HMRC has a word for the figure you file when the records are gone for good, and it is not the one most people use. Get it wrong and you tick a box that invites an enquiry you never needed.
GOV.UK puts it in a line: “‘Provisional’ means you'll be able to get paperwork to confirm your figures later. ‘Estimated’ means you will not be able to confirm the figures.” Its Self Assessment manual agrees: an estimate is one the taxpayer wishes accepted as final “because it is not possible to provide an accurate figure for example where the records have been lost”.
A dead exchange and a lost CSV is an estimate, and the 2026 return notes are blunt: “Do not put ‘X’ in box 20 if you've used estimated figures, but tell us in the ‘Any other information’ box why you have.” Tick it and you have promised HMRC final figures you cannot produce.
Where the disclosure goes
The box that counts for a capital gain is the one least often used: box 53, ticked “if your computations include any estimates or valuations”. Its notes then require you to say where and why you estimated and “how you worked out these amounts” — either in box 54 or in your computation. Follow only the general GOV.UK page and you file an unflagged computation no caseworker will see marked.
This is not tidiness. The Compliance Handbook's definition of careless carries the sentence everything turns on: a person still unsure after taking care “should draw attention to the entry and the uncertainty when they send the return”, and then “will have taken reasonable care… and if they are wrong they will not have been carelessly so”. Where an inaccuracy is made despite reasonable care, “no penalty will be due”.
The penalty is the price of silence, not the price of being wrong.
The limit is equally clear. HMRC's own example of carelessness is someone who makes “no attempt to check the balances” and “just estimates the amounts”. The test is not whether your number is right, but whether you tried for the real one first.
What an unevidenced cost is worth
There is no authority for the thing everyone fears. Nothing in the Cryptoassets Manual, the Capital Gains Manual, the Compliance Handbook or the Taxation of Chargeable Gains Act 1992 treats a cost you cannot evidence as nil; the market-value substitutions that exist turn on the nature of the transaction, never a missing receipt.
It also matters less than the panic suggests. Take a pool of six units costing £88,777, of which £9,720 rests on a bank debit and a reconstructed price rather than a receipt, and sell two for £138,000 in December 2025 on a £46,000 salary.
Taxable income after the £12,570 personal allowance is £33,430, leaving £4,270 of basic rate band below the £37,700 limit. Pooled cost is £14,796.17 a unit, so the gain is £108,407.67; after the £3,000 allowance, £4,270 falls at 18 per cent and £101,137.67 at 24, giving £25,041.64.
Strike the £9,720 out and the tax becomes £25,819.24 — £777.60 more, or 3.11 per cent. The reason is pooling: the disallowed cost spreads across all six units and only two were sold, so £3,240 of extra gain surfaces now and £6,480 stays against the four still held. On that £777.60 a careless inaccuracy at the prompted minimum of 15 per cent is £116.64, and disclosed in box 54 or the computation the penalty line disappears.
What the rules cost, and what they allow
In your favour: the guidance contemplates estimates, the valuation page asks only for “a consistent methodology” whose details “should be kept”, and the statute conditions the deduction on what you gave, not what you can prove. Against you: HMRC publishes no method for rebuilding a cost basis, and interest runs regardless of care. Registration closed today; the return is due by 31 January 2027.
One last thing to strike out: moving coins between wallets you control is not a disposal. It is the commonest reconstruction error, and it inflates the gain before any estimate is made.
| Pool, six acquisitions | 6.00 units, £88,777 |
| Of which unevidenced | £9,720 |
| Gain, reconstructed costs accepted | £108,407.67 |
| £4,270 at 18%, £101,137.67 at 24% | £25,041.64 due |
| CGT if the £9,720 is disallowed | £25,819.24 |
| Cost of the nil-cost outcome | £777.60 (3.11%) |
| Penalty at 15%, interest at 7.75% | £116.64, £60.26 |
Retrieved from GOV.UK, HMRC and legislation.gov.uk on 5 October 2026. The annual exempt amount and the 18 and 24 per cent rates are from HMRC's Capital Gains Tax rates and allowances guidance, last updated 13 April 2026, which gives £3,000 for 2024/25, 2025/26 and 2026/27 alike; the personal allowance and £37,700 basic rate limit from GOV.UK income tax rates; late payment interest of 7.75 per cent effective 9 January 2026 from HMRC's published rates. The estimated and provisional distinction is from GOV.UK “Keeping your pay and tax records” and Self Assessment manual SAM121190; box wording from the SA100 and SA108 for 2025/26 and their notes; reasonable care from Compliance Handbook CH81120, CH81130 and CH81145, whose carelessness example is drawn from an inheritance tax account; valuation from CRYPTO23000; pooling from CRYPTO22200 and sections 38 and 104 of the Taxation of Chargeable Gains Act 1992. Unverified: HMRC manual pages show a manual-wide update date, not a page date, so no claim is made about when any page changed. HMRC publishes no method for rebuilding a cost basis, and no authority treats an unevidenced cost as nil. All arithmetic was computed for this article. General information, not tax advice. Not a recommendation to buy, sell or hold anything.