Part of our complete guide to Corporation Tax.
The deadline that comes before filing
Corporation tax is due 9 months and 1 day after the end of your company's accounting period, which is earlier than the 12-month deadline for actually filing the Corporation Tax return itself. This surprises a lot of new directors, you must calculate and pay what you owe based on your own figures, before the formal return documenting those figures is even submitted.
Verify before relying on this: the 9-month-and-1-day payment deadline is widely cited but hasn't been freshly re-confirmed against gov.uk at the time of writing. Check your specific accounting period with us.
Why the two deadlines are different
It feels backwards, pay first, file later, but that's simply how the rules are structured. The logic is that HMRC wants the cash sooner than it needs the full, finalised paperwork. In practice this means your accountant needs to finalise your numbers well ahead of the 12-month filing deadline, in time to calculate and pay by the earlier 9-month-and-1-day point.
What if the estimate is slightly wrong
You pay based on your best calculation at the time of payment. If the figures in your eventually-filed return differ slightly, you either pay the shortfall (with interest backdated to the original due date) or reclaim any overpayment. This is a normal part of the process, not a sign something went wrong, but it's another reason to get your bookkeeping current well before the deadline rather than estimating in the dark.
Use our calculator to know the number now
Our corporation tax calculator applies the current rates and marginal relief automatically, see our guide on corporation tax rates for 2026/27, so you can get a reliable figure well ahead of the payment deadline, not at the last minute.
Want your deadline tracked for you? Book a free consultation, our limited company accounts service keeps your corporation tax payment and filing deadlines on separate, correctly-tracked clocks.
Frequently asked questions
When is corporation tax actually due?
9 months and 1 day after your company's accounting period ends, which is earlier than the 12-month deadline for filing your Corporation Tax return itself.
Why would I pay before filing the actual return?
The payment deadline and filing deadline are simply different dates set in the rules, payment is due sooner (9 months and 1 day) than the return itself (12 months), so you must calculate and pay based on your figures before the formal return is submitted.
What if my profit estimate turns out to be wrong?
You pay based on your best calculation at the time, if the actual figure in your filed return differs, you pay any shortfall (with interest from the original due date) or claim back any overpayment.
Do large companies have different payment rules?
Yes, companies with profits above certain large thresholds must pay corporation tax in quarterly instalments rather than a single payment, this affects very few small businesses but is worth knowing about if you're growing quickly.