Tracking what your assets cost you
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 7 min
In short
Cost basis can only be captured accurately at the moment of purchase. Reconstructing it later means hunting down the price, quantity and fee for every acquisition. Which calculation methods apply and what documentation is needed are set by the rules — check the National Tax Agency's guidance and a tax professional.
Key points
- Capture date, quantity, total paid and fee at the moment of acquisition
- Swapping one crypto for another is an acquisition too — yen movements alone are not enough
- Assets you did not buy, such as airdrops and rewards, still need a recorded basis
- Which method applies, and whether an election must be filed, is set by rules — ask a tax professional
Definition
Keeping an ongoing record of what each holding cost you to acquire — the starting point for any gain or loss calculation.
A gain is the disposal amount minus what the asset cost you. The disposal side sits in exchange history; the cost side turns difficult as soon as several venues and several years are involved. And being unable to evidence a cost basis can force you onto an unfavourable assumption. This makes acquisition data the most valuable thing in your records.
Capture four fields at the moment of acquisition: date, quantity, total paid in yen terms, and the fee. Keep the fee separate because whether it forms part of the basis depends on the type of transaction. Even if you leave that judgement to a professional, they cannot make it without the underlying number.
The acquisitions people miss are the ones that involve no yen. Swap one coin for another and the coin you received was acquired at that moment. The same goes for staking rewards, airdrops, and tokens received as proceeds from an NFT sale. Skipping these because no cash left your bank account is how quantities stop reconciling later.
In practice a single sheet holding acquisitions only is the easiest thing to maintain. Kept apart from disposals, it lets you read the history of one asset in order. Once a year, reconcile it against the CSVs you exported and check that the quantities agree.
Whether the total average or moving average method applies, whether you may switch, and whether an election must be filed are all set by rules that get amended. This page does not assert what is permitted — confirm with the National Tax Agency's guidance and a tax professional.
Watch out for
- · This page is a general orientation, not tax advice
- · Whether a method may be chosen or changed, and any filing needed, is set by the rules — ask a tax professional
- · Without evidence of a cost basis you may be pushed onto an unfavourable assumption
Frequently asked questions
I cannot work out the cost basis of a purchase from years ago. What now?
Gather whatever survives — exchange history, bank records, confirmation emails from the time — and then take it to a tax professional. Do not simply pick a number yourself.