There are many reasons why companies must disclose greenhouse gas (GHG) emissions today. Beyond the obvious environmental responsibility, there's a growing web of regulations, investor expectations, and supply chain requirements that make accurate carbon accounting essential.

Whether you're reporting under the EU CSRD, responding to customer questionnaires, or simply trying to understand your own footprint, you'll quickly encounter a foundational question: how should you actually calculate your emissions?

This guide explains the main carbon calculation methods available, when to use each, and how companies can evolve their approach over time.

Why calculation methodology matters

The methodology you choose affects the accuracy of your results, the resources required to calculate them, the credibility of your disclosures, and your ability to identify and reduce emissions over time.

Most companies start with simpler approaches and move toward more precise methods as they build internal capabilities and supplier relationships. The GHG Protocol — the globally accepted framework for corporate emissions accounting — explicitly allows this iterative approach.

The main carbon calculation methods

1. Spend-based method

The spend-based method calculates emissions by multiplying the money your company spends on goods and services by an average emission factor for each spending category.

How it works: Financial spend data (from accounting systems) is matched to industry-average emission factors from databases like Exiobase or USEEIO. These factors express emissions in kg CO2e per unit of currency spent in a given sector.

Pros: Relatively quick to implement; uses data already available in your finance systems; covers all Scope 3 categories.

Cons: Less precise than activity-based methods; doesn't capture supplier-specific performance; can be misleading if spend patterns change.

Best for: Initial baseline assessments; Scope 3 categories where activity data is hard to obtain; smaller companies with limited sustainability resources.

2. Activity-based method

The activity-based method calculates emissions using physical activity data (e.g. kWh of electricity consumed, litres of fuel burned, tonnes of material purchased) multiplied by specific emission factors for those activities.

How it works: You collect data on actual activities — energy bills, fuel receipts, freight volumes, flight distances — and apply emission factors from authoritative sources (national grid averages, IPCC factors, DEFRA conversion tables).

Pros: More accurate than spend-based; reflects actual operational data; better for identifying and tracking reductions.

Cons: More data-intensive; requires cooperation from suppliers and internal teams; some categories are still difficult to measure precisely.

Best for: Scope 1 and 2 emissions; high-impact Scope 3 categories once you've identified them; companies with mature data collection processes.

3. Supplier-specific method

The supplier-specific method uses actual emission data provided directly by your suppliers for the goods or services they provide to you.

How it works: Suppliers calculate and share their product-level or company-level emission data, which you then incorporate into your Scope 3 inventory. This is the most accurate approach for purchased goods and services (Scope 3 Category 1).

Pros: Highest accuracy; reflects real supplier performance; helps identify suppliers with lower emissions.

Cons: Requires supplier engagement and capability; not all suppliers can provide this data; significant coordination effort.

Best for: Large or strategically important spend categories; companies with supplier engagement programmes; CSRD reporting where Scope 3 accuracy is scrutinised.

4. Hybrid method

Most companies use a hybrid approach — applying more precise activity-based or supplier-specific data for their highest-impact categories, while using spend-based estimates for the rest.

The GHG Protocol recommends this approach as a practical balance between accuracy and effort. Coolset is designed to support hybrid calculations, allowing you to combine spend-based estimates with activity-based data and supplier-specific inputs in a single inventory.

How Coolset's TÜV-certified methodology works

Coolset's carbon calculation methodology is certified by TÜV Rheinland, one of the world's leading independent testing organisations. The certification confirms that Coolset's approach meets the requirements of the GHG Protocol Corporate Standard and the GHG Protocol Corporate Value Chain (Scope 3) Standard.

The platform supports all three calculation methods and allows companies to evolve from spend-based to activity-based and supplier-specific approaches as their data quality improves. All calculations include detailed audit trails, emission factor references, and methodology documentation — making them suitable for third-party assurance under CSRD and other frameworks.

Previously, Coolset's certification was announced in a dedicated article. The methodology underpins Coolset's GHG Protocol alignment and is regularly reviewed to reflect the latest scientific guidance.

Which method should you use?

The right starting point depends on where your company is today:

• If you're measuring your footprint for the first time, start with spend-based for a fast baseline.
• If you've already done a baseline and want more accuracy, move to activity-based for your top emission categories.
• If you have supplier engagement capability and CSRD obligations, work toward supplier-specific data for your most material Scope 3 categories.

The most important thing is to start — and to document your methodology clearly so that your results are defensible and auditable.

To see how Coolset supports all three calculation methods in practice, book a free demo with our team.

Also useful: why finance teams shouldn't rely on spreadsheets for sustainability reporting.