Calculate Scope 3: Measure the Emissions from Your Suppliers and Purchases

Quick answer. Scope 3 covers 15 categories of indirect emissions throughout your value chain: upstream purchases, transport, business travel, waste and sold products. Unlike scopes 1 and 2, it is calculated using two approaches: physical (measured data × ADEME emission factor) for greater accuracy, or spend-based (purchase amount € × sector-specific ratio) for rapid estimation. For an SME, scope 3 typically accounts for 70 to 90% of total emissions, making it the real lever for decarbonization.

Scope 3 has become a pillar of emissions reporting since decree n° 2022-982 of 1 July 2022, which introduced "significant indirect emissions" into the mandatory assessment (BEGES) under article L229-25 of the French Environmental Code. Since January 2023, companies with more than 500 employees (250 in overseas territories) subject to the BEGES must include their significant indirect emissions; in practice, the aim is to cover the main contributors, usually at least 80% of scope 3.

For SMEs without immediate regulatory obligations, scope 3 remains essential: major customers, financiers and investors now increasingly integrate the value chain into their ESG ratings. Anticipating scope 3 calculation prepares you for EcoVadis audits, sustainability requests and partner requirements.

The GHG Protocol, the international standard adopted by ADEME methodology, divides scope 3 into 15 distinct categories: 8 upstream (categories 1–8, "upstream") and 7 downstream (categories 9–15, "downstream"). Each covers a flow or activity in your value chain.

Upstream categories (1 to 8)#

Downstream categories (9 to 15)#

Scope 3 calculation relies on two methodologies, each with its own advantages and limitations.

Physical approach (activity data)#

You collect measured data in physical units (tonnes, kilometres, square metres, units) which you convert to tCO2e using emission factors from the ADEME Base Empreinte.

Formula: Activity data (tonnes, km, kWh) × Emission factor (kg CO2e/unit) = Emissions (tCO2e)

Examples:

• Purchase 50 tonnes of steel × 2.3 tCO2e/tonne = 115 tCO2e
• Transport 100 tonnes over 500 km × 0.080 tCO2e/tkm = 4 tCO2e
• Flight: 10 passengers × 2,000 km × 0.190 kgCO2e/pkm = 3.8 tCO2e

Advantages:

• More accurate (error margin ±15%)
• Rewards suppliers' specific decarbonization efforts
• Relevant for categories where physical data exists (transport, materials)

Limitations:

• Requires detailed supplier data collection (often time-consuming)
• Incomparable across periods if volume inflation occurs

Spend-based approach (monetary)#

You start with invoiced amounts in euros to apply a sector-specific carbon intensity ratio, expressed in kg CO2e/€ or kg CO2e/k€.

Formula: Purchase amount (€) × Sector-specific monetary ratio (kg CO2e/€) = Emissions (tCO2e)

Examples:

• Purchase €500k of machined parts × 0.15 kgCO2e/€ = 75 tCO2e
• Transport €200k of freight × 0.05 kgCO2e/€ = 10 tCO2e

Advantages:

• Fast (based on invoices)
• Applicable immediately for screening
• Covers all suppliers without detailed tracking

Limitations:

• Less accurate (error margin ±30 to ±50%)
• Penalizes greener, more expensive purchases (paradox: buying decarbonized steel 30% more expensive increases your reported emissions)
• Sensitive to inflation (prices rise = reported emissions rise)

1. Classify your scope 3 flows#

List all external spending and activities, then assign each item to its scope 3 category. Use your invoices, travel records, logistics data and waste registers.

Simplified example for an SME:

• Categories 1 & 2: Purchases from suppliers (materials, supplies, machinery)
• Categories 4 & 9: Inbound and outbound transport
• Categories 6 & 7: Employee travel

2. Define coverage: 100% or materiality threshold#

BEGES requires covering significant emissions (≥80% of total scope 3). You are not required to measure each category, only those relevant and material to your operations.

Common practice:

• Start with the three to five largest categories (purchases, inbound/outbound transport, travel)
• Estimate others by default or exclude them
• Clearly document your scope

3. Collect and structure data#

Each calculation item must be based on reliable primary data.

For the physical approach:

• Detailed invoices by material or product category
• Delivery slips with weight/volume
• Transport kilometres provided by carriers
• Employee travel invoices (flight tickets, train, car rental)
• Weighed waste tonnages

For the spend-based approach:

• Net amounts of purchases by supplier or category (accounting code)
• Transport invoices
• Business travel amounts

4. Apply ADEME emission factors#

The ADEME Base Empreinte (https://base-empreinte.ademe.fr) is the French reference. You will find:

• Physical factors: by material (steel, concrete, paper, etc.), by transport mode (truck, train, plane, ship), by energy type, etc.
• Monetary ratios: by industry sector (e.g. "Metallurgy", "General Distribution", "Food Service")

Indicative table (verify on 2026 Base Empreinte):

5. Consolidate and analyse#

Total by category, supplier or flow. Identify the three to five largest sources ("hot spots"). Document all assumptions and data sources.

Example consolidation:

• Upstream purchases (cat. 1): 450 tCO2e (52%)
• Transport (cat. 4, 9): 180 tCO2e (21%)
• Travel (cat. 6, 7): 110 tCO2e (13%)
• Waste (cat. 5, 10): 90 tCO2e (10%)
• Other (cat. 2, 8, 11, 12, 13, 14, 15): 40 tCO2e (4%)
• Total scope 3: 870 tCO2e

Use physical approach for:#

• Large, measurable items: raw materials, transport, travel (data often available in invoices or management systems)
• Critical suppliers: negotiate activity data with them
• Reduction actions: physical approach demonstrates real impact

Use spend-based approach for:#

• Diverse purchases from many small suppliers (too costly to track individually)
• Initial screening and sector benchmarking
• Capital goods (category 2) or intangibles (services)

Hybrid approach (recommended in practice):#

Start with spend-based for global screening, then refine with physical approach on your three to five largest categories. This is the best balance of accuracy and feasibility.

SME: what scope 3 coverage?#

If your SME has no BEGES obligation (<500 employees), you are not legally required to measure scope 3. However, major customers (>500 employees) and financiers increasingly request this data. Starting with scope 3 of upstream purchases (category 1) and transport (categories 4 & 9) meets 80% of expectations.

Challenge: uncooperative suppliers#

If a supplier refuses to share data, use the spend-based approach on their industry sector. Base Empreinte provides ratios by NACE sector. Document this default approach.

Transport: FTL vs LTL#

Factors vary greatly depending on whether transport is "full truck load" (FTL, lower factor) or "less than truck load" (LTL, higher factor). Check with your logistics providers.

Plane vs train#

Air travel is 10 to 100 times more carbon-intensive than rail. Encourage rail for trips >500 km. Data must be accurate (ticket = best source).

Waste: estimation or measurement#

Weigh flows where possible (especially industrial waste). Otherwise, estimate based on your activity (ratio of tonnes waste / € sales or / tonne produced).

• Do not neglect scope 3: it typically accounts for more than two-thirds of an SME's footprint. A partial but rigorous measurement is better than none.
• Distinguish measured data from estimated: document clearly. An estimate using sector ratios is more reliable than invented data.
• Watch the cost paradox: buying a greener product at higher cost increases your reported emissions in the spend-based approach. Use the physical approach on these items to avoid penalizing ESG efforts.
• Update your factors: ADEME Base Empreinte evolves each year. Recalibrate annually to avoid comparability gaps.
• Ensure traceability: each amount or tonnage must trace back to an invoice, ticket or reading. This facilitates internal and external audit.
• Link scope 3 to your carbon strategy: identifying "hot spots" is not enough. Engage suppliers in reduction plans (procurement criteria, relocation, material substitution).

As a chartered accountant registered with the Professional Order and statutory auditor, we regularly support SMEs on scope 3. A recent case involved an industrial SME with 180 employees that assumed scope 3 was limited to logistics transport. Detailed analysis showed upstream raw material purchases actually accounted for 65% of scope 3 emissions (category 1), ahead of transport (20%). The initial calculation had relied on a naive spend-based approach. Re-examining with the physical approach — material weighings, decarbonized RFQs — revealed that a simple supplier switch (from standard European steel to recycled) would reduce 100 tCO2e/year for just +2% cost. This is a lever that only physical data and detailed analysis can reveal.

This experience shows scope 3 is not an ESG formality: it is a strategic management tool. Numbers drive action, and action generates returns — material cost reduction, supply resilience, access to green finance.

Hayot Expertise Advice. Launch scope 3 calculation this year, even if you are not required to. A solid baseline prepares you for customer requests, financiers and growing sustainability demands. Our experts provide a targeted diagnostic: mapping of material categories, choice between physical and spend-based approach based on available data, structured collection, then an action plan by supplier. Combined with our ESG audit and reporting offering, this feeds your CSRD reporting and your shadow carbon price to steer decarbonization. Let's discuss your scope 3 flows.