Sourced statutory deadline
Calculate the ordinary CT600 filing deadline from the Corporation Tax accounting-period end, after confirming the supported facts.
HMRC CTSA filing rules, Finance Act 1998 Schedule 18 paragraph 14. Last reviewed 28 September 2026. This is not the Companies House accounts deadline or the tax payment date.
Calculate a standard Company Tax Return filing date using HMRC's complete twelve-month rule, including February/leap-year boundaries. Exceptional or uncertain cases require review.
The ordinary deadline is the final day of the complete twelve-month period after the CT accounting-period end. The scope confirmation must also establish that no other statutory period or special rule gives a later date.
For a period ending 28 February 2027, that filing period runs from 1 March 2027 to 29 February 2028. HMRC CTM93040 explicitly confirms this leap-year outcome; generic corresponding-day addition would give the wrong date.
Rule summary
Finance Act 1998 Schedule 18 paragraph 14 selects the latest applicable period end: twelve months from the CT accounting-period end, three months from service of the notice, and the relevant period-of-account rule. That accounts rule is twelve months from its end if it lasts at most 18 months, or 30 months from its beginning if longer.
HMRC CTM93040 explains the ordinary twelve-month date: normally the corresponding numbered day/month next year; 28 February immediately before a leap year gives 29 February, and 29 February gives 28 February next year.
A Companies House accounts period is not necessarily one CT accounting period. In HMRC CTM93050's 15-month accounts example, the return for a CT period ending 31 December 2018 is due 31 March 2020 because the whole accounts period ends 31 March 2019. Do not apply this calculator's ordinary rule to that case.
If you cannot confirm the notice and period facts, select Unsure: the result is review required, without an invented filing date. A longer or exceptional period is not automatically overdue twelve months after the first CT period ends.
Worked examples
One CT accounting period and the whole period of account end on 4 April 2026; the notice-service rule does not give a later date and no exception applies.
The complete twelve-month filing period runs from 5 April 2026 through 4 April 2027. The standard CT600 filing deadline is 4 April 2027.
An ordinary supported CT period ends on 28 February 2027, immediately before the leap year 2028.
The filing period runs from 1 March 2027 through 29 February 2028. The deadline is 29 February 2028, not 28 February. This follows HMRC CTM93040's explicit February rule.
HMRC CTM93050 example 2 has a period ending 31 December 2018 and a notice served on 31 October 2019. Alternatively, the user does not know when the notice was served.
The official example's deadline is 31 January 2020 because the notice-service period ends later. Select No / unsure in this calculator: it returns review required, not 31 December 2019. Establish actual notice-service and accounts-period facts before determining the applicable date.
Confirm the actual CT period, whole accounts period and HMRC notice-service facts before relying on a date.
Track the filing date separately from the Corporation Tax payment date; if filing is late, use the reviewed CT600 penalty tool with the verified deadline.
Read the paired guide and clickable HMRC sources for the latest-of-periods rule and exceptions.
Read the guide
Read the supported rule, assumptions and worked examples behind this tool.
Assumptions and limits
This tool applies a published rule set directly and is designed for standard scenarios.
Reviewed on . Preserved source review from the accepted scoped statutory repair; this contract migration does not re-date that review.
Applies to: Current CTSA ordinary filing-period rules; Finance Act 1998 Schedule 18 paragraph 14.
Reviewed scope: One CT period of at most twelve months coinciding with the whole period of account; served notice does not give a later deadline; no extension/special date.
Reviewed model: ct600-filing-deadline-calculator: Current CTSA ordinary filing-date rule; Finance Act 1998 Schedule 18 paragraph 14; reviewed 28 September 2026
No. In the ordinary filing case, HMRC CTM93040 gives 29 February 2028 because the following year is leap.
Not from an end date alone. A long accounts period can contain multiple CT periods and the relevant accounts-period rule may give a later deadline. Select No / unsure and review the full facts.
Yes. The three-month period after actual service may end later than the ordinary twelve-month period. Missing or late notice facts are outside the calculator's supported case.