Beta This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

Company Taxation Manual

CTSA: quarterly instalments: amount due at each instalment: formula

You calculate the amount of tax to be paid at each instalment date by applying the formula:

3 x CTI


  • ‘CTI’ is the amount of the company’s total liability, as described in CTM92520, for the accounting period that is payable by instalments.
  • ‘n’ is the number of months in the accounting period. You calculate part months by dividing the number of spare days by 30 and expressing the sum as a decimal to two places, rounded arithmetically.