The rule
look at the first digit dropped: 5 or more rounds up
Why it works
Rounding replaces a number with a nearby simpler one. Which way to go is only genuinely ambiguous at exactly half, and the different rounding rules are precisely different answers to that one question.
How to do it by hand
- Decide how many decimals you want
- Look at the first digit being dropped
- 5 or more rounds up, less than 5 rounds down
- Banker's rounding sends exact halves to the nearest even digit
What is worth knowing
Banker's rounding exists because always rounding halves up introduces a systematic upward bias, and over millions of transactions that bias becomes real money. Sending halves to the nearest even digit cancels it out on average, which is why it is the IEEE 754 default and what most financial software uses. Truncating is different from rounding down for negatives: truncating −3.7 gives −3, while rounding down gives −4, and confusing the two is a common source of off-by-one errors in code.