Handling decimals and rounding
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 5 min
In short
Crypto quantities carry many decimal places, and rounding partway through lets error accumulate. The practical rule is to keep quantities at full precision and round only at the final step. How figures are rounded on the return itself is addressed by the rules — check the National Tax Agency's guidance and a tax accountant.
Key points
- Do not round mid-calculation; round only at the end
- What a spreadsheet displays and what it holds are two different things
- Check that trailing digits survived the CSV import
- Rounding on the return itself follows the rules, not your preference
Definition
Setting a consistent policy for where and how to round, given crypto quantities with many decimals and the yen amounts derived from them.
Crypto quantities run to eight decimal places or finer, depending on the asset. Treat that lightly and round partway, and the error accumulates as transaction counts grow until your closing balance no longer matches what you actually hold. A mismatched balance then costs an enormous amount of time to trace, regardless of how small the discrepancy is.
The commonest cause is spreadsheet import. Opening a CSV can turn a quantity into scientific notation or round it to the displayed precision. What makes this awkward is that display and stored value do not always agree: sometimes the full value survives behind a rounded display, sometimes the value itself was lost on import. Importing the column as text avoids the whole class of problem.
The working principle is simple: hold quantities at full precision through the calculation and round exactly once, at the end. Never round an intermediate result and feed it into the next step. Following that alone prevents most reconciliation failures. If your tool exposes a setting for intermediate rounding, check it.
The same thinking applies to yen amounts, except that the return itself has prescribed rounding. What unit figures go in, and how sub-yen amounts are handled, are set out in the rules, so follow the National Tax Agency's guidance and the instructions in the return preparation screens. This site will not assert a specific treatment.
Finally, write your own policy down — something as brief as 'quantities held at the precision of the source CSV; yen values computed per transaction and handled at the total stage per the return's instructions'. Using the same policy next year keeps the cost basis that carries across years from drifting. If the treatment is unclear, ask a tax accountant.
Watch out for
- · This page is a general orientation, not tax advice
- · When a balance will not reconcile, do not nudge decimals to make it agree
- · Rounding on the return follows the rules — check National Tax Agency guidance
Frequently asked questions
What if a tiny quantity difference remains?
Look for the cause first — lost digits on import, an unrecorded fee, a mis-deduplicated row. There is usually a reason. Rather than adjusting it away unexplained, record what you investigated and take the treatment to a tax accountant.