Two calculators can both be right about the same 40 km trip and still differ by more than a factor of five. In the UK government's 2026 conversion factors, published on 11 June 2026, a medium petrol car emits 0.17411 kg CO2e per vehicle-kilometre and national rail 0.03092 kg CO2e per passenger-kilometre (the DESNZ conversion factors for 2026), so one 40 km leg is 6.96 kg for the whole car, 3.48 kg each if two people are in it, and 1.24 kg for one rail passenger. The condition that changes every one of those numbers is what is being counted: the vehicle or the person, one way or return, and whether the fuel supply chain is inside the boundary.
Five right answers to one journey
Each row below uses the 2026 factors (DESNZ) and the same 40 km, one way. The factors are for UK activities, so they do not describe a journey in another country.
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| What is counted | Factor used | Result |
|---|---|---|
| Whole car, tailpipe only | 0.17411 kg CO2e per vehicle-km | 6.96 kg |
| Car shared by two, tailpipe only | the same, divided by two | 3.48 kg each |
| Car shared by two, tailpipe plus fuel supply | 0.17411 + 0.04957 = 0.22368 per vehicle-km | 4.47 kg each |
| One rail passenger, direct emissions | 0.03092 kg CO2e per passenger-km | 1.24 kg |
| One rail passenger, direct plus fuel supply | 0.03092 + 0.00897 = 0.03989 per passenger-km | 1.60 kg |
The arithmetic: 40 × 0.17411 = 6.9644, and half of that is 3.4822. Then 40 × 0.22368 = 8.9472, and half is 4.4736. For rail, 40 × 0.03092 = 1.2368, and 40 × 0.03989 = 1.5956. Every figure in the table is defensible and none of them is the answer on its own.
Vehicle-kilometre or passenger-kilometre
This is the distinction that produces most of the disagreement. The car factors are per vehicle-kilometre: they describe the car, whoever is in it, and you divide by the occupants yourself. The rail and bus factors are already per passenger-kilometre, so dividing them again by the number of people travelling counts the same reduction twice.
Two people in a car halve the per-person figure and change nothing about the car. Adding a third occupant moves the 6.96 kg to 2.32 kg each and leaves the vehicle emitting 6.96 kg. Say which of those two sentences a number is answering before you compare it with anything.
What well to tank adds
An emissions factor converts an activity, such as distance travelled or kWh purchased, into estimated emissions. The 2026 set separates direct emissions from well-to-tank emissions, which cover extracting, refining and delivering the fuel, and publishes them as separate lines in the full spreadsheet (the conversion factors 2026 full set). For a medium petrol car the well-to-tank line adds 0.04957 kg CO2e per vehicle-km on top of 0.17411, a 28% increase; for national rail it adds 0.00897 on top of 0.03092, a 29% increase.
Do not add a stage yourself unless the method says it is missing. A calculator that already includes the fuel supply chain will quietly double-count when you bolt another factor on, and the resulting number is worse than either of the honest ones.
One way or return, and who is counted
Distance is the other half of the mismatch. The same rail leg is 1.24 kg one way and, over 80 km there and back, 80 × 0.03092 = 2.47 kg. A screen that shows 1.24 and a screen that shows 2.47 may be describing an identical journey.
Mode categories hide assumptions too. In the 2026 set an average local bus is 0.10151 kg CO2e per passenger-km, so the same 40 km is 4.06 kg by bus, while a coach at 0.03948 gives 1.58 kg (the full set) — both are "the bus" in ordinary speech, and they differ by a factor of two and a half. Match the category, the country and the year before you match the numbers.
What to write beside the number
Record six things with every estimate: the unit, the distance basis, the occupancy, the mode category and country, the stages included, and the version year of the factors. Give each person a column and compare notes; if two rows differ, the disagreement is now locatable instead of a matter of trust. That habit is the same one used in reading an environmental claim together, and the equivalent care with axes and baselines belongs to reading a chart before debating its headline.
Keep the scale of the thing in view as well. The EEA's 2025 assessment of Europe's mobility systems reports that transport accounted for almost a third of the EU's total greenhouse gas emissions in 2023, that passenger cars are around 72% of transport activity in passenger-kilometres, and that aviation and maritime transport are projected to rise from about 26% of Europe's transport emissions in 2023 to over 47% by 2050 (the EEA report). A per-trip figure is a tool for choosing between two options in front of you, which is what comparing one everyday journey door to door is for; it is not a measurement of anybody's share of that third.
Sources
- Greenhouse gas reporting: conversion factors 2026Accessed September 17, 2026
- Conversion factors 2026: full set (for all users)Accessed September 17, 2026
- Sustainability of Europe's mobility systems 2025Accessed September 17, 2026