The rule
truncate = drop every digit past the chosen position
Why it works
Truncating simply discards the extra digits. Unlike rounding it never looks at what it is throwing away, so it always moves the number towards zero, and it always loses at least as much as rounding would.
How to do it by hand
- Decide how many decimals to keep
- Delete everything after that point
- Do not adjust the last kept digit
- The result is always closer to zero than the original
What is worth knowing
Truncation has a systematic bias that rounding does not: it always moves towards zero, so an average of truncated values comes out low. That makes it the wrong choice for money in most contexts, and the right one wherever the convention demands it — tax withholding and interest accrual are often specified as truncated by law, precisely so the result is predictable rather than fair. The distinction matters most with negatives, where truncating and rounding down go opposite ways.