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Corporation Tax late filing penalty and interest calculator

Accounting period end date

The last day of the period the CT600 covers — for example, 30 4 2025.

Date the CT600 was, or will be, filed

For example, 15 8 2026.

In pounds. Leave blank or enter 0 if there's no tax — the late filing penalty still applies.

Date the tax was, or will be, paid (optional)
Was the company liable to a late filing penalty for each of its two previous accounting periods?

Answer yes only if the returns for both of the last two periods were late too, and neither penalty was cancelled. A third late return in a row costs more.

Do your accounts cover more than 12 months?
Last day of the accounts period (optional)

Only for accounts of 13 to 18 months, which need two CT600s: enter the last day the accounts cover. Both returns are then due 12 months after it.

For a company that doesn't pay Corporation Tax by quarterly instalments.

Example

Worked example

Period end: 30 April 2025; Filed on: 15 August 2026; Tax due: £10,000; Paid on: 1 August 2026; Late previous two: no. Enter your own details to replace this example.

For an accounting period ending 30 April 2025, the CT600 deadline is 30 April 2026. Filing on 15 August 2026 is more than 3 months late: a £400 penalty (doubled for deadlines from 1 April 2026). Interest on £10,000 paid 1 August 2026, 181 days after the 1 February 2026 due date, is £384.32. Total: £784.32.

CT600 filing deadline
30 April 2026
Return filed
15 August 2026 107 days late
Late filing penalty
£400 filed after 31 July 2026 (more than 3 months late), doubled amount (deadline on or after 1 April 2026)
Tax-geared penalty (10% or 20%)
£0 filed by 31 October 2026 (18 months after the period ends)
Corporation Tax payment due
1 February 2026
Late payment interest
£384.32 181 days on £10,000 to 1 August 2026
Interest at 7.75%, 2 February 2026 to 1 August 2026
£384.32 — 181 days
Total
£784.32
  • Late filing penalties apply even if there is no Corporation Tax to pay.
  • The deadline assumes HMRC sent its notice to file in good time. If the notice was served less than 3 months before the deadline, the company has 3 months from when it was served.
  • No flat-rate penalty is charged if the return is filed by the Companies House deadline for the same accounts and that deadline is later, for example after an extension (Finance Act 1998, Schedule 18, paragraph 19).
  • If you had a reasonable excuse you can appeal a late filing penalty, but you must file the return first.
  • If a return is still missing, HMRC can estimate the tax it thinks is due (a determination) and charge penalties on that. Filing the return replaces the estimate.
  • Interest is simple interest for each day after the due date, up to and including the day of payment, at the HMRC rate for that day. It uses a 365-day year: HMRC does not publish whether it divides by 366 in a leap year, so leap-year figures may differ by a few pence.
  • Interest is not a penalty, so a reasonable excuse does not remove it. You can object if you think HMRC has worked it out wrongly, and it is tax deductible.
  • It assumes the tax is paid in one go. Part payments reduce both the interest and the 10% or 20% penalty; they are not modelled here.
  • Large companies that pay Corporation Tax by quarterly instalments are charged interest differently and are not covered.

Sources: GOV.UK: Company Tax Returns — penalties for late filing GOV.UK: Increases to Corporation Tax late filing penalties (filing dates from 1 April 2026) Finance Act 1998, Schedule 18, paragraph 17 (flat-rate penalty) Finance Act 1998, Schedule 18, paragraph 18 (tax-related penalty) Finance Act 1998, Schedule 18, paragraph 14 (filing date) GOV.UK: HMRC interest rates for late and early payments GOV.UK: Pay your Corporation Tax bill (9 months and 1 day) GOV.UK: Corporation Tax interest charges

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