A company car tax checker is only as good as its inputs, and there are five: the P11D value, the CO2 band and electric range, the fuel type and whether a diesel meets RDE2, the driver's marginal rate, and the months the car is available. Give it those and HMRC's method produces one figure, the cash equivalent, from which the driver's tax and the employer's Class 1A follow. This page is written for the driver checking a figure they have been given, on a quote, on a P11D or in a tax code: where each input is found on the paperwork, what a checker should show for the method, and what to do when the figure does not match. The company car tax calculator on this site is the checker.
Where the five inputs are
The P11D value is the list price when first registered plus factory options, on the manufacturer's price list or the P11D itself, not the invoice. The CO2 figure and, for a plug-in hybrid, the electric range are on the V5C and the certificate of conformity. The fuel type and RDE2 status are on the V5C. The marginal rate is the driver's own, from their salary and the benefit together. The months available are the dates the car was delivered and returned.
What a checker should show
The appropriate percentage it applied and the band it read it from; the cash equivalent before and after the reduction for months unavailable and private-use payments; the tax at the driver's rate per year and per month; and the Class 1A for the employer. A checker that shows a monthly figure and nothing else cannot be checked against the P11D, which is where the figure has to agree.
Checking a tax code
The benefit appears in the code as a deduction from the personal allowance equal to the cash equivalent, so a code that has fallen by roughly the cash equivalent is right. A code that has fallen by more may include last year's underpayment or private fuel; one that has fallen by less may be waiting for the P11D. The driver can see the figure HMRC holds in their personal tax account and ask for it to be corrected.
When the figure does not match
The usual causes are a P11D value taken from the invoice rather than the list price, a diesel taxed without the RDE2 check, a hybrid's electric range read from the wrong document, or a car available for part of the year taxed for the whole of it. Rework it with the right inputs on the calculator on this site, and take the corrected figure to payroll for the P11D.
Questions people ask about company car tax checker
Can I check company car tax before I accept the car?
Yes. The P11D value and CO2 figure are on the quote or the manufacturer's specification, and the calculator on this site works the figure from those with your rate.
Why does my checker figure differ from HMRC's?
Most often the P11D value or the band. Check the list price against the manufacturer's list for the date of first registration and the CO2 figure against the V5C.
Does the checker work for a car provided for part of the year?
Yes: enter the months available and the cash equivalent is reduced in proportion, which is the sheet's approximation of HMRC's thirty-day unavailability rule.