The carbon dioxide figure on a vehicle record is a regulatory number, not a reading. It comes from the type approval testing the model went through before it was sold, it is expressed in grams per kilometre, and it is the input the tax system uses to place a car in a band. Your own car has never been measured, and the figure will not change however you drive.
Where to read it
The DVLA vehicle enquiry returns the CO2 figure against the registration, and it is printed on the V5C as well. It sits alongside the taxation class and the cylinder capacity, and between them those three fields explain almost every vehicle tax question people ask. If a car is old enough to predate the emissions based system, the record may carry no figure at all, and the vehicle is taxed on engine size instead.
Which test produced the number
Cars approved under the older laboratory cycle and those approved under the newer, more realistic procedure carry figures produced by different tests, and the newer numbers are generally higher for the same car. That is a change in the measurement, not in the exhaust. When comparing two cars of different ages on their published figures, check that you are comparing like with like, because a model that looks worse on paper may simply have been tested more honestly.
What the figure decides
It sets the first year rate for a new car and, for cars registered since the emissions based system began, the band the vehicle sits in afterwards. It feeds benefit in kind calculations for company cars. It is used by some local clean air and low emission schemes, although several of those work on the Euro standard rather than the carbon figure, which is a different number with a different meaning. Read the scheme's own rules rather than assuming the CO2 band decides it.
Why a check sometimes returns nothing
A vehicle registered before the emissions based system, an older import whose approval paperwork did not carry a figure, and some specialist or rebuilt vehicles can all show an empty field. That is not an error in the lookup. Where there is no figure the tax class falls back to engine size, and the V5C will show which basis applies. If you need a number for a fleet or reporting purpose, the manufacturer's type approval documentation is the place it actually lives.
Questions people ask about co2 emission checker
Can I check CO2 emissions from a number plate?
Yes. The free DVLA vehicle enquiry returns the CO2 figure used for taxation, and the same figure is printed on the V5C registration certificate.
Is the CO2 figure a measurement of my car?
No. It is the type approval figure for the model and derivative, produced in laboratory testing before sale. Your individual car has never been measured.
Why do two sources give different CO2 numbers?
Usually because they come from different approval tests. The newer procedure produces higher figures than the older one for the same vehicle, so check which test each number came from.
Does a low CO2 figure mean I can enter a clean air zone free?
Not necessarily. Many schemes work on the Euro emissions standard rather than the carbon figure. Check the individual scheme's rules against your vehicle.