Every car has a BIK percentage band. This is based on CO2 emissions, and a P11D value, which is the list price, including extras and VAT, but without the first year registration fee and vehicle tax.
To calculate the BIK tax, multiply the P11D value by the BIK percentage banding, then multiply that figure by your tax band - i.e. 20% or 40%.
For a simpler way, use our Company Car Tax Calculator that will allow you to easily compare different cars against each other.