Round a number to a chosen number of significant figures, and count those in the original. Enter the number and how many sig figs.
Usage Tip
In calculations, the result should carry no more significant figures than the least precise input.
Rounding keeps the most significant ones
Leading zeros never count; trailing zeros after a decimal point do.
The original significant-figure count is also shown.
Trailing zeros in whole numbers are treated as not significant here.
Significant figures show how precisely a number is known. Every non-zero digit counts, zeros between digits count, leading zeros never count, and trailing zeros count only when the number has a decimal point. A value written as 1500 is ambiguous, while 1.500 times ten to the third clearly has four significant figures.
When calculating, multiplication and division should be reported with the fewest significant figures of the inputs, while addition and subtraction follow the fewest decimal places. Round only at the end of a calculation, since rounding at every step lets small errors accumulate.
The number 0.004560 has four significant figures, because the leading zeros only place the decimal point while the 4, 5, 6, and trailing 0 all count. The number 12.00 also has four, while 1200 on its own is ambiguous. Rounding 3.14159 to three significant figures gives 3.14, and rounding 0.0004567 to two gives 0.00046.
Scientific notation removes the ambiguity, since 1.20 times ten to the third shows three significant figures and 1.2 times ten to the third shows only two. Exact counts and defined constants, such as 12 eggs in a dozen, have unlimited significant figures and should not limit the precision of a result.