Count the significant figures in any number and round it to a chosen number of sig figs — with the counting rules explained.
| Input | Result |
|---|---|
| 0.04560 | 4 sig figs; to 3 → 0.0456 |
| 1200 | 2 sig figs (ambiguous); to 2 → 1200 |
| 3.14159 | 6 sig figs; to 4 → 3.142 |
| 0.05 | 1 sig fig; to 2 → 0.050 |
The digits in a number that carry real measurement information. Leading zeros only position the decimal point and never count; trailing zeros count only when the number includes a decimal point.
Written plainly, you can't tell if the zeros are measured or placeholders — by convention we count 2. Scientists write 1.2×10³ (2 sig figs) or 1.200×10³ (4) to remove the doubt.
You round to a number of significant digits, not a place value: 3.14159 to 3 sig figs is 3.14, but 0.00314159 to 3 sig figs is 0.003142 — the decimal point moves, the precision doesn't.
Science labs, chemistry and physics homework, and engineering — anywhere a result shouldn't claim more precision than the measurements behind it.