Producing a product carbon footprint you can stand behind

How do I calculate a product carbon footprint?

Five steps. Choose the product and the boundary. List everything that goes into one unit (the bill of materials) and the energy and waste per unit from your own processes. Put an emission factor against each line, your suppliers' own figures where they exist and published factors where they do not. Add it up and state the data quality. Check it against a standard before it goes to the customer. A first one takes weeks and most suppliers get help; the second takes days.

Step 1: which product, which boundary?

One product, the one the customer buys most of or a representative of a family. The boundary is where the footprint stops: cradle-to-gate (raw materials to leaving your site) is what most customers ask suppliers for, because they add their own stages after your gate. Cradle-to-grave adds use and end of life and is asked for less often. State the boundary on the first line of the footprint; it is the first thing a customer checks. The GHG Protocol Product Standard defines both.

Step 2: what data do I need?

Allocation is the one judgement call: if a process makes several products, its energy has to be split between them (by mass, by units, by value). Choose one basis, state it, and keep it the same across products.

Your customer will accept: a cradle-to-gate footprint to the GHG Protocol Product Standard with the boundary, the allocation basis and the factor source per line stated, and a data quality indication. A third-party check is better and comes later for most suppliers.

Step 3: which factors?

The ladder from the Measure stage applies line by line. Your supplier's own product footprint for a material is the best factor; their company-level figure applied to what you buy is next; a published factor for the material is the fallback. Many governments publish these free: Germany's ProBas database from the Umweltbundesamt and Taiwan's product carbon footprint factor database from its environment ministry are two examples, and commercial databases go further at a cost. Label each line with which you used. For your own energy, use the grid factor for your country and your market-based figure if you buy renewable electricity, as in your company footprint.

Step 4: how do I present it?

As a short document a customer can check and, increasingly, a data file their system can read. The document: product name and code, functional unit (one unit, one kilogram, one pallet), boundary, standard, reporting period, the total in kg CO2e per unit, a breakdown by stage (materials, energy, transport, packaging, waste), the allocation basis, the factor source per major line, a data quality statement and whether it has been verified. The data file: the PACT Methodology sets out the common format large customers now ask for, so that a footprint can be exchanged between systems without re-typing. If you can produce both, do.

Step 5: how do I check it before sending?

Two checks. First, plausibility: compare your total with published figures for similar products; a result far above or below is usually a unit error (grams against kilograms, a factor per tonne applied per kilogram). Second, completeness and format: run it through the free PCF Evaluator, which grades a footprint or EPD on data quality, whether the figure sits in the expected range, and alignment with the PACT format, and tells you which fields are missing. Customers run exactly this kind of check on what they receive, so running it first means fewer questions back.

Do I need it verified?

Not for a first PCF, unless the customer has asked. Verification against ISO 14067 or the Product Standard is a paid third-party check and is worth it once a footprint is being used in several customers' numbers or in your own marketing claims. Say on the document whether it has been verified; an unverified footprint with its method stated is accepted by most customers as a first submission.

Do this week: pull the bill of materials for your most-bought product and last year's energy per unit. Email the suppliers of the three largest material lines and ask for their product footprint or company figure.

What should I do next?

Asking your own suppliers covers how to get those factors without recreating the burden you have been through. If your customer uses DitchCarbon, claim your profile and upload the footprint, where it is checked and matched to what the customer buys.