How it works
Significant figures are the digits in a number that carry meaning for precision — every non-zero digit, plus any zero sitting between them or confirmed by a decimal point. Leading zeros never count, and trailing zeros only count when a decimal point makes them unambiguous.
3456 → 3 sig figs → 3,460 (the 4th digit, 6, rounds the 5 up)
0.0025 has 2 significant figures (leading zeros don't count)
1.50 has 3 significant figures (the trailing zero is confirmed by the decimal point)
Worked example
Rounding 3,456 to 3 significant figures.
- Keep the first 3 significant digits: 3, 4, 5.
- Look at the next digit to decide rounding: 6 rounds the 5 up to 6.
3,456 rounds to 3,460 at 3 significant figures.
Common questions
Why is "150" ambiguous?
Without a decimal point, you can't tell whether the trailing zero was measured precisely or just happens to be zero — it could mean 2 or 3 significant figures. This calculator follows the common textbook default of treating a bare trailing zero as not significant (2 sig figs for "150"); write "150." or "1.50 × 10²" to make 3 sig figs explicit.
Do zeros between digits count?
Yes — "captive" zeros between two non-zero digits are always significant. 1,005 has 4 significant figures; the two zeros in the middle both count.
What happens if I round to more sig figs than the number has?
Rounding only ever removes precision, so asking for more significant figures than the number already provides just returns the number itself — you can't manufacture precision that isn't there.