Free tool
Corporation Tax Deadline Calculator
Enter the company's accounting period end to see both Corporation Tax deadlines: the tax payment is due 9 months and 1 day after the period end, and the CT600 return is due 12 months after — three months later.
How the two deadlines work
- Corporation Tax payment is due 9 months and 1 day after the end of the accounting period.
- CT600 return is due 12 months after the end of the accounting period — three months after the payment.
- Large companies (profits over £1.5 million) pay in quarterly instalments rather than a single payment date.
- An accounting period longer than 12 months (common for a first set of accounts) requires two CT600 returns: one for the first 12 months and one for the remainder.
Source: GOV.UK Corporation Tax dates guidance. General information, not filing or tax advice. PenaltyProof does not file returns or pay tax for you.
Missing the CT600 deadline is expensive
For CT600 returns with a filing date on or after 1 April 2026, HMRC's flat penalty is £200 on day one and £400 at three months — rising to £1,000 and £2,000 for companies with three successive late returns. Tax-based penalties of 10% of the unpaid tax apply at six and twelve months, plus interest on late-paid tax from the day after the payment deadline.
CT600 penalty increase 2026 › or Corporation Tax return deadline guide ›Never miss a client's Corporation Tax deadline again
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