Significant Figures Calculator

All non-zero digits are significant, zeros sitting between non-zero digits are significant, and trailing zeros after a decimal point are significant. Leading zeros, such as those in 0.0045, are never significant and are only there to place the decimal point.

It reads the number in exponential form and counts the mantissa digits. Exponential form drops leading zeros and any insignificant trailing zeros automatically, so 12345 shows as 1.2345 and counts as 5, while 0.0045 shows as 4.5 and counts as 2.

The input is read as a numeric value, so a figure like 0.004560 is stored as 0.00456 and the final zero is lost, giving 3 significant figures. Trailing zeros on a whole number, such as 1500, are genuinely ambiguous, and this tool treats them as not significant, so 1500 counts as 2.

The calculator locates the place value of the leading digit, then rounds so exactly the requested number of significant digits remain. Rounding 12345 to 3 sig figs finds the digit in the ten-thousands place and produces 12300; rounding 0.0789 to 2 gives 0.079.

The rounded value is shown as a string with exactly the requested number of significant digits, so trailing zeros are kept to make the precision explicit. Rounding 2 to 3 sig figs is shown as 2.00, which clearly signals three significant figures rather than one.

Rounding is standard nearest-value rounding, with a digit of exactly 5 pushed up to the next value. Note that because decimals are stored in binary, a number that looks like an exact half may be a hair above or below, so the result can occasionally surprise you at the boundary.

Zero is a special case with no non-zero digit to anchor the count. By the common convention used here, this calculator reports zero as having 1 significant figure, and rounding zero returns zero.

Yes. The minus sign is ignored for counting, so -12345 has the same 5 significant figures as 12345. You can type input in scientific form like 4.56e-3, and very large or tiny rounded results are collapsed back into plain decimal notation where that is reasonable.

No. The counting and rounding run entirely in your browser. Nothing you enter is uploaded or stored, so the tool works offline once the page has loaded.