Count significant figures and perform arithmetic with proper sig-fig rules. Color-codes significant vs non-significant digits.
Type a value and the tool counts its significant figures, coloring every digit that counts and dimming the ones that do not: leading zeros, ambiguous trailing zeros. Enter an arithmetic expression and it applies the two rules of measurement math. Multiplication and division round to the fewest significant figures; addition and subtraction round to the fewest decimal places. The thing most users get wrong is the second rule: 12.11 + 0.3 rounds to 12.4, not 12.41, because sums care about decimal places, never about significant-figure counts.
A concrete input and expected output from the current implementation.
Input
0.00430
Expected output
3 significant figures - the 4, the 3, and the trailing 0 count; the two leading zeros do not
The zeros before the 4 only fix the position of the decimal point, so they are placeholders, not measured digits. The trailing zero comes after a decimal point and is measured, so it counts, giving 3 significant figures in total.
The tool scans the digits left to right. Zeros before the first non-zero digit are placeholders and are marked non-significant. Every non-zero digit counts, zeros trapped between non-zero digits count, and trailing zeros that follow an explicit decimal point count. A trailing zero with no decimal point, as in 1500, is ambiguous, so the tool flags it rather than inventing a count.
The tool computes the exact result first, then rounds it. Products and quotients round to the same number of significant figures as the operand with the fewest. Sums and differences round to the fewest decimal places instead, because in a sum the coarsest operand fixes the last honest digit. Operands in scientific notation are handled the same way, since the exponent never affects the significant-figure count.
Because addition and subtraction round to the fewest decimal places, never to the fewest significant figures. The 0.3 is only known to the tenths place, so the sum cannot honestly report a hundredths digit and rounds to 12.4. The fewest-significant-figures rule applies to multiplication and division only.
Use scientific notation. 1.5 x 10^3 means 2 significant figures, 1.50 x 10^3 means 3, and 1.500 x 10^3 means 4. A bare 1500 carries none of that information, which is exactly why the tool flags trailing zeros that lack a decimal point instead of guessing at the count.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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