customer wants to know CO2 emissions per turnover, how do I calculate that

Emissions divided by revenue, no new measurement

This ratio is not an additional measurement point but a calculation based on existing data: total emissions in tonnes CO2-equivalent, divided by annual turnover in million euros. The numerator is already in your supply chain file if you the question on scope 1, the question on scope 2 and possibly scope 3 have already been answered. The denominator is the turnover from your financial statements, the same figure that the customer often requests separately. The result is called "carbon intensity" and is usually expressed as tCO2e/mln euro turnover.

The choice you make yourself is which scopes you include in the numerator. If the customer asks only for scope 1 and 2, that is a smaller and often more precise numerator than when scope 3 must be included. Many SMEs do not have a complete picture of scope 3 — in that case you calculate with what you have and state which scopes are included. A list that asks for "total CO2 emissions per euro of turnover" without specifying scope usually means scope 1 and 2 together; if you are in doubt, asking the customer themselves is a more realistic option than guessing. If you do not yet have a scope 3 estimate, then the overview of scope 3 items is a more logical starting point than this ratio itself.

Where this classification comes from

The scope breakdown on which the numerator of this calculation is based comes from the GHG Protocol; this is the standard breakdown into scope 1, 2 and 3 that both EFRAG's VSME standard and virtually every customer questionnaire adopt. EFRAG's VSME basic module, the voluntary reporting standard for non-listed SMEs, mentions emissions per turnover as one of the key indicators, but does not prescribe a fixed formula for anything more than emissions divided by turnover — the scope choice and the unit of the denominator are therefore not fixed in a standard, only in the customer's question.