Rounding makes numbers easier to read, but done carelessly it introduces errors that compound. Significant figures are the companion idea that keeps a rounded number honest about how precise it really is.
The rounding rule
Look at the digit right after the place you are keeping. If it is 5 or more, round up; if it is 4 or less, round down. So 3.47 to one decimal is 3.5, and 3.44 is 3.4. The common tie-breaker for exactly 5 is “round half up,” though scientists sometimes “round half to even” to avoid bias.
What significant figures capture
Significant figures are the digits that carry real meaning. 0.00420 has three (4, 2, and the trailing 0), while leading zeros are just placeholders. Reporting a measurement as 4.20 rather than 4.2 signals you are confident to the hundredths place.
Why it matters
If you measure a length to the nearest centimeter, reporting it to the millimeter fakes a precision you do not have. And rounding only at the end of a calculation — never in the middle — prevents small errors from snowballing into a wrong final answer.
Frequently asked questions
How many decimals should I keep?
Match the precision of your least precise input. A result cannot be more accurate than the data it came from.
When should I round?
Only at the final step. Keep full precision through intermediate calculations.
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Results are general information only and not professional financial, medical, or legal advice.