Business Travel (Air) Emissions

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Why Method Selection Matters

Business air travel is, for most service-sector organisations, the largest single line in the Scope 3 Category 6 inventory and the one most sensitive to methodology choices. The same itinerary can be quantified three ways, and the figure each produces reflects the data the organisation holds rather than any disagreement about the underlying flights.

Air travel is only one line of the Category 6 boundary: business travel by car and hotel stays are quantified under their own methodologies within the same category, while employee commuting and homeworking sits in Scope 3 Category 7.

The choice has three downstream effects. It sets the disclosed figure: a spend-based estimate and a distance-based calculation of the same travel programme rarely match, and the gap can be material. It sets the data-quality position under frameworks such as PCAF, where activity-based methods score better than spend-based proxies. And it sets the verification posture under the GHG Protocol Scope 3 Standard, which expects the most granular method the available data supports.

The Three Methods at a Glance

Before each method is worked through in detail, the table sets out how they compare on input, data burden, and result granularity. The framing that matters: all three are accepted, but a verifier expects an organisation with itinerary-level booking data to use the distance-based method rather than fall back on spend.

Distance-based is the corporate default because itinerary data is widely available from travel-management companies and the DEFRA factor set is purpose-built for it. Spend-based is the fallback rung when only financial data exists. Supplier-specific is the most accurate but depends on carriers reporting verified fuel allocation, which remains uneven. The sections below take each in turn, then apply all three to one itinerary in the worked example.

Method 1: Distance-Based (Passenger-km × Factor)

The distance-based method multiplies passenger-kilometres by an emission factor specific to the flight’s haul band and cabin class. The arithmetic is simple; the precision lives in selecting the correct factor and the correct distance.

emissions (kg CO₂e) = distance (passenger-km) × haul-and-class factor (kg CO₂e per passenger-km)

The factors come from the DEFRA 2026 conversion factor set, split by haul and cabin class, each available with and without a radiative-forcing uplift. DEFRA factors are published on an AR5 100-year GWP basis by convention; corporate inventories that default to AR6 elsewhere do not restate these aviation factors.

Great-circle distance and the ICAO v13.1 correction

A flight’s straight-line great-circle distance (GCD) understates the path actually flown. Routing, holding, and approach stacking add measurable distance, particularly on congested corridors. DEFRA allows for this inside its factors: every DEFRA flight factor already includes an 8% uplift on the great-circle distance (DEFRA 2026 methodology, paragraph 8.38), so the DEFRA method applies the factor to the GCD and adds nothing. The ICAO Carbon Calculator v13.1 instead adds a distance-band correction in kilometres before its own per-passenger calculation, which is why the ICAO route-level result can differ slightly. Never add a separate uplift to DEFRA factors: it counts the detour twice. Where offsetting is in scope, the same correction underpins CORSIA route accounting.

Haul-band factors

The headline DEFRA 2026 average-passenger factors (all cabins combined), radiative forcing included, are the current applied values below. Domestic carries the highest per-km factor because take-off and climb — the most fuel-intensive phases — dominate a short total distance; long-haul amortises that climb energy across thousands of cruise kilometres.

Values read live from the MasterBrain (business_travel.air.*.average.with_rf), AR5 100-year GWP basis. Without-RF variants and per-aircraft fuel-burn rows are held in the same section.

Cabin-class allocation

DEFRA assigns higher factors to premium cabins because they occupy more aircraft floor area per passenger, distributing total fuel burn across fewer seats. The multipliers below are relative to the economy factor for the same haul band:

Long-haul cabin ratios; short-haul distinguishes economy and business only.

Radiative forcing

Method 2: Spend-Based (Travel Spend × EEIO Intensity)

The spend-based method multiplies travel expenditure by an environmentally-extended input-output (EEIO) intensity for the air-transport sector. It requires only general-ledger spend data, which makes it the fastest method to deploy and the only option where itinerary data is unavailable.

emissions (kg CO₂e) = air-travel spend (currency) × sector EEIO intensity (kg CO₂e per currency unit)

Its weakness is granularity: spend is a poor proxy for distance, because fare varies with booking timing, class, and route competition far more than with emissions. A last-minute economy fare and an advance-purchase business fare can cost the same and emit very differently. Under PCAF data-quality scoring, spend-based sits at Score 4–5 — the fallback rung — and the Scope 3 Standard expects an organisation to move to activity data as soon as it is available.

Method 3: Supplier-Specific (Carrier Fuel Allocation)

The supplier-specific method uses a carrier’s own reported fuel consumption, allocated to the organisation’s travellers. It is the most accurate where the data exists, because it reflects the actual aircraft, load factor, and routing flown rather than a class-average default. The fuel-to-CO₂ conversion rests on the IATA RP1726 jet fuel factor of 3.16 kg CO₂ per kg of Jet A/A-1.

emissions (kg CO₂) = allocated jet fuel (kg) × 3.16

The constraint is availability: carrier fuel allocation reporting is voluntary and uneven, and an organisation can rarely cover its whole travel programme this way. In practice supplier-specific data covers the carriers that report it, with distance-based factors filling the remainder — a hybrid that the GHG Protocol Scope 3 Standard explicitly permits.

Decision Framework: Which Method Should You Use?

Method selection follows from two questions: whether itinerary or origin–destination data exists, and whether any carrier supplies verified fuel allocation. The tree traces the path; the matrix resolves the same logic against data availability.

            ┌─────────────────────────────────────────────┐
            │ Itinerary / origin–destination data exists? │
            └───────────────────┬─────────────────────────┘
                      NO ◄───────┴───────► YES
                      │                     │
                      ▼                     ▼
            ┌──────────────────┐   ┌──────────────────────────────┐
            │ Spend-based      │   │ Carrier-reported fuel        │
            │ (EEIO intensity, │   │ allocation available?        │
            │  PCAF Score 4–5) │   └──────────────┬───────────────┘
            └──────────────────┘     NO ◄─────────┴─────► YES
                                     │                     │
                                     ▼                     ▼
                          ┌──────────────────┐  ┌──────────────────┐
                          │ Distance-based   │  │ Supplier-specific│
                          │ (DEFRA factors,  │  │ for those carriers│
                          │  the calculator) │  │ distance for rest │
                          └──────────────────┘  └──────────────────┘

Worked Example: Same Itinerary, All Three Methods

To show how the methods diverge on identical travel, consider one return business-class trip, London to New York, one traveller. The same trip is run through all three methods. Input figures are illustrative; the GWP basis is the DEFRA AR5 convention.

The supplier-specific figure is unavailable for this trip because the carrier does not report fuel allocation, so the hybrid falls back to the distance-based result for this route. The three figures will not match, and the table below shows the shape of the divergence:

Worked-example figures are fixed as an audit record: they must reconcile to the values the calculator returned on the page’s last-reviewed date. The calculator’s default DEFRA tier uses the same great-circle distance, because DEFRA’s factors already include the routing uplift, so it returns the same figure.

Verification and Data Infrastructure

Each method rests on a different data foundation. The table summarises what each requires before the layers are detailed.

The itinerary feed is the prerequisite for the distance-based method: a per-segment record of origin, destination, and cabin class, ideally piped directly from the travel-management company rather than reconstructed from expense claims. Spend-based depends only on coded general-ledger travel spend, which is why it is the screening fallback. Supplier-specific depends on the carrier supplying verified fuel allocation, which most do not, and which must be checked for double-counting against any distance-based segments for the same flights.

A verifier under the GHG Protocol Scope 3 assurance approach tests whether the figure traces to source records — the itinerary feed for distance-based, the GL extract for spend-based — and whether the haul-band classification, cabin mapping, and RF basis are applied consistently. The recurring findings are predictable: RF basis undeclared, spend-based used where itinerary data was in fact available, and codeshare segments double-counted across two carriers’ reports.

Edge Cases and Second-Order Effects

The three methods handle the common case well. Several situations sit at their edges:

RF basis mismatch. The single most consequential edge case is inconsistency: a programme that applies with-RF factors to some routes and without-RF to others produces a figure no verifier can reconcile. Declare the basis once and apply it across the whole programme.

Codeshare double-count. A flight sold by two carriers under a codeshare is one physical flight. Distance-based accounting attributes it once to the operating carrier; supplier-specific accounting can double-count it if both carriers report the same fuel. Reconcile to the operating carrier.

Air freight is not Category 6. Cargo moved by air is upstream transport under Category 4 and uses GLEC tonne-km freight intensities, covered in the GLEC Framework reference. Only passenger travel by employees belongs in Category 6.

Multi-leg layovers. A trip with a connection should be summed segment by segment, not estimated from endpoint GCD, which understates the routed distance through the hub.

Implementation Roadmap: Spend to Distance to Supplier

Most organisations cannot stand up itinerary-level infrastructure in their first reporting year, and they do not need to. The defensible path establishes a baseline quickly and improves data quality over subsequent cycles.

Year 1 closes the gap with whatever financial data exists. Year 2 stands up the itinerary feed and switches to the distance-based method — the largest single improvement in data quality and the point at which the air-travel calculator becomes the working tool. Year 3 layers supplier-specific data over the carriers that report it, reconciling against the distance-based figures for the same flights to avoid double-counting.

Sources and Further Reading

The methods, factors, and frameworks on this page trace to the governing sources below. Current applied factors are read live from the MasterBrain; worked-example values are fixed as an audit record.

• DEFRA 2026 conversion factors — distance-based passenger-km factors by haul and class — UK DEFRA emission factors
• GHG Protocol Scope 3 Standard, Category 6 — boundary and method hierarchy — GHG Protocol Scope 3 Standard
• ICAO Carbon Calculator v13.1 — great-circle distance correction and route methodology
• IATA RP1726 — jet fuel allocation methodology (3.16 kg CO₂ per kg Jet A/A-1)
• CORSIA — aviation offsetting context — CORSIA aviation carbon offsetting
• PCAF Financed Emissions Standard — data-quality scoring for spend-based proxies — PCAF financed emissions standard