Significant Figures Calculator — Sig Fig Counter and Rounder

Significant figures (sig figs) are the digits in a measured value that carry real information. This calculator counts them using the standard rules, marks which digits count, rounds a number to any number of significant figures — with both ordinary half-up rounding and banker's half-even rounding — and applies the sig-fig rules to the result of adding, subtracting, multiplying or dividing two measurements. Numbers are read as text, so trailing zeros such as the last 0 in 0.004050 are never lost.

How to use this tool

1. Type the number exactly as written, including any trailing zeros and decimal point.
2. Count mode shows how many figures are significant and brackets them.
3. Round mode rounds to the number of significant figures you choose.
4. Calculate mode takes two measurements and an operation and rounds the answer by the sig-fig rules.

Formula

Non-zero digits count · zeros between them count · leading zeros never count · trailing zeros count only with a decimal point

So 0.004050 has 4 significant figures (4, 0, 5, 0), 100.0 has 4 and 1500 is ambiguous — 2, 3 or 4 — unless written as 1500. or 1.500 × 10³. In scientific notation every digit of the mantissa counts. When multiplying or dividing, the answer keeps as many significant figures as the least precise input; when adding or subtracting, it keeps the fewest decimal places.

Example

Multiplying 2.5 × 3.42 gives 8.55 exactly, but 2.5 has only 2 significant figures, so the answer is 8.6. Adding 12.11 + 18.0 gives 30.11; 18.0 is known only to the tenths place, so the answer is 30.1.