CORSIA Offsetting Requirement: How the Calculation Works

The full calculation with worked examples — filtering to covered emissions, applying the growth factor, subtracting the CORSIA Eligible Fuels reduction, and building a forecast before ICAO confirms the numbers.

An operator that waits for ICAO to hand it a number has already lost its options. By the time the figure is confirmed, the compliance period is closing and the whole sector is buying in the same window. Modelling the requirement in advance is what makes a purchasing strategy possible.

The CORSIA compliance cycle

The Formula

Offsetting requirement = (covered emissions × applicable growth factor) − CORSIA Eligible Fuels reduction

Each of the three terms carries more complexity than it looks.

Term 1: Covered Emissions

Filtering happens in stages, and each stage removes tonnes.

Filter	Effect
International flights only	Domestic emissions removed entirely
Aeroplanes above 5,700 kg MTOM	Lighter aeroplanes and all rotorcraft removed
Exclude exempt flight types	Humanitarian, medical, firefighting, State aircraft
Covered route pairs only	Both origin and destination States must participate
Correct operator entity	Wet leases, code shares and group certificates allocated properly

Emissions are derived from fuel mass:

Fuel	Factor
Jet-A and Jet-A1	3.16 kg CO2 per kg fuel
Jet-B	3.10 kg CO2 per kg fuel
Aviation gasoline (AvGas)	3.10 kg CO2 per kg fuel

The route-pair filter is where forecasts most often go wrong, because State participation changes — and changes structurally in 2027. See CORSIA scope and thresholds.

Term 2: The Growth Factor

Sectoral only through 2029, then blended with individual growth from 2030. Covered in full in baseline and growth factors.

Term 3: The CORSIA Eligible Fuels Reduction

Qualifying sustainable aviation fuel and lower carbon aviation fuel reduce the requirement directly, reflecting the lifecycle saving against conventional jet fuel.

The claim requires all three of:

The fuel meets the CORSIA Eligible Fuels sustainability criteria
It is certified under an approved Sustainability Certification Scheme
The chain of custody is documented from production through to uplift

Burning qualifying fuel without the certification and documentation produces an environmental benefit and no CORSIA claim. See CORSIA Eligible Fuels.

Worked Example

Illustrative figures, chosen to show the mechanics.

An operator reports 900,000 tonnes of CO2 across all flights in a year.

Step	Tonnes	Note
Total emissions, all flights	900,000	Starting point
Less domestic	−400,000	Outside CORSIA entirely
International subtotal	500,000
Less non-covered route pairs	−180,000	Far-end State not participating
Covered emissions	320,000	The base the factor applies to
× sectoral growth factor 6%	19,200	Gross obligation
Less CEF reduction	−1,200	Certified lifecycle saving
Net obligation	18,000 t	For that year

Three things this example shows:

The covered-emissions filter does most of the work. Total emissions were 900,000 tonnes; the figure driving the obligation was 320,000. An operator budgeting from total emissions overstates by nearly threefold.

Small factor changes move large numbers. Moving the growth factor from 6% to 9% adds 9,600 tonnes — a 50% increase in the obligation from a three-point move in a variable the operator does not control.

Second-phase coverage is the step change. If mandatory participation from 2027 brings the previously uncovered 180,000 tonnes into scope, covered emissions rise to 500,000 and the same 6% factor yields 30,000 tonnes rather than 19,200. That is why generic percentage uplifts fail and network-level modelling is necessary.

Accumulating Across a Compliance Period

Requirements are calculated annually and settled per three-year compliance period.

Year	Covered emissions	Factor	Annual obligation
2024	300,000	5.0%	15,000 t
2025	320,000	6.0%	19,200 t
2026	340,000	6.5%	22,100 t
Period total			56,300 t

The full 56,300 tonnes must be cancelled and reported by the deadline following the period's close.

There is no carry-forward. Cancelling more than you owe does not build a balance against a future period — the surplus is simply spent. Precision in this calculation therefore has direct financial value.

Building a Forecast

Step 1 — establish a live covered-emissions base

From your verified reporting, isolate international flights on covered route pairs. Maintain this as a running figure through the year rather than reconstructing it each spring. Operators who rebuild it annually spend the same effort repeatedly and introduce inconsistency between years.

Step 2 — model route coverage scenarios

Run two scenarios against your actual network: current participation, and second-phase mandatory participation from 2027. The delta between them is frequently the largest single number in the whole forecast, and it is entirely specific to where you fly.

Step 3 — estimate the growth factor as a range

Published industry traffic and emissions data supports a reasonable estimate ahead of ICAO confirmation. Produce low, central and high cases with the assumptions written down.

Step 4 — layer in individual growth from 2030

Using your own fleet and network plans against the weighting schedule. This connects growth decisions to their CORSIA cost while those decisions are still being made.

Step 5 — model the fuels reduction realistically

Base it on fuel you can actually obtain with certification and chain of custody you can actually produce — not on a sustainability target. An aspirational SAF assumption understates the obligation and the budget.

Step 6 — re-run annually

Participation changes, factors are confirmed, your network changes, and the ICAO Council periodically adjusts parameters. A forecast built once and left alone will drift.

Documenting the Calculation

Whatever you calculate will be examined — by a verifier, possibly an auditor, quite likely someone who was not present when the decisions were made. It must be reproducible from your records alone.

Retain: the covered-emissions derivation with its route filtering logic, the participation list captured as at the date relied on, the growth factors applied and their source, the fuels claim with supporting certification, and the resulting figure.

Capture rather than link. The published participation list changes, and a URL is not evidence of what it said on the day.

Common Errors

Error	Consequence
Using total instead of covered emissions	Obligation overstated, often by multiples
Assuming current participation persists	Understates the 2027 step change
Applying a generic uplift for the second phase	Wrong for almost every network
Treating the sectoral factor as your growth	Misreads a shrinking year as zero obligation
Assuming SAF claims will be available	Overstates the reduction, understates the budget
Reporting under the wrong group entity	Misallocated emissions, painful to correct

Where to Go Next

Baseline and growth factors — term 2 in detail
CORSIA Eligible Fuels — term 3 in detail
Compliance periods and deadlines — when it settles
CORSIA credit pricing — turning tonnes into a budget

DSTechnoverse builds documented, reproducible requirement models for operators. Talk to our carbon markets team or apply as a CORSIA buyer or seller.