How to Count Significant Figures

Significant figures are the digits in a number that carry real information about its precision. They tell a reader how confidently a measurement is known. A length recorded as 12.30 centimeters claims more precision than 12.3, because the trailing zero says the measurement was pinned down to the hundredths place.

Getting the count right matters most in science, engineering, and any field where measurements are combined. Reporting too many digits implies a precision you did not actually have, while reporting too few throws away information you legitimately captured.

All non-zero digits are always significant. Zeros are where people get tripped up. Zeros between non-zero digits, called captive zeros, always count: 1005 has four significant figures. Leading zeros never count; they only set the decimal place, so 0.0042 has just two significant figures.

Trailing zeros are the subtle case. In a number with a decimal point, trailing zeros count: 4.500 has four significant figures. In a whole number with no decimal point, trailing zeros are ambiguous. Writing 1500 could mean two, three, or four significant figures, which is exactly why scientific notation exists. Expressing it as 1.50 times ten cubed makes the three-figure precision explicit.

The Sig Fig Calculator counts the significant figures in any number and explains why each digit does or does not count, then lets you round to a precision you choose. It runs completely in your browser, so nothing you type is sent anywhere.

To round to a given number of significant figures, keep that many significant digits and look at the next digit to decide whether to round up or down. Rounding 3.14159 to three significant figures gives 3.14, while rounding 0.0027465 to two gives 0.0027. Remember that leading zeros still do not count toward the total.

When rounding a whole number down to fewer significant figures, you may need to add trailing zeros as placeholders and, ideally, switch to scientific notation to show which zeros are meaningful. This keeps your stated precision honest.

When multiplying or dividing, the result should carry as many significant figures as the least precise input. If you multiply 2.0 by 3.14159, the answer is limited to two significant figures, so you report 6.3. When adding or subtracting, you instead match the least precise decimal place rather than the count of significant figures.

A practical habit is to keep extra digits through the middle of a calculation and round only at the very end. Rounding too early lets small errors accumulate and can shift the final reported value.