- 1Supplier data with no method or boundary is worth less than a good average factor.
- 2The GHG Protocol requires you to publish the share of supplier data in scope 3.
- 35 criteria judge a data point: technology, year, location, completeness and reliability.
- 4Check first the suppliers that account for 80% of purchases, then the highly emitting materials.
The same tonne of steel emits 2.34 tonnes of CO2 when it comes out of a blast furnace and 0.69 tonnes when it comes out of a scrap-based electric arc furnace, according to the worldsteel 2024 global averages. A buyer that applies the factor of the wrong route to its steel purchases distorts that line by a factor of more than 3. Its supplier's data can correct the gap, provided it arrives with what is needed to verify it.
Supplier data replaces the average factor only if its boundary and method hold up
The most common case is shown: an unverified product footprint.
A company's scope 3 rests mostly on average factors, and each piece of data received from a supplier can either refine it or distort it. The sections below describe this data, then how to judge it and how to check it.
1What data can a supplier provide?
A supplier usually provides one of 3 things: the footprint of the product sold, the carbon footprint of its company or its answers to a questionnaire. The GHG Protocol scope 3 standard, the international reference for carbon accounting, ranks data from the most precise to the least precise. The more the data relates to the product purchased, the less it needs to be allocated among the supplier's customers.
The footprint of the product purchased
A product's carbon footprint counts its emissions from cradle to factory gate: extraction of materials, upstream transport and manufacturing. It generally follows ISO 14067, confirmed in 2024, or the PACT methodology (Partnership for Carbon Transparency), backed by the World Business Council for Sustainable Development (WBCSD), whose version 3 dates from 30 April 2025. The buyer includes it by multiplying it by the quantities purchased, with no allocation key, like a product footprint calculated by life cycle assessment (LCA).
The company-wide footprint, to be allocated
A supplier with no product footprint often sends the carbon footprint of its whole company, covering scopes 1 and 2 and sometimes part of its scope 3. This total then has to be allocated among its customers, using a physical key such as tonnes delivered or an economic key such as turnover. The GHG Protocol allows the supplier to do the allocation itself and state its key, or to send its total together with the share of its output that was purchased.
This overall figure holds only for a homogeneous supplier. The standard takes the example of a service provider that also owns a factory: its company-wide footprint describes the service purchased poorly, and data by site or by activity is needed. The standard even accepts that average data specific to the activity may be worth more than a supplier's overall company footprint.
The answer to a questionnaire
Much of the data arrives by questionnaire. The CDP supply chain programme approached more than 45,000 suppliers in 2025, on behalf of more than 200 client organisations. EcoVadis for its part ranks the reliability of the carbon data it collects into 4 levels.
For transport, the data is a legal obligation. Under French law, Article L1431-3 of the Transport Code requires any transport provider to inform its customer of the quantity of greenhouse gases emitted by its service. Since 1 January 2025, failure to comply has been punishable by a fine of up to €3,000, which makes the data easy to obtain and useful for a haulier's carbon footprint assessment as well as for its customer.
2Is verification of supplier data required?
No text requires supplier data to be verified by a third party. The frameworks do, however, require you to say where the figures come from and what they are worth, and some ask for verification on the supplier's side.
What the GHG Protocol asks for
The scope 3 standard requires a company that publishes to state, for each category, the share of emissions calculated with data obtained from its suppliers. It must also describe the quality of the data used. The standard recommends asking suppliers for the documentation of their method and sources, and allows you to ask them for a verification, internal or by a third party.
This data is not outside the scope of a check. When the buyer has its own carbon footprint assessment verified by a third party (in French) (in French), the verifier also traces the lines fed by supplier data back to their supporting documents.
What PACT and EcoVadis provide
The PACT methodology goes further for a product's carbon footprint, which it requires to be verified by an independent third party. Until 2030, the verification covers the calculation model that produces the footprints, then the programme that governs them. Each footprint exchanged carries its share of primary data, and a quality rating from 2027. When the share of an upstream component is unknown, it counts as 0%.
EcoVadis ranks carbon data into 4 levels, from "Low" to "Third-Party Verified". Only the 2 top levels are presented as usable for external reporting and for scope 3, as our article on EcoVadis and the carbon footprint assessment recalls.
What the CSRD allows you to request
For a company subject to the Corporate Sustainability Reporting Directive (CSRD), the Omnibus Directive of 24 February 2026 sets a limit. A supplier with up to 1,000 employees may refuse information that goes beyond the voluntary standard, and no contrary clause in a contract is binding. A buyer that asks for more must tell the supplier so, together with its right to refuse. The directive must be transposed by 19 March 2027 at the latest, and our article on the revised European Sustainability Reporting Standards (ESRS) describes the standard that serves as the ceiling.
3How do you judge the quality of supplier data?
The GHG Protocol asks the buyer to assess supplier data against 5 indicators, the same as for its own data. Its example grid scores each on 4 levels, from "very good" to "poor", and is the basis of the tool that opens this article.
The 5 GHG Protocol criteria
A supplier's data is judged on 5 points, each scored from the best to the worst level:
- Technology: does the data describe the process used in practice to make the product purchased, a similar process or an unknown process?
- Year: the GHG Protocol grid rates data very good when it is less than 3 years from the activity, and poor beyond 10 years.
- Location: data from the site or country of manufacture is worth more than an average from another region, especially where electricity weighs heavily.
- Completeness: does the data cover all the sites concerned, over a period long enough to smooth out seasonal variations in activity?
- Reliability: measured and verified data gets the best score, an estimate with no justification or qualification gets the worst.
These scores retain an element of judgement. The standard itself notes that a combustion factor more than 10 years old can remain accurate, because some fuels have barely changed.
Boundary and declared unit
Before any comparison, 2 items need to be read on the declaration. The boundary says whether the footprint runs from cradle to factory gate or covers only the supplier's plant, in which case all the raw materials are missing. The declared unit says what the figure refers to: a kilogram, a piece, a tonne-kilometre or a square metre. Mixing up the piece and the kilogram can multiply a line without anything in the total giving a warning.
3 adjustments that distort a footprint
3 calculation choices, commonly found in declarations, make a figure unfit to be added up in scope 3:
- Offsetting deducted, whereas ISO 14067 places offsetting outside a product's footprint and the GHG Protocol counts scope 3 independently of carbon credits.
- Biogenic carbon subtracted: PACT requires 2 footprints to be declared, with and without the removal of biogenic CO2, and the GHG Protocol asks for this CO2 to be reported separately.
- So-called green electricity: PACT accepts guarantees of origin only if they meet the GHG Protocol quality criteria, and otherwise applies the residual mix, or failing that the average grid mix.
These 3 points show up in the supplier's method note, rarely in its figure. A footprint received on its own should therefore be treated as an estimate, and a declared carbon neutrality changes nothing about what the supplier emits. The declaration below, constructed for this article, shows where these points can be read.
On this declaration, 2 deductions and 1 "green" electricity claim with no proof distort the total
Example constructed for the article, with fictitious values. Points 4 to 6 distort the total.
Share of primary data: not stated. Verification: none.
Row 6, offsetting
To add back
ISO 14067 places offsetting outside the footprint of a product, and the GHG Protocol counts scope 3 independently of the credits purchased. The deducted amount is added back.
4How do you check your suppliers' data?
The check starts with a triage, goes through the supporting documents and recalculates only what changes the result.
Choosing which suppliers to check
The GHG Protocol proposes ranking direct suppliers by purchase value, selecting those that account for 80% of spending, a threshold it gives as an example, then adding those that each weigh more than 1%. It adds suppliers of highly emitting materials and those that produce or emit fluorinated gases, even for small amounts. The other purchases continue to be calculated with average factors, such as those in the Base Empreinte of ADEME (the French Agency for Ecological Transition).
The standard advises starting with the suppliers over which the company has the most influence, such as a subcontractor that manufactures for it. The ranking of the items that weigh most shows where to focus effort, in particular for an industrial site.
Checking against supporting documents
For each data point retained, the check relies on 4 steps recommended by the GHG Protocol:
- Ask for the method note, with the standard followed, the sources of the emission factors, the inclusions, the exclusions, the assumptions and the year of the data.
- Compare the figure with the average factor for the same product family, and ask for a written explanation of any gap that the technology does not justify.
- Ask preferably for the activity data, kilowatt-hours or kilograms of fuel, and calculate the emissions yourself, which limits conversion errors.
- Check the allocation key of a company-wide footprint and the share of the supplier's output that you buy in the year of your carbon footprint assessment.
A footprint received without its method is only an estimate, whatever the number of decimal places.
The targeted recalculation
The recalculation covers only the data that carries weight. The example below, constructed for this article, starts from a declared footprint and corrects it for the adjustments found in the supplier's method note.
Corrected for its 3 adjustments, the declared footprint is 4 times higher
Example constructed for the article, with fictitious values: the cardboard box from the declaration above, in kgCO2e per box.
Declared footprint
The figure received
Offsetting added back
Carbon credits, outside the footprint
Biogenic carbon added back
Reported separately
Electricity at residual mix
Guarantees of origin not proven
Recalculated footprint
The one that goes into scope 3
On 200,000 boxes purchased a year, 24 tCO2e declared become 96 tCO2e. The corrected footprint goes into the carbon footprint assessment.
Each data point ends up in one of 3 decisions: include it as it is, include it corrected, or keep the average factor and ask for it again the following year. The GHG Protocol recommends improving low-quality, high-emission data first. It also asks for the base year to be recalculated when a change of method significantly alters it, a point that a transition plan takes up.
No public price list sets the price of this check. It depends on the number of suppliers selected and the quality of their answers, and is most often priced within a carbon footprint assessment engagement, with or without software.
5What should you ask a supplier for before using its data?
The GHG Protocol lists what a questionnaire can ask of a supplier, and its list comes to 5 points:
- The footprint of the product sold, calculated under the GHG Protocol's product standard or an equivalent standard such as ISO 14067, with its boundary and unit.
- Failing that, the supplier's scope 1 and 2 emissions for the year, and the activities upstream of the supplier when they weigh in the product purchased.
- A description of the methods and data sources, including the emission factors and the global warming potentials used for each gas.
- The allocation method the supplier used, or the information the buyer needs to allocate the emissions among customers itself.
- Whether a verification, internal or by a third party, exists, and the level of assurance obtained.
What a supplier can provide also depends on what it sells. A carrier, a construction products manufacturer, a cardboard maker or a host of digital services often already has a format, and that is the format to ask for, with the check points specific to each.
For 4 common purchases, the data often already exists at the supplier, with its weak point
For 4 common purchases, what the supplier can provide and what to look at before including it.
GHG information, mandatory
What the supplier can provide
The quantity of greenhouse gases emitted by each service, which the French Transport Code requires every carrier to give its customer.
What to check
The level of values used: default values set by the ministry, company averages or measurements made for each service.
Since 1 January 2025, a breach is punishable by a fine of up to €3,000.
A simple, common format
The standard recommends a request that is as simple as possible, on a single template, coordinated with the other questionnaires the supplier receives. A shared format spares the supplier from answering the same question several times, and PACT claims more than 50 compliant solutions able to exchange product footprints in a single format. Packaging thus lends itself to the same requests as its eco-design.
What the contract can provide
A confidentiality agreement answers the most frequent concern, that of revealing production data. The GHG Protocol also suggests asking for a third-party verification rather than the detail of activity data when the supplier refuses to open it. For a supplier with up to 1,000 employees, the clause stays within the limits of the voluntary standard, and the same logic applies to an LCA review (in French) (in French) requested from a supplier.
For us at Projet Celsius, the useful question to put to a supplier concerns how its figure was calculated, before the figure itself. A footprint accompanied by its method can be checked and tracked from one year to the next. In our carbon footprint assessment engagements, we sort the data received, recalculate the data that weighs most and draft the following year's request. The share of supplier data thus grows from one assessment to the next and is then used to reduce emissions.
6Key takeaways
- A supplier's data replaces an average factor only if it arrives with its method, boundary, unit and year.
- The GHG Protocol requires you to publish, category by category, the share of scope 3 calculated with supplier data and to describe its quality.
- A footprint that deducts carbon credits, biogenic carbon or poorly justified green electricity has to be corrected before it is added up.
- The check focuses first on the suppliers that account for 80% of purchases, then on the most highly emitting materials and fluorinated gases.
- For its customer's sustainability statement, a supplier with up to 1,000 employees may refuse any request that goes beyond the voluntary standard.
For the scope 3 calculation method itself, see our scope 3 guide, and to find out whether it is required, the question of mandatory scope 3.
