The 2026 UK Government GHG conversion factors, still widely called the "DEFRA factors", were published on 11 June 2026 by the Department for Energy Security and Net Zero (DESNZ). Air travel factors are unchanged from last year, but several ground transport figures have shifted significantly.
If you report business travel emissions year on year, knowing how emission factors have changed is essential to explaining the movement in your numbers. Here’s what you need to know.
What are DEFRA emission factors and why do they matter?
DEFRA emission factors convert real-world activities, like travel, into carbon estimates. They are updated annually to reflect scientific findings, industry trends and operational data, so up-to-date factors are critical for accurate carbon reporting.
When a factor is updated, the emissions estimate changes with it.
What changed in the 2026 DEFRA emission factors?
The 2026 update has brought some of the biggest year-on-year changes to land travel factors we’ve seen in a while. Coach and international rail factors rose steeply, but factors for national rail, light rail, tram, the Underground, and plug-in hybrid and electric cars all fell.
Two forces are driving most of this change: continued decarbonisation of the UK electricity grid, which feeds through into every factor with an electric component, and a refresh of the underlying activity data for several land travel modes, much of which had not been updated since before the COVID-19 pandemic.
Category | Change vs 2025 | Reason for change |
Air | 0% | No Change |
Cars – small/mini PHEVs and all BEVs | -12% to -37% | Driven by a cleaner UK electricity grid plus efficiency improvements in EVs and PHEVs. |
Coaches | +42% to +43% | New data from a large coach operator replaces earlier estimates that implied unrealistically high occupancy rates. |
National rail | -13% | Passenger-km and fleet composition data refreshed for the first time since 2021, when estimates were still based on pre-pandemic patterns. |
International rail | +154% to +156% | A large swing due to changes in service patterns and rolling stock utilisation versus the pre-pandemic baseline data previously used. |
Light rail and tram | -26% | Reflects the cleaner electricity grid together with updated passenger-km data, which also replaces earlier pre-pandemic estimates. |
London Underground | -44% to -45% | Same drivers as light rail and tram: grid decarbonisation plus updated passenger-km data. |
Why did rail emission factors change in 2026?
National rail factors decreased by 13%. The previous factors were based on pre-pandemic estimates; this year’s update refreshes passenger-km (the total distance travelled by all passengers) and fleet composition (the types of train in service) for the first time since 2021.
International rail factors increased by 154% to 156%. Rail factors allocate a train’s emissions across its passengers, so they are sensitive to how services operate. The update reflects current service patterns (routes and frequency) and rolling stock utilisation (how trains are deployed and how full they are), correcting outdated pre-pandemic data rather than signalling a fall in demand.
Light rail and tram and London Underground factors have fallen by 26% and 44% to 45% respectively, both driven by a cleaner electricity grid and updated figures on passenger numbers and distances travelled. For a step-by-step method, see our guide to calculating rail emissions.
Why did coach emission factors increase?
Coach factors rose by 42% to 43% because new data from a large coach operator replaced earlier estimates. Those earlier estimates implied unrealistically high occupancy rates.
Our guide to calculating coach emissions explains how the factor is applied.
Did the well-to-tank (WTT) factors change?
There are no major changes to the WTT factors for air or land travel in the 2026 update. WTT factors cover the indirect, upstream emissions from extracting, processing, transporting and distributing fuel before combustion.
How will the 2026 factors affect my business travel emissions?
Most organisations will see only minor changes at programme level, because the updates largely affect ground transport, which is typically a relatively small part of total travel programme emissions. The exception is organisations that rely heavily on rail or coach travel, which should expect a noticeable shift in reported emissions for those categories.
To understand how the update affects your reporting year, it’s important to check when the new factors were applied. DEFRA publishes its factors mid-year, so unless your full year’s data has been recalculated, some 2026 travel may still be calculated using the 2025 factors. Mixing old and new factors within one reporting year makes year-on-year comparisons harder to interpret.
Do I need to restate previous years' emissions?
As these changes reflect improved data inputs rather than a change in methodology, no rebaselining or adjustment of historical reporting is necessary.
What you do need is a clear explanation.
How should I explain the 2026 factor changes to stakeholders?
It’s important that stakeholders can see whether emissions changed because of what your organisation did or because emission factors changed. Reporting the two separately keeps your figures transparent.
To quantify the impact of the updated factors, recalculate this year’s travel activity using the 2025 factors and compare the result with your 2026 figures. The difference shows how much is due to the new factors; everything else reflects how your travel has changed.
Suggested wording you can adapt:
Our 2026 rail and coach emissions have changed due to updates to DEFRA's underlying input data, which now reflects current passenger loads and fleet composition rather than pre-pandemic patterns, alongside a cleaner UK electricity grid. These updates account for [X]% of the change in our reported emissions, with the remainder due to operational changes.
For how last year’s factor changes affected travel emissions reporting, see our 2025 DEFRA emissions factors update.