Keeping your own transaction log
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 5 min
In short
Alongside exchange history and tool exports, keep one log of your own. Six fields — date, asset, quantity, value, fee, venue — are enough to make later calculations solid. What documentation a return requires is a matter for the National Tax Agency's guidance and a tax professional.
Key points
- Six fields: date, asset, quantity, value at the time, fee, venue
- Written at the time it is accurate; reconstructed from memory it is not
- Log transfers between your own venues too, marked as moves rather than trades
- Your own log is the yardstick for checking what a calculation tool imported
Definition
A running list you maintain yourself, adding each trade, swap, transfer or reward as it happens.
The point of keeping your own log when the exchange already has one is that you can cross-check. Import a CSV into a calculation tool and compare the count and quantities against your log: mismatches expose missed rows and double-counted ones. With only one source of truth, there is nothing to check against.
Resist adding fields. Date (ideally with the time), asset, quantity, value in yen at that moment, fee, and where it happened. Those six let you redo almost any calculation. A single spreadsheet tab is plenty; elaborate systems do not survive contact with a busy month, and a log you stop maintaining is no log at all.
The entries people forget are the ones that are not trades: moving coins from your exchange account to your own wallet, consolidating onto a hardware device, depositing into a DeFi protocol. None of these are sales, but if they are absent from the log, a later reader sees the balance drop and reads it as one. A single note saying it was a move prevents that.
Write the entry immediately. Left to the end of the month, the exact swap amount and the value at the time are already gone. Adding one line before you close the exchange tab is the version of this habit that actually lasts. As for which documents are needed as support at filing time, that is set by the rules — confirm with the National Tax Agency's guidance and a tax professional.
Watch out for
- · Do not keep passwords or seed phrases in the same sheet as your log
- · This page is a general orientation, not tax advice
- · Required supporting documents are set by the rules — check with the National Tax Agency and a tax professional
Frequently asked questions
If I use a calculation tool, do I still need my own log?
Not mandatory, but useful as a check on what the tool imported — especially for cross-venue transfers and transaction types the tool does not handle, where nothing else would reveal the error.