There is no single road tax table. There are several systems running in parallel, and which one applies to a vehicle depends on the day it was first registered. That is why a calculator that asks only for the engine size gives the wrong answer for a modern car, and why the only reliable input is the registration itself, which resolves the date and the emissions figure together.
The date decides the system
Vehicles registered before the emissions based system began are taxed on engine size, in two bands split at a capacity threshold. Vehicles registered after it began and before the current system are taxed on their carbon dioxide figure, in a lettered band that stays with them for life. Vehicles registered under the current system pay a first year rate based on emissions and then a standard rate, with an additional rate for cars above a list price threshold for a number of years. One car, three possible regimes.
What the record gives a vehicle tax calculator
The DVLA vehicle enquiry returns the date of first registration, the carbon dioxide figure, the fuel type, the cylinder capacity and the taxation class. Between them those fields determine which system applies and which band inside it. That is why entering a registration is more reliable than entering a make and model: the model name does not determine the derivative, and the derivative determines the emissions figure that determines the band.
The things a calculator will get wrong
The additional rate for expensive cars depends on the list price when new including options, which is not on the vehicle record at all, so any calculator working from the registration alone can miss it. Alternative fuel vehicles have their own treatment. Historic vehicles above an age threshold are exempt, and so are vehicles registered with certain disability classes, and both of those are recorded as a taxation class rather than inferred. Read the class on the V5C before you trust an estimate.
Car tax band check: what a specific vehicle is paying now
The free DVLA enquiry returns the tax status, the expiry date and the band. That is the authoritative answer for a car you are looking at, and it is better than a calculation because it reflects whatever exemptions and classes actually apply. Use a calculator when you are comparing vehicles you do not yet own, and use the enquiry when the question is about a particular car with a particular registration.
Questions people ask about check road tax cost
Why does road tax cost differ between two identical looking cars?
Usually the first registration date, which decides which system applies, or the derivative, which decides the emissions figure. A facelift or a different engine can move a car between bands.
Can I calculate road tax from the registration alone?
You can check the band and the current status free from the DVLA enquiry. A full calculation may also need the list price when new, which is not on the vehicle record.
Are older cars exempt from vehicle tax?
Vehicles above a set age qualify for the historic class, which is an exemption applied by taxation class rather than automatically assumed. The V5C shows the class that applies.
Does the tax band change if I modify the car?
The band follows the emissions figure recorded at type approval, so ordinary modifications do not move it. Changing the fuel type or the engine is notifiable and can change the basis.