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Self Assessment With Provisional Figures: Unknown Profit

General information, not tax, legal or financial advice. We are not HMRC. Check GOV.UK or ask a qualified adviser. Sources checked on 17 October 2026. Dates and amounts after that date are expected and may change. Examples use made-up numbers. This page contains no affiliate links.

Short answer: You still need to send your return by the deadline if you do not know your profit for the whole tax year. GOV.UK says you should work out what it is likely to be (known as "provisional figures"), include those, and tell HMRC that you have used provisional figures when you submit. When you find out the real figure, change your return; you have 12 months from the Self Assessment deadline to do so (for 2025-26, that is expected to run to 31 January 2028, our calculation from the rule).

When it can happen

GOV.UK's examples: your accounting period ends at a different time from the end of the tax year, your accounting period differs from your basis period, or you are waiting for a valuation.

What you need to do

Estimate the profit for the whole tax year as best you can.
Submit the return by the deadline: online returns are due by 11:59pm on 31 January 2027 for 2025-26 (paper returns were due by 31 October 2026, so for most people the online date is the relevant one).
Say on the return that provisional figures were used.
When the final figures are known, change the return within the 12 months.

Tax and interest

GOV.UK says that if more tax is due, you pay interest on the difference between your estimates and the final figures, worked out from the original due date for payment. If you have overpaid, you get interest paid to you. For the current interest rate, see HMRC's late payment interest.

Example (made-up numbers)

A freelance designer estimates £30,000 profit and pays tax on that by 31 January. In March she learns the real figure is £32,000. She amends the return and pays the extra tax, plus interest from 31 January.

What this page does not cover

How to calculate a reasonable estimate, and the accuracy standard HMRC expects, are not described on the page we read. A careless or deliberate error is a different matter: see penalties for errors.

FAQ

Can I wait until I know the figure to file? GOV.UK says you must file by the deadline; late filing has penalties: missed deadline.

Do I need a special form? The page says to tell HMRC you used provisional figures when you submit; it names no separate form.

What we could not verify

How HMRC wants the provisional figures flagged in each software product or on paper (not detailed on the page read).
The accuracy HMRC expects of an estimate.
Effects on payments on account when a return is amended (not read).

Sources (official)

GOV.UK, Self Assessment tax returns: Sending a return — https://www.gov.uk/self-assessment-tax-returns/sending-return (read in full, 17 October 2026)
GOV.UK, Self Assessment tax returns: Deadlines — https://www.gov.uk/self-assessment-tax-returns/deadlines (read in full, 17 October 2026)

Deadlines calendar → Penalty estimator →

Not tax advice. See the disclosure.