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HMRC internal manual

COTAX Manual

HM Revenue & Customs
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Returns / notices: notices and returns: computations

CT return form CT600 includes calculations of the amount of tax chargeable.

The company must also provide computations linking the entries in the return to the figures in the accounts where it is not readily apparent. Online filed computations must be in iXBRL (inline Extensible Business Reporting Language) format for APs ending after 31 March 2010. Accounts must be in iXBRL format, unless the return is for an unincorporated charity, club or society, in which case they may be in iXBRL or PDF format.

Computations normally include a detailed profit and loss account.

Companies may submit tax computations rounded to the nearest £1,000 provided their turnover exceeds certain limits. For more information see the Business Income Manual at BIM31047.

No rounding should take place on the return form itself, or in any arithmetic that precedes the entries made on that form. In practice, online validation prevents rounding.


  • COM130021 for a list of functions to use in particular situations
  • COM130012 for legislation relevant to this subject
  • COM130011 for a list of forms relevant to this subject.