This calculator works out three dates from your accounting period end: when your accounts are due at Companies House, when Corporation Tax must be paid, and when the Company Tax Return (CT600) is due. A private company files its accounts 9 months after the end of the accounting period and a public limited company 6 months after. Corporation Tax is due 9 months and 1 day after the period ends, and the CT600 is due 12 months after.
Quick answers
- Private company accounts: due at Companies House 9 months after the accounting reference date, under section 442 of the Companies Act 2006.
- Plc accounts: due 6 months after the accounting reference date.
- First accounts over 12 months: due 21 months from incorporation for a private company (18 months for a plc), or 3 months after the period end if that is later.
- Month-end rule: if the period ends on the last day of a month, the deadline is the last day of the relevant month, so a 30 April year end files by 31 January.
- Corporation Tax payment: due 9 months and 1 day after the end of the accounting period for most companies.
- CT600: the Company Tax Return is due 12 months after the end of the accounting period it covers.
How the calculator works
Enter the last day of the accounting period, the company type and whether these are the first accounts.
- Normal accounts. The calculator adds 9 months (private) or 6 months (plc) to the period end. It keeps the same day number, unless the period ends on a month end or that day does not exist in the target month, in which case it uses the last day of the month.
- First accounts. Add the incorporation date. If the first period is more than 12 months long, the deadline is the later of 21 months (plc 18 months) from incorporation and 3 months after the period end. If the first period is 12 months or less, the normal 9 or 6 months applies.
- Corporation Tax payment. 9 months after the period end, plus 1 day.
- CT600. 12 months after the period end.
Worked example
A private company has a year end of 31 March 2026 and is not filing its first accounts.
| Deadline | Rule | Date |
|---|---|---|
| Accounts at Companies House | 9 months, month-end rule | 31 December 2026 |
| Corporation Tax payment | 9 months and 1 day | 1 January 2027 |
| Company Tax Return (CT600) | 12 months | 31 March 2027 |
A second company was incorporated on 15 March 2025 and makes up its first accounts to 31 March 2026, a period of just over 12 months. 21 months from incorporation is 15 December 2026; 3 months after the period end is 30 June 2026. The later date applies, so the first accounts are due on 15 December 2026.
First accounts and other dates that catch directors out
First accounts are where most late filing penalties start, because the deadline runs from incorporation, not the year end.
- Count from incorporation. A company formed on 15 March has an accounting reference date of 31 March, but its first accounts are due on 15 December 21 months later, not on 31 December.
- Expect two tax returns. A first period longer than 12 months is split for Corporation Tax into a 12-month period and a short second period. Each has its own CT600 and payment date; the calculator shows the dates for the period end you enter.
- Plan for large company instalments. Companies with taxable profits over £1.5 million pay Corporation Tax in quarterly instalments instead of 9 months and 1 day after the year end.
- Do not leave it to the last day. Companies House says failing to deliver acceptable accounts on time is a criminal offence, and late filing penalties for a private company start at £150 for up to a month late.
If you will miss the date, apply before it passes: see how to extend your Companies House filing deadline. Our guide to company accounts filing deadlines covers changing your year end, and filing accounts after CATO closed explains the filing routes now available.
Sources
- Life of a company: annual requirements, part 1 accounts (Companies House)
- Companies Act 2006, section 442: period allowed for filing accounts (legislation.gov.uk)
- Pay your Corporation Tax bill (HMRC)
- Company Tax Returns (HMRC)
Frequently asked questions
When are accounts due for a 31 March year end?
For a private company, by 31 December of the same year. The Corporation Tax is due on 1 January and the CT600 on 31 March the following year.
How long do I have to file first accounts?
If the first period is more than 12 months, a private company has 21 months from incorporation, or 3 months from the period end if later. A plc has 18 months.
Is the Corporation Tax payment date the same as the filing date?
No. Corporation Tax is due 9 months and 1 day after the period end, which is 3 months before the CT600 is due.
Does a period ending on 30 June file on 30 March or 31 March?
31 March. When the period ends on the last day of a month, the deadline is the last day of the relevant month.
Do dormant companies have the same deadline?
Yes. A dormant company still files accounts at Companies House 9 months after its year end, even if it has nothing to report.