Significant Figures Calculator
Round any number to a chosen number of significant figures, count the significant figures it already has, and see it in scientific notation.
The number rounded to the figures that actually mean something.
How the significant figures calculator works
Significant figures are the digits that carry measurement information: all non-zero digits, zeros between them, and trailing zeros after a decimal point — but not leading zeros. 0.004560 has four. Rounding to n figures keeps the first n significant digits and rounds the rest; the calculator also reports how many the number you typed has and writes it in scientific notation, where every digit shown is significant.
Formula: keep the first n significant digits; round the remainder
Worked examples
| Inputs | Rounded value | Note |
|---|---|---|
| 0.0045678 to 3 s.f. | 0.00457 | 0.00457 |
| 123456 to 2 s.f. | 120000 | 120000 |
| 9.99 to 2 s.f. | 10.0 | rounds up to 10 |
| 0.004560 — how many s.f.? | 0.004560 | four (the trailing zero counts) |
FAQFrequently asked questions
Do leading zeros count?
No — 0.0045 has two significant figures. They only place the decimal point.
Do trailing zeros count?
After a decimal point, yes (2.50 has three). In a whole number like 1200 it is ambiguous — write 1.2 × 10³ or 1.200 × 10³ to be clear.
How many figures should a result have?
For multiplication and division, as many as the least precise measurement; for addition and subtraction, round to the least precise decimal place.
Why does 9.99 to 2 s.f. become 10?
Rounding carries over: 9.99 → 10. — which has two significant figures written as 10. or 1.0 × 10¹.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.