# We buy billets from several mills. How does the UK CBAM average them?

Canonical: https://senseible.earth/answers/uk-cbam-billets-from-several-mills

By weight. HMRC's rules apply weighted averages where precursors come from several installations and periods, so each mill's billet tonnes are multiplied by that mill's figure and added. You may instead isolate the emissions where billets come from one installation or a subset. Each mill's data is recorded separately.

## What you record for each mill

- Country of origin and the installation: identifier if any, UN/LOCODE, address and coordinates.
- Production route.
- Emissions intensity averaged over its most recent monitoring period, and that period's start and end dates.
- Tonnes used in each of your production processes.

## When a mill gives you nothing

For any quantity with incomplete or inconclusive data, use HMRC's default value for that precursor. You can mix: actual figures for mills that are verified, defaults for those that are not.

## One limit

HMRC allows defaults for precursors even when your own rolling uses actual data. The reverse is not allowed: if the importer uses a default for your bar, actual precursor data cannot be used.

## Three billet suppliers in one year

- Mill A: 5,000 t at 2.10000 tCO2e/t = 10,500 tCO2e.
- Mill B: 3,000 t at 1.80000 tCO2e/t = 5,400 tCO2e.
- Mill C (no verified figure): 2,000 t at a default of 2.50000 tCO2e/t = 5,000 tCO2e.
- Precursor emissions: 10,500 + 5,400 + 5,000 = 20,900 tCO2e for 10,000 t, a weighted 2.09000 tCO2e/t.
- Add your rolling emissions, then divide by bars produced.

Example figures; look up the actual default for your billet's commodity code.

## What Senseible does here

Senseible reads billet purchase invoices by supplier and month, so the tonnes per mill come straight from your bills. We build the figure from your records and check every figure in-house; for the UK it must then be verified by a verifier accredited for UK CBAM. Have your own verifier? Send it to them.

- [Mixing defaults and actual data](https://senseible.earth/answers/uk-cbam-default-values-mixing-rules)
- [Which year's precursor data](https://senseible.earth/answers/uk-cbam-precursor-monitoring-period)
- [CBAM Suite](https://senseible.earth/cbam-suite)

Sources: [HMRC, Carbon Border Adjustment Mechanism (CBAM): Policy summary (updated 2 November 2026)](https://www.gov.uk/government/publications/carbon-border-adjustment-mechanism-cbam-policy-summary/carbon-border-adjustment-mechanism-cbam-policy-summary), [HMRC, UK CBAM monitoring, reporting and verification technical guidance (October 2026)](https://assets.publishing.service.gov.uk/media/6ad219fe0f7b53973e02680f/CBAM_technical_guidance.pdf)

Last reviewed 2026-11-03.