CT600 Filing Deadline Calculator: When Is Your Return Due?
Your CT600 filing deadline and Corporation Tax payment deadline are different dates. Here's how to work out yours.
The Two Deadlines
| Obligation | Deadline | Consequence of Missing |
|---|---|---|
| Pay Corporation Tax | 9 months + 1 day after period end | Interest (7.75% per annum) |
| File CT600 | 12 months after period end | Penalties (£200 initial, then escalating) |
You should pay before filing — the payment deadline comes first.
Quick Calculator
| Period End | Payment Due | Filing Due |
|---|---|---|
| 31 March 2025 | 1 January 2026 | 31 March 2026 |
| 30 June 2025 | 1 April 2026 | 30 June 2026 |
| 30 September 2025 | 1 July 2026 | 30 September 2026 |
| 31 December 2025 | 1 October 2026 | 31 December 2026 |
| 31 March 2026 | 1 January 2027 | 31 March 2027 |
How to Calculate Your Deadline
Filing Deadline
12 months after your accounting period ends.
Period ends 31 March 2025 → File by 31 March 2026.
Payment Deadline
9 months and 1 day after your accounting period ends.
Period ends 31 March 2025 → Pay by 1 January 2026.
First Period (New Companies)
Your first accounting period starts on your incorporation date. Your first year-end defaults to the anniversary of the month-end after incorporation.
Example: Incorporated 15 June 2025 → First year-end 30 June 2026.
But you can change your year-end (once) to make it more convenient — many companies choose 31 March to align with the tax year.
Short Accounting Periods
If your accounting period is shorter than 12 months (e.g., first period or changing year-end), the deadlines are calculated from the period end date, not a full 12 months.
Example: Period 1 July 2025 to 31 December 2025 (6 months)
- Payment due: 1 October 2026
- Filing due: 31 December 2026
What If the Deadline Falls on a Weekend?
Both deadlines are fixed regardless of weekends or bank holidays. Online CT600 filing is available 24 hours a day, 7 days a week, so there is no statutory extension if the filing deadline falls on a weekend or bank holiday. For the payment deadline, HMRC requires funds to clear your account by the due date — plan ahead if making a bank transfer.
Late Filing Penalties
HMRC increased late filing penalties for returns with a filing deadline on or after 1 April 2026:
| When the CT600 is late | Penalty |
|---|---|
| Filed after deadline | £200 |
| Still outstanding at 3 months late | £200 |
| 3rd consecutive late return | £1,000 per penalty |
| Unpaid tax outstanding at 6 months late | +10% of unpaid tax |
| Unpaid tax outstanding at 12 months late | +10% of unpaid tax |
For returns with a filing deadline before 1 April 2026, the penalties were lower: £100 initial + £100 at 3 months (£500/£500 for the third consecutive late return). The interest charge on late payment is separate and applies from the payment due date.
HMRC Free Filing Service Has Closed
HMRC's free CT600 filing service closed on 31 March 2026. You now need commercial software to file your Corporation Tax return.
Taxpipe is £59 per filing — no subscription, no hidden fees.
Don't miss your deadline. File your CT600 with Taxpipe today.