This transport methodology sets out how transport emissions are calculated, providing a consistent approach for estimating carbon dioxide emissions from transport activity. It can also be adapted to assess transport-related energy use where required.

This guidance is based on the Greenhouse gas reporting: conversion factors 2025 for carbon dioxide equivalentsitres, kg, kWh and CO2e/kWh). These factors are published annually.

The factors used must reflect the reporting year. For example, if your organisation is reporting for the 2020/21 financial year, it should use the 2020 greenhouse gas (GHG) conversion factors published by the Department for Business, Energy and Industrial Strategy. Complete datasets are available from 2014 onwards, and new datasets are published each June.

The methodology covers all vehicles up to 3.5 tonnes. Refer to Table 2, ‘GHG and kWh calculation methods’, for a detailed overview of the five methods used.

• Methods one and two are based on data about the actual fuel used or the actual vehicle used.
• Methods three to five are based on UK national fleet data combined with information about the vehicle used.

You can use methods three to five to estimate GHG emissions and energy use if you do not have specific data for the vehicle. However, any improvements made to vehicle efficiency or fleet composition will not be tracked accurately.

Method one: Fuel use

This method is the most accurate way to determine energy use and greenhouse gas (GHG) emissions. Refer to Table 3, ‘GHG factors’, for more information on GHG emissions and energy use from fuels.

If you operate a directly managed fleet vehicle, such as a light commercial vehicle (LCV) or heavy-duty vehicle (HDV), you should already have access to fuel consumption data. This data may also be available for company cars and lease cars through fuel cards, employee fuel benefits or mileage reimbursement schemes.

For opt-out, salary sacrifice and grey fleet vehicles, fuel data is not usually available. In these cases, you can use Method 5.

When accurate mileage data is available, fuel consumption can be used to measure energy efficiency. In the UK, this is commonly expressed as miles per gallon (mpg), but it can also be measured as miles/kWh, litres/100km or kWh/km. Using kWh provides a fuel-independent measure of efficiency regardless of whether the energy source is measured in litres, kilograms or kWh. Electric vehicles typically measure efficiency in watt-hours per mile (Wh/mile) or miles per kWh.

Method two: mileage and published g/km

If you do not have fuel consumption data, you can use the published greenhouse gas (GHG) emissions of the vehicle in gCO2/km. This provides the next most accurate estimate of energy use and GHG emissions. Emissions data has been available for all cars since 2001, for some cars since 1997, and for all vans since 2009.

You will need to submit a P11D form to HMRC for every employee who has received benefits in kind or expenses. Vehicle emissions data can be obtained from the DVLA through providers such as Carweb using the vehicle registration mark.

You must apply an uplift factor to emissions published by manufacturers based on WLTP tests. This is because these figures are often lower than real-world driving emissions.

The uplift factor reflects real-world vehicle use and accounts for variations in vehicle type, driving behaviour, traffic conditions and weather. For example, CO2 emissions in congested traffic or adverse weather conditions are typically higher than those recorded during manufacturer testing.

For cars, the gap between published emissions data and real-world emissions has increased over time. This has been reflected in the DBEIS/DESNZ GHG factors since 2014. The most accurate approach is to apply an age-related uplift based on DBEIS/DESNZ data. Refer to Table 4, ‘DBEIS/DESNZ real-world uplift by year of registration’, for more information.

The calculation is based on:

Published g/km × (1 + % uplift for year) × distance travelled (km) = gCO2e

The GHG fuel factor (see Table 2, ‘Overview of GHG and kWh calculation methods’) expressed as kgCO2e/kWh is then used to convert the calculated GHG emissions into kWh of energy consumed. You must also know the fuel type to make this calculation. Where the year of registration is unknown, an appropriate average uplift factor can be used.

Method three: mileage plus fuel and a size factor

You can use this method when you do not have published emissions data for your vehicles but do know the fuel type, engine size or vehicle size.

Method four: mileage plus fuel type

If you only know the fuel type, the following factors are used:

Method five: UK fleet average

If mileage data is the only information available, you can use average UK fleet data to estimate emissions and energy use. At present, separate national fleet datasets for England, Scotland and Wales are not available.