CT600 deadline calculator

By Max Szolcek, founder of Dormant Return. Last updated 3 September 2026

Enter the date your accounting period ends and the calculator gives you the CT600 filing deadline, the Corporation Tax payment date and the Companies House accounts deadline. Every rule it uses is written out below, so the page is just as useful if you prefer to count the months yourself.

The rules the calculator applies

CT600 filing deadline: 12 months after the end of the accounting period

gov.uk states that the deadline for a Company Tax Return is 12 months after the end of the accounting period it covers. A period ending 31 March 2026 therefore has a filing deadline of 31 March 2027. Where a month has fewer days than the one you started from, the deadline falls on the last day of that month: a period ending 29 February 2028 is due by 28 February 2029.

The deadline applies whether or not any tax is owed. A dormant company that has been sent a notice to deliver a return must file by the same date as a trading one, and the late-filing penalties are the same: £200 the day after the deadline, another £200 three months later, and £1,000 each if the return is late three times in a row. See late CT600 penalties for dormant companies.

Corporation Tax payment: 9 months and 1 day after the end of the period

For companies with taxable profits up to £1.5 million, gov.uk says Corporation Tax must be paid 9 months and 1 day after the end of the accounting period. That date comes before the filing deadline, which surprises people: the tax is due three months before the return. A period ending 31 March 2026 has a payment date of 1 January 2027. A dormant company has nothing to pay, but the date still exists, and HMRC expects to be told that no payment is due, which is one of the things the return itself does.

Companies House accounts: 9 months after the accounting reference date, or 21 months from incorporation for first accounts

Companies House is a separate regulator with its own deadline. gov.uk states that annual accounts are due 9 months after the company's financial year ends, and a company's first accounts are due 21 months after the date it registered with Companies House. If you give the calculator an incorporation date and no separate start date, it treats the period as the first one and applies the 21-month rule. Dormant company accounts still have to be filed at Companies House even though HMRC gets a CT600 with its own set of accounts; the two do not talk to each other. See dormant for Companies House vs dormant for Corporation Tax.

Periods longer than 12 months mean two CT600s

A Corporation Tax accounting period cannot be longer than 12 months. gov.uk's guidance on accounting periods says that if your accounts cover more than 12 months you must file two returns. This catches most new companies: Companies House sets the first accounting reference date to the end of the month a year after incorporation, so the first set of accounts covers slightly more than a year, and HMRC expects one CT600 for the first 12 months and a second for the few remaining days. The calculator flags this whenever the period end is 12 months or more after the start date. The detail is in your first Company Tax Return: why HMRC expects two CT600s.

Confirmation statement: not tied to the accounting period

The calculator leaves this one out because it does not depend on your accounts. gov.uk says every company must file at least one confirmation statement every 12 months, and it can be filed up to 14 days after the review period ends. The review period ends 12 months after incorporation for the first statement, and 12 months after the date on the previous statement for later ones.

Deadline coming up for a dormant company? Dormant Return prepares the CT600 and iXBRL dormant accounts and files them straight to HMRC under your own Government Gateway sign-in. About five minutes, £5 one-off, refunded automatically if HMRC rejects the return.

Start your return