TL;DR – Article Summary:
- DEFRA’s emissions factors are updated annually to reflect scientific findings, industry trends, and operational data. These factors convert real-world activities, like travel, into carbon estimates. Up-to-date emissions factors are critical for accurate carbon reporting.
- The 2025 DEFRA update has substantially lowered aviation emissions factors, with reductions ranging from 16% to 42% across all flight categories. This decrease is primarily attributed to the post-COVID recovery of air travel. With more seats occupied per flight load factors have reduced, causing emissions per passenger to drop.
- DEFRA’s emissions factors rely on data that typically lags by 1 to 3 years, limiting proactive reporting. Instead of depending on these delayed updates, businesses seeking real-time emissions insights should consider adopting fuel-based methodologies, which capture more specific travel changes, such as aircraft carrier type.
First things first: What are Emissions Factors?
Emissions factors—also known as conversion factors—are numerical values that translate real-world activities like travel into estimates of carbon dioxide equivalent (CO₂e) emissions. They allow businesses to calculate environmental impacts in a standardised and measurable way.
The DEFRA methodology uses emissions factors as the basis for quantifying greenhouse gas emissions from specific activities like passenger flights.
Emissions = Activity Data × Emissions Factor
Example calculation: A long-haul business class flight of 3,000 km × DEFRA's 2025 factor of 0.41077 kg CO₂e per passenger km = 1,232.3 kg CO₂e.
* Calculations include RF and WTT.
The 2025 Update: Three Key Changes
1. Post-Pandemic Normalisation
The 2024 emissions factors were based on 2021 data — a year when COVID-19 led to unusually low passenger loads. As a result, emissions per passenger-kilometre were inflated. In contrast, the 2025 update, based on 2023 data, reflects a return to typical passenger load levels.
2. Updated Aircraft Data
DEFRA relies on aircraft data, specifically flight distance and duration, to estimate aviation emissions. The 2025 update uses current, publicly available fleet information, better representing today’s more efficient aircraft and current operational patterns, replacing confidential Civil Aviation Authority data from 2012.
3. Improved Calculation Methods
The EUROCONTROL Small Emitters Tool (SET) estimates fuel use and emissions for entire flights using real-world data from airlines across Europe. DEFRA uses the SET to estimate fuel burn for flights under the UK Emissions Trading Scheme (ETS). The 2025 update includes revisions to SET, resulting in more accurate calculations.
Significant Emissions Reductions Across all Flight Categories
The above updates, absent from previous factor calculations, have led to substantial reductions across all flight types:
Flight Distance | Short-haul | Long-haul | Domestic |
Reduction in emissions factor | 27.81% | 36.96% | 14.14% |
**Calculations using RF, and full lifecycle (combustion + WTT). Data sources linked below.
Why Updates Matter for your Business:
DEFRA (now DESNZ) has published the UK’s official emissions factors annually since 2002, providing the standard methodology for corporate greenhouse gas (GHG) reporting. These annual updates ensure that carbon accounting reflects more recent data and operational efficiencies. Using outdated factors can lead to over- or underestimating emissions, potentially undermining environmental targets and misleading stakeholders.
Actions for Businesses using DEFRA:
• Use the latest DEFRA emissions factors to calculate your travel emissions for current reporting year to stay accurate.
• Communicate changes with your team and stakeholders, explaining any significant shifts in reported emissions that result from factor updates rather than actual operational changes.
• Adjust your targets and strategies as needed because updated factors may change your progress toward net zero goals.
The Critical Limitation: Time Lag in Data
DEFRA’s standardised approach to emissions reporting offers consistency but is fundamentally limited by a significant time lag. Emissions factors are based on industry data that is typically 1–3 years old, which presents several challenges:
• Delayed updates to passenger capacity changes (load factors).
• Delayed recognition of innovation: Advancements such as sustainable aviation fuels (SAF) or next-generation aircraft technologies take years to be reflected in the official emissions factors.
• Reduced strategic value: Reliance on historical data limits the ability to make informed, forward-looking decisions around sustainability.
Ultimately businesses looking to be proactive should be using a fuel based methodology.
The Case for Real-Time Data
For organisations committed to driving real environmental impact—not just reporting it—the choice of emissions methodology is critical. Accurate, real-time data is also essential for driving sustainable investment across the aviation industry.
Distance-based methods like DEFRA’s offer consistency, but fall short in accuracy and responsiveness to innovation. In contrast, estimated fuel-based methodologies provide a more precise, dynamic, and actionable understanding of emissions.
Fuel-based methodologies offer significant advantages:
Aspect | DEFRA methodology | Fuel-Based methodology |
Data Currency | 1-3 year lag | Real-time |
Specificity | Generic flight/distance groups | Aircraft-specific calculations |
Operational Factors | Standardised assumptions | Actual operational data |
Strategic Value | Compliance reporting | Strategic decision making |
Innovation Tracking | Limited | Adapts to current developments |
Fuel- or quantity-based calculations are considered the most accurate method for estimating emissions, as they rely on the actual or estimated amount of fuel consumed and take into account detailed technical characteristics of the specific aircraft.
There are a number of fuel-based methodologies in the market, all with different benefits and limitations. Leading methodologies, all offered by Thrust Carbon, include: ICAO+, Google TIM (Travel Impact Model), and IATA CO2 Connect.
Making the most of Fuel-based Methodologies:
Fuel-based methodologies offer granular real-time data, enabling business to make strategic decisions that can drive meaningful emissions reductions. By incorporating details such as aircraft-specific emissions and airline efficiency, these methods reveal reduction opportunities that would otherwise be obscured by DEFRA.
For example, with a fuel based methodology you can choose the least carbon intensive flight on a specific route. When applied at scale across an organisation, these choices can lead to significant emissions savings.
The Path Forward
The 2025 DEFRA changes highlight both the importance of emissions factors and their limitations. The delay in these changes underscores why organisations committed to meaningful climate action need tools that are proactive, not reactive.
Source:
Calculation data sets:
• 2024 Data: https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2024
• 2025 Data: https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2025
