UK CBAM — Methodology and Liability Calculation Approach
A UK distributor imports £600,000 of aluminium extrusions a year from three countries. From 1 January 2027 it must register with HMRC, calculate the embodied emissions in every consignment, work out a quarterly carbon charge against them, deduct any carbon price already paid abroad, and file a tax return — for a tax with no certificates, no transitional phase, and a rate for each sector that HMRC resets every quarter.
The UK CBAM is a carbon tax on imports, and getting the embodied-emissions number right is the whole game.
The UK CBAM is a carbon tax on imported aluminium, cement, fertiliser, hydrogen and iron & steel, live from 1 January 2027. Liability is embodied emissions × a sector-specific quarterly rate, less relief for a qualifying overseas carbon price.
What the UK CBAM is, and the leakage problem it addresses
The UK Carbon Border Adjustment Mechanism is a tax on the carbon embodied in certain imported goods, taking effect from 1 January 2027. It is designed to ensure that carbon-intensive goods entering the UK face a carbon price comparable to the one domestic producers pay under the UK Emissions Trading Scheme — closing the gap that would otherwise let importers undercut decarbonising UK manufacturers.
The problem it targets is carbon leakage. UK producers of steel, cement, aluminium and similar goods pay for their emissions through the UK ETS and, for some, Carbon Price Support. A foreign producer in a jurisdiction with no equivalent carbon price faces no such cost, so its goods can land in the UK cheaper — not because they are more efficient, but because their carbon is unpriced. Left unaddressed, that incentivises production and emissions to move offshore, undermining domestic decarbonisation without reducing global emissions. The UK CBAM levels that field by charging imported goods the carbon price they would have faced had they been made in the UK, net of any carbon price already paid in the country of origin.
The defining design choice is that the UK CBAM is a tax, not a certificate scheme. There is no registry, no purchase or surrender of CBAM certificates, no quarterly certificate-holding obligation. A liable importer self-assesses embodied emissions, applies the published sector rate, deducts any qualifying overseas carbon price, and pays HMRC through an ordinary tax return. This is the single biggest structural difference from the EU CBAM, which operates through certificates, and it shapes everything downstream — the compliance burden is a tax-accounting burden, not a trading one.
The UK CBAM charges imported goods a carbon price comparable to what a UK producer pays under the UK ETS, less any carbon price already paid abroad. It is administered by HMRC as a self-assessed tax — there are no tradable certificates, and there is no transitional reporting-only phase before the charge begins.
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Scope: sectors, goods and emissions in the charge
Scope is the first compliance question, and the UK design is narrower than early proposals. The mechanism applies to specific goods — identified by commodity code on HMRC’s list of goods in scope — within five industrial sectors. HMRC’s default value tables cover 570 codes, 478 of them iron and steel. Glass and ceramics, originally proposed, are out from 2027 (kept under review for later inclusion), and so is imported electricity.
| Sector | In scope from 2027 | Greenhouse gases charged | Notable exclusions |
|---|---|---|---|
| Aluminium | Yes | CO₂ + PFCs (perfluorocarbons from smelting) | Scrap aluminium excluded |
| Cement | Yes | CO₂ | — |
| Fertiliser | Yes | CO₂ + N₂O (nitrous oxide from nitric-acid production) | — |
| Hydrogen | Yes | CO₂ | — |
| Iron & steel | Yes | CO₂ | Scrap steel excluded |
| Glass | No — dropped from 2027 | n/a | Under review for later inclusion |
| Ceramics | No — dropped from 2027 | n/a | Under review for later inclusion |
| Electricity | No — excluded | n/a | Reflects the UK’s distinct electricity market |
Direct, indirect and precursor emissions
The charge applies to the embodied emissions of a CBAM good — but which emissions count is precisely bounded, and it does not map cleanly onto the GHG Protocol scope framework. UK CBAM uses the regulatory categories of direct and indirect emissions, defined by reference to UK ETS system boundaries, rather than Scope 1, 2 and 3.
- Direct emissions — emissions from the production process of the good itself, including heating and cooling, within the UK ETS system boundary. In scope from 2027.
- Precursor emissions — emissions embodied in an in-scope CBAM good used as an input to a more complex CBAM good (for example, the steel embodied in a steel product). In scope from 2027, so the charge follows emissions up the value chain.
- Indirect emissions — emissions from the electricity consumed in production. Delayed until 2029 at the earliest, to align with continued support for energy-intensive industries.
UK CBAM “direct” emissions are not identical to Scope 1, and CBAM “indirect” is not identical to Scope 2. The boundaries are set by the UK ETS monitoring rules, and precursor emissions pull in value-chain emissions that would sit in Scope 3 under corporate accounting. Map carefully; do not assume an existing corporate inventory can be reused unadjusted.
Registration, the £50,000 threshold and the tax point
Liability sits with the importer of the CBAM good — the “liable person.” Whether an importer is in scope at all turns on a single value test.
An importer must register with HMRC once the value of its CBAM goods crosses £50,000, assessed through two tests applied to goods passing the tax point on or after 1 January 2027:
- Forwards test — on any given day, whether the importer expects the value of CBAM goods passing the tax point over the next 30 days to meet or exceed £50,000. Liability to register begins the day the test is met.
- Backwards test — on the first of each month, looking back over the preceding 12 months (or back to 1 January 2027 during the first year), whether the value met or exceeded £50,000.
The £50,000 figure was raised from an originally-proposed £10,000 specifically to spare smaller importers — the Government estimated the higher threshold removes over 80% of potentially-liable businesses while still capturing around 99% of imported emissions. The tax point — when liability crystallises — is the point the good is released into UK free circulation (or enters the UK where no customs controls apply). A registered importer must keep records for six years, submit returns, and pay the tax due; fraudulent evasion is a criminal offence.
The threshold is by value, not weight or consignment. A single high-value steel shipment can put an importer over £50,000 on its own. Monitor the rolling 12-month total continuously — crossing the threshold even slightly triggers full registration and return obligations.
The liability calculation: rate, embodied emissions and relief
The UK CBAM liability for an accounting period reduces to one equation, applied per CBAM good and summed:
CBAM liability = (embodied emissions × sector CBAM rate) − Carbon Price Relief
Where embodied emissions = emissions intensity (tCO₂e per tonne of good) × weight of the good imported (tonnes); the sector CBAM rate is HMRC’s quarterly £/tCO₂e rate for that sector; and Carbon Price Relief is the deductible carbon price already paid overseas on those same emissions.
Three inputs drive the number, and each is governed by its own set of rules: the embodied emissions (which the importer determines using actual data or default values), the sector CBAM rate (which HMRC sets quarterly), and Carbon Price Relief (which the importer claims with evidence). The sections below take each in turn. The arithmetic is straightforward; the difficulty is sourcing and verifying the inputs.
Determining embodied emissions: actual data vs default values
Embodied emissions are the heart of the calculation, and the importer has two routes to them.
Actual verified emissions data
Installation-level emissions intensity for the specific good, calculated by the producer following the UK CBAM system-boundary and monitoring rules, and independently verified by an accredited third party. Generally lower than the default where the producer is efficient — and the route to minimising liability.
Default emissions values
One HMRC value per CN code, the same for every country of origin, published 16 October 2026. Used when actual verified data is unavailable. In force from 1 January 2027 to 31 December 2028 and includes a 10% mark-up in every sector except fertilisers; country-specific defaults are planned from 2029.
The mechanics: the producer’s installation calculates a verified emissions intensity — tCO₂e per functional unit (per tonne of the good for most sectors; per tonne of clinker for cement and per tonne of nitrogen for fertilisers, converted to per tonne of the good using its clinker or nitrogen content) — through a defined seven-step process. The liable importer then reports that emissions intensity and the weight of the imported good to HMRC, which converts kilograms to tonnes and multiplies to give total embodied emissions. Importers may link the emissions intensity either to the date of importation or the date of production of the good.
HMRC’s default value tables give one value per CN code for all 570 goods in scope; the UK CBAM calculator looks them up by code or name. The force of law notice that gives them legal effect was still marked draft on 22 October 2026, and the sector rates published alongside are illustrative: HMRC publishes the official January to March 2027 rates before the end of 2026.
How the sector CBAM rate is set
The CBAM rate is what converts embodied emissions into a charge, and it is engineered to track the effective carbon price a UK producer in that sector actually faces — not the headline UK ETS price. A separate rate is set for each sector and updated quarterly.
HMRC’s formula:
Sector CBAM rate = average UK ETS price (preceding quarter) × [1 − (sector free allocation share × reduction factor)]
HMRC’s cement example, 16 October 2026: £58.15 (July to September 2026 average) × [1 − (87.1% × 97.5%)] = £8.78 per tonne of CO₂e. The illustrative rates for the other sectors are iron & steel £9.69, aluminium £9.79, hydrogen £9.24 and fertilisers £1.45.
The rate references the average UK ETS price over the quarter preceding the quarter of import, then applies a sector-specific Free Allocation Adjustment — a reduction reflecting the free allowances that sector receives under the UK ETS. The adjustment is derived from baseline sectoral emissions and free allowances (UK ETS data for 2019, 2022 and 2023) and is tapered each year by a reduction factor mirroring the nine-year phase-out of UK ETS free allowances.
The free-allocation adjustment is the subtle and essential part. UK producers do not pay the full UK ETS price on every tonne — they receive free allowances as carbon-leakage protection. If the CBAM rate charged the full ETS price on imports, importers would be over-charged relative to domestic producers, giving UK industry double protection and distorting the carbon price. Adjusting the rate down for free allocation keeps imported and domestic goods on a genuinely comparable footing. As UK ETS free allowances phase out over nine years, the adjustment shrinks and the effective CBAM rate rises toward the full ETS price.
Because the rate references the preceding quarter’s ETS price, an importer can know the rate that will apply before the quarter of import begins — useful for pricing and planning. The first accounting period (calendar year 2027) uses the relevant quarterly rates across that year.
Carbon Price Relief
Carbon Price Relief (CPR) prevents double carbon taxation. If the embodied emissions in an imported good have already been subject to a qualifying overseas carbon price, the importer can deduct that price from the UK CBAM charge, provided it holds the necessary verified evidence.
The rules are deliberately tight on what counts:
- Only explicit, deductible carbon prices qualify — an emissions trading scheme with a market-determined price, a fixed-price carbon tax, or another CBAM. Implicit pricing (such as fuel duties or non-pricing carbon policy) does not count.
- The price must relate to the same embodied emissions being charged under UK CBAM, and overseas prices are referenced on a quarterly basis.
- The onus is on the importer to hold verified evidence that a qualifying carbon-price liability was incurred — including through the supply chain where the importer did not produce the good. The evidence must show the liability was incurred, not necessarily that it has been paid.
CPR is the mechanism most relevant to imports from jurisdictions with their own carbon pricing — notably the EU, where goods may already have borne an EU ETS or EU CBAM cost. The UK has also extended CPR to recognise carbon prices incurred under other CBAMs, reducing the risk that a good is charged twice as it crosses successive borders.
Worked example — an aluminium importer’s liability
The following is an illustrative UK CBAM liability calculation using HMRC’s published default value and HMRC’s illustrative aluminium rate of 16 October 2026. Only the overseas carbon price is assumed. Inputs, formula and result reconcile exactly; substitute the official rate for the quarter of import when HMRC publishes it.
A UK importer brings in 500 tonnes of unwrought, non-alloy aluminium (CN 7601 10 90) in an accounting period from a country whose producers paid a partial domestic carbon price. The importer uses the default emissions value (actual verified data unavailable) and claims Carbon Price Relief for the qualifying overseas price.
| Step | Input (assumed where noted) | Formula | Result |
|---|---|---|---|
| Embodied emissions | 500 t × 2.136 tCO₂e/t (HMRC default value) | weight × emissions intensity | 1,068 tCO₂e |
| Gross CBAM charge | 1,068 tCO₂e × £9.79/t (HMRC illustrative rate) | embodied emissions × sector rate | £10,455.72 |
| Carbon Price Relief | 1,068 tCO₂e × £4.00/t (assumed qualifying overseas price) | embodied emissions × deductible overseas price | −£4,272.00 |
| Net CBAM liability | — | £10,455.72 − £4,272.00 | £6,183.72 |
Net liability for the accounting period: £6,183.72, an effective net rate of £5.79 per tonne of embodied CO₂e (£9.79 sector rate less £4.00 overseas carbon price relief). The rate is HMRC’s illustrative figure and the overseas price is assumed. A producer with verified emissions below HMRC’s default would lower the charge.
The composition below shows where the charge comes from on this single dataset: the gross charge against embodied emissions, and how Carbon Price Relief reduces it to the net liability actually owed to HMRC.
Illustrative — derived from the worked example above (single reconciled dataset). CPR + net liability = gross charge (£4,272.00 + £6,183.72 = £10,455.72).
UK CBAM vs EU CBAM
UK importers, and producers selling into both markets, need the differences between the two mechanisms clear. They share a foundational architecture — charge imported goods a carbon price comparable to the domestic one — but diverge sharply on instrument, scope, timing and threshold.
| Dimension | UK CBAM | EU CBAM |
|---|---|---|
| Instrument | A tax — self-assessed, paid to HMRC. No certificates. | Certificates — purchased and surrendered via a registry. |
| Start | 1 January 2027, charge live immediately | Transitional reporting Oct 2023–2025; definitive charge from 2026 |
| Transitional phase | None — no reporting-only run-in | Yes — reporting-only phase preceded the charge |
| Sectors | Aluminium, cement, fertiliser, hydrogen, iron & steel | Iron & steel, cement, aluminium, fertiliser, hydrogen, electricity |
| Electricity | Excluded | Included |
| Indirect emissions | Delayed to 2029 at the earliest | Included for certain products |
| Threshold | £50,000 of CBAM goods over rolling 12 months | 50 tonnes of CBAM goods a year (from 2026) |
| Reference carbon price | Average UK ETS price of the preceding quarter, free-allocation adjusted | EU ETS allowance price (weekly average from 2027) |
| Rate cadence | Quarterly, sector-specific | Weekly certificate price (from 2027) |
The UK and EU have agreed to work toward linking their carbon markets, with the intent that linkage would exempt UK products from the EU CBAM and vice versa. As of 22 October 2026 no linking agreement had been published. It is a development worth tracking, as it would materially change the calculus for goods moving between the two. Until any linkage is in force, a good crossing from a third country into the UK and then into the EU could face both mechanisms, with Carbon Price Relief the route to avoiding genuine double-charging.
Compliance workflow and timeline
For a UK importer of CBAM goods, the compliance path is a tax-registration and return cycle layered on top of an emissions-data-gathering exercise.
- Identify CBAM goods. Map imports against HMRC’s list of commodity codes in scope for the five sectors. Exclude scrap aluminium and steel. This determines whether the mechanism applies at all.
- Monitor the £50,000 threshold. Run the forwards (30-day) and backwards (rolling 12-month) value tests continuously. Register with HMRC when either is met.
- Source embodied-emissions data. Obtain verified actual emissions intensity from producers where possible; otherwise use HMRC default values. Engage suppliers early — verified data is the lever on liability.
- Gather Carbon Price Relief evidence. Where a qualifying overseas carbon price was incurred, collect the verified documentation needed to claim CPR, including through the supply chain.
- Calculate liability. For each good: embodied emissions × the relevant quarterly sector rate, less CPR. Sum across the accounting period.
- File and pay. Submit the online CBAM return and pay HMRC. The first accounting period is the calendar year 2027, with return and payment due 31 May 2028; periods become quarterly from 2028. Retain records for six years.
The timeline compresses fast. CBAM goods passing the tax point from 1 January 2027 count; the first return covers all of 2027 and is due 31 May 2028; from 1 January 2028 accounting periods are quarterly. HMRC’s registration service opens on 1 January 2028, with a registration deadline of 31 January 2028 for importers who passed the threshold during 2027. HMRC published illustrative rates on 16 October 2026 and publishes the official January to March 2027 rates before the end of 2026. Indirect emissions enter scope no earlier than 2029.
Need to understand the carbon-pricing system the CBAM rate references?
Edge cases and common pitfalls
Several issues separate a clean first UK CBAM return from one that triggers HMRC scrutiny or an over-payment.
- Assuming EU CBAM data transfers unchanged. The two mechanisms share commodity-code scope in places but differ on sectors (no UK electricity), instrument (tax vs certificates) and rate construction. EU CBAM figures are a starting point, not a UK answer.
- Treating the threshold as per-consignment. The £50,000 test is a rolling 12-month value total, not a per-shipment de minimis. Several sub-threshold consignments can aggregate over the line.
- Reusing a corporate GHG inventory as embodied emissions. CBAM direct/indirect categories follow UK ETS system boundaries, not GHG Protocol scopes, and precursor emissions pull in value-chain emissions. The boundaries do not match — re-derive against the CBAM rules.
- Defaulting to default values when actual data would be cheaper. Default values are global production-weighted averages and can overstate an efficient producer’s emissions. Where a producer can supply verified actual intensity, it often reduces liability — but it must be independently verified.
- Over-claiming Carbon Price Relief. Only explicit, deductible carbon prices qualify, on the same embodied emissions, with verified evidence. Implicit pricing and unverified claims will not survive challenge.
- Forgetting the nil return. A registered liable person with no liability in a period — including where CPR reduces it to zero — must still file a return for that period.
- Missing indirect-emissions timing. Indirect (electricity) emissions are out until 2029 at the earliest. Including them in a 2027 or 2028 calculation over-states the charge.
Frequently asked questions
1 January 2027. Unlike the EU CBAM, there is no transitional reporting-only phase — the charge applies from the start. CBAM goods passing the tax point on or after 1 January 2027 count toward liability. The first accounting period runs the full calendar year 2027, with the return and payment due by 31 May 2028; from 1 January 2028 accounting periods become quarterly.
Five sectors: aluminium, cement, fertiliser, hydrogen, and iron & steel. Within them, only specific goods identified by commodity code on HMRC’s list are covered. Glass and ceramics were dropped from the 2027 scope (kept under review), imported electricity is excluded, and scrap aluminium and steel are excluded. The greenhouse gases charged are CO₂ across all sectors, plus N₂O for fertilisers and PFCs for aluminium.
Liability equals the embodied emissions of the good multiplied by the sector-specific quarterly CBAM rate, minus Carbon Price Relief for any qualifying carbon price already paid overseas. Embodied emissions are the verified emissions intensity (tCO₂e per tonne) times the weight imported. The sector rate references the average UK ETS price of the preceding quarter, adjusted down for the free allowances that sector receives under the UK ETS. The importer self-assesses and pays HMRC — there are no certificates.
An importer must register with HMRC for CBAM once the total value of its CBAM goods meets or exceeds £50,000 over a rolling 12-month period. Two tests apply: a forwards test (expected value over the next 30 days) and a backwards test (value over the preceding 12 months, checked monthly). The threshold was raised from a proposed £10,000 to spare smaller importers — the Government estimated it removes over 80% of potentially-liable businesses while still capturing around 99% of imported emissions. It is a value test, not a per-consignment or weight test.
No. The UK CBAM is structured as a tax. There is no registry, no purchase or surrender of CBAM certificates, and no quarterly certificate-holding requirement. A liable importer self-assesses embodied emissions, applies the sector rate, deducts Carbon Price Relief, and pays HMRC through an online tax return. This is the single biggest structural difference from the EU CBAM, which operates through tradable certificates.
Not from the 2027 start. Direct emissions from production and precursor emissions embodied in in-scope inputs are in scope from 1 January 2027. Indirect emissions — those from the electricity consumed in production — are delayed until 2029 at the earliest, reflecting continued support for energy-intensive industries through the EII Compensation Scheme. Including indirect emissions in a 2027 or 2028 calculation would over-state the charge.
Yes, through Carbon Price Relief, where the embodied emissions were subject to a qualifying explicit carbon price — an emissions trading scheme, a fixed-price carbon tax, or another CBAM. Implicit pricing such as fuel duties does not qualify. The importer must hold verified evidence that the liability was incurred on the same embodied emissions, including through the supply chain where it did not produce the good. CPR is most relevant to imports from carbon-pricing jurisdictions such as the EU, and has been extended to recognise prices paid under other CBAMs.
Two routes. Use the producer’s independently-verified actual emissions intensity, calculated under the UK CBAM system-boundary and monitoring rules — generally the route to a lower, defensible figure for an efficient producer. Or use HMRC’s default emissions value for the product, one value per CN code, the same for every origin, used where actual verified data is unavailable. HMRC published the default values for all 570 codes on 16 October 2026, in force for 2027 and 2028, with illustrative sector rates; official rates for January to March 2027 follow before the end of 2026.