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HMRC internal manual

Company Taxation Manual

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

The amount of tax to be paid at each instalment date is:

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.

    Part months are calculated by dividing the number of spare days by 30 and expressing the sum as a decimal to two places, rounded arithmetically.