When part of your transaction history is missing
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 8 min
In short
If part of your history cannot be retrieved, the answer is never to put in a number that seems about right. The practical approach is to work out a figure by a method you can defend from the material you still have, and to document that method and its basis. Which method is appropriate depends on the facts, so take it to a tax accountant.
Key points
- First confirm it is genuinely unavailable — support can often still retrieve it
- Pin down exactly what is missing: which account, which period, which record type
- Work out a figure by a method you can justify from the material that remains
- Document the method and its basis, and consult a tax accountant
Definition
The practice of arriving at a defensible cost basis or result from the remaining material when part of the record cannot be obtained, and preserving the reasoning.
First, establish that it really is unavailable. Data that cannot be exported from the screen can often still be produced by support, and some operators will provide history for a closed account after identity checks. Trade notification emails may still be in your inbox; an old spreadsheet may still exist. It takes time, but actual data beats any estimate. Exhaust these routes before giving up.
If gaps remain, define them precisely: which account, which dates, which type of record. 'Somewhere in there I don't know' gives you nothing to work with. Once the range is pinned down, the balances either side of it often narrow considerably what can have happened inside it.
Then lay out what you still have: bank statements showing yen movements; on-chain records, which do not disappear where self-custody was involved; notification emails; screenshots; the spreadsheet you kept at the time; the papers from a year you already filed. Fragments combined can outline the shape of the missing piece.
Now choose a method of estimation, and judge it by one test: can you explain it to someone else? You should be able to write out the assumptions and the arithmetic — 'from the yen deposited to this exchange over this period and the quantity held at the end of it, the cost basis was estimated as follows'. Putting in a convenient number with nothing behind it not only leaves you unable to explain it later, it produces an incorrect return. There is no version of this where an arbitrary figure is acceptable.
Finally, keep the method and its basis alongside the result: what was missing, what you tried in order to retrieve it, what material remained, what method you adopted and why. With that on file you can respond calmly if you are later asked. Which method is appropriate varies with the facts, and this site cannot give a general answer — where the amounts or the gaps are significant, agree the approach with a tax accountant before you file.
Watch out for
- · This page is a general orientation, not tax advice
- · Putting in an unsupported figure produces an incorrect return — never do it
- · Whether a method is appropriate is a case-by-case judgement; consult a tax accountant
Frequently asked questions
What happens if the cost basis genuinely cannot be established?
The rules set out how an unestablished cost basis is handled, and the result can be an assumption unfavourable to the taxpayer. Exactly how it applies depends on the facts, so rather than deciding yourself, read the National Tax Agency's guidance and consult a tax accountant.