, methods and data sources are disclosed so others can check them |
| Accuracy | Emissions are neither over- nor under-stated, and uncertainty is cut as far as practical |
The GHG Protocol publishes free calculation tools and guidance, including cross-sector tools (for example, stationary combustion and transport), sector-specific tools and country-specific tools. They are a useful check on your own calculations, though large companies usually outgrow them.
Pokka used Terrascope to calculate product carbon footprints for 4,980 SKUs. When checked against 700 products, the results were about 92% accurate. That shows how activity-level data can be applied at scale without building each footprint by hand. Read the Pokka case study.
Terrascope's corporate carbon footprint platform supports all four methods in one inventory.
Not sure which method fits your inventory? Talk to our team.
What is the most accurate carbon accounting method? The supplier-specific method, because it uses your suppliers' own emissions data. In practice, most companies only have it for a share of their suppliers, so they combine it with other methods.
Is spend-based carbon accounting accepted for reporting? Yes. The GHG Protocol accepts spend-based estimates, especially for Scope 3. But it is the least precise method, so companies are expected to improve data for their largest sources over time.
What is the difference between spend-based and activity-based methods? Spend-based uses money spent and an average factor per dollar. Activity-based uses physical quantities, such as kWh or tonnes, and a factor per unit. Activity-based is more accurate and responds to real operational changes.
Which method does the GHG Protocol recommend? The GHG Protocol does not require one method. It asks companies to choose methods that give accurate, relevant results for their largest sources and to disclose the methods used.
What is an emission factor? An emission factor converts activity into emissions, for example kg CO2e per kWh of electricity or per dollar spent on a category. Factors come from government databases, life cycle databases or suppliers.
Can I change carbon accounting methods between years? Yes, and you should as your data improves. Document the change and, if it shifts results significantly, recalculate your base year so comparisons stay fair.