Sig Fig Calculator

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.