Sig Fig Calculator
Count the significant figures in any number with every digit classified by rule, or round a number to 1–15 significant figures with exact string-based math.
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About this tool
This significant figures calculator does two jobs. In counter mode it takes any number and reports how many significant figures it has — but unlike a bare answer, it highlights every digit and states the rule that applies to it: nonzero digits are always significant, zeros trapped between significant digits (captive zeros) count, leading zeros never count, and trailing zeros count only when the number has a decimal point. In rounding mode it rounds any number to between 1 and 15 significant figures.
The classic trap is a whole number ending in zeros. Written as 1200, the two trailing zeros are ambiguous: by convention they are placeholders and the number has 2 significant figures, but if the value was actually measured to the ones place the true count would be 4. The calculator does not hide this — it reports the conventional count and shows the note, along with the scientific notation (1.200 × 10³) that would make the intended precision explicit. Compare 0.004560: the three leading zeros never count, while the final zero after the 6 does, giving 4 significant figures.
Rounding is done on the digits you typed, never on a binary float, so the output keeps exactly the right digits: 3.14159 to 3 sig figs is 3.14, and 0.0019997 to 3 sig figs is 0.00200 — with the trailing zeros preserved, because at that precision they are significant. A checkbox switches the output to scientific notation, which is the only unambiguous way to write results like 990000.
Significant figures are about measurement precision, not decimal places — rounding 1234.5 to 3 sig figs gives 1230, while rounding it to 3 decimal places would leave it untouched. If what you actually need is rounding to a place value like tenths, hundredths or thousands, use the separate Rounding Calculator on this site; this tool is the one for lab reports, chemistry and physics homework, and anywhere precision is counted in figures.
Example. The number 0.00450 is classified as three significant figures.
Limit. Rounding is offered for 1 through 15 significant figures and will not invent digits past the supplied numeral.