DEFRA road transport methodology by vehicle size



This methodology represents greenhouse gas emissions associated with the use of road vehicles as differentiated by their size. The data and calculation methodology originates from the UK government department DEFRA, as published in their most recent greenhouse gas (GHG) conversion factors documentation.

The methodology

The emissions methodology is based upon emissions factors which describe the rate at which road vehicles emit greenhouse gases according to distance travelled. These emissions factors represent emissions associated with entire vehicles, which can be contrasted with similar transport-related emissions factors which describe emissions on per-passenger or per-unit-of-freight bases. Users can nevertheless choose to 'share' vehicular emissions amongst passengers by specifying the vehicle occupancy.

Emissions - conventionally expressed in terms of mass (e.g. kg) - are calculated by simply multiplying these rates (mass emitted per distance; e.g. kg CO2 per km) by a distance travelled (e.g km).

Model data

The rate at which road vehicles emit greenhouse gases varies depending on factors such as the type of fuel used, and the fuel 'efficiency' of the vehicle (i.e. the distance acheived per unit of fuel consumed). Therefore, emissions factors for a broad range of generalised vehicle types are provided.

A total of 36 specific types of road vehicle are represented and are differentiated by their vehicle type (e.g. car, motorbike or van), their fuel type (e.g. petrol, diesel, or several others), and a measure of their size. In the case of petrol and diesel cars and motorbikes, 'size' represents engine size (in litres and cubic centimetres respectively), while vans are differentiated on the basis of their gross vehicle weight. Cars which run on compressed natural gas (CNG), liquefied petroleum gas (LPG), or hybrid or unknown energy sources are differentiated according to qualitative size categories (e.g. 'small', 'medium').

Each vehicle type is represented by six distinct emissions factors which differentiate greenhouse gas emissions into the following types:

  • direct CO2 emissions: carbon dioxide emissions produced during vehicle usage, i.e. fuel combustion
  • direct CH4 emissions: methane emissions produced during vehicle usage, i.e. fuel combustion
  • direct N2O emissions: nitrous oxide emissions produced during vehicle usage, i.e. fuel combustion
  • total direct emissions: all direct emissions, i.e. CO2 + CH4 + N2O
  • indirect emissions: emissions associated with stages in the fuel production chain such as raw material extraction and fuel delivery
  • total or 'life cycle': the total of direct and indirect emissions

Each of these emissions factors are expressed in terms of kg CO2e per km.

Activity data required

According to this methodology, greenhouse gas emissions are directly proportionate to distance travelled for a specified (optional) vehicular occupancy for a given type, fuel and size of vehicle.

Calculation and result

The returned emissions quantities for this methodology are inclusive of CO2, CH4 and N2O. The following discrete amounts are returned:

  • CO2: CO2 emissions
  • methaneCO2e: CH4 (methane) emissions
  • nitrousOxideCO2e: N2O (nitrous oxide) emissions
  • totalDirectCO2e: total direct emissions
  • indirectCO2e: Indirect emissions
  • lifeCycleCO2e: Full life cycle emissions

All quantities (including the separated CH4 and N2O emissions) are expressed in terms of CO2e - the quantity of CO2 which would exert the same atmospheric warming effect and the emissions quantity.

Related methodologies

Other DEFRA vehicle-based methodologies available represent cars differentiated by market segment and heavy goods vehicles. Methodologies oriented around passenger and freight transport are also available.


This methodology represents per vehicle emissions and therefore the returned emissions should be considered - by default - to represent those attributable to an entire single vehicle, or its sole occupant depending on the viewpoint of the user (the default value for vehicle occupancy is 1). If multiple vehicle occupancy is specified - by setting the occupancy profile item value to a positive value other than 1 - total vehicular emissions are shared between all occupants and therefore the returned emissions quantity represents the emissions attributable to each occupant.

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