Significant Figures Calculator

Significant figures (or significant digits) are the digits in a number that carry real, measured meaning. They include every non-zero digit, any zeros trapped between non-zero digits, and trailing zeros when a decimal point is present. Counting them tells you how precise a measurement is, which is why they matter in science and engineering.

Start at the first non-zero digit and count every digit to the right, including captive zeros. Leading zeros never count, so 0.0025 has 2 significant figures. Trailing zeros count only when a decimal point is shown: 1200 has 2 sig figs, but 1200.0 has 5. To force a specific count, write the number in scientific notation, for example 1.20 x 10^3 for 3 sig figs.

Keep that many significant digits starting from the first non-zero digit, then look at the next digit to decide rounding. If it is 5 or more, round the last kept digit up; if it is 4 or less, leave it. For example, 3.14159 rounded to 3 sig figs is 3.14, and 12345 rounded to 2 sig figs is 12000. This calculator rounds half away from zero and shows the scientific-notation form too.

For multiplication and division, the answer keeps the same number of significant figures as the operand with the fewest. So 2.5 (2 sig figs) times 3.42 (3 sig figs) gives 8.6, not 8.55. For addition and subtraction, the answer keeps the same number of decimal places as the operand with the fewest, so 12.11 plus 18.0 gives 30.1. Round only at the final step to avoid losing accuracy.

By the strict rule, no. A whole number with no decimal point, such as 1500, is treated as having 2 significant figures because the trailing zeros are ambiguous. To make them count, add a decimal point (1500.) for 4 sig figs, or use scientific notation like 1.500 x 10^3. This calculator follows that convention, so type a decimal point or use scientific notation when the zeros are measured.

They communicate how precise a value is and stop you from overstating accuracy. If you measure a length as 4.5 cm, reporting a calculation result as 4.531879 cm implies precision you never had. Following sig-fig rules keeps your reported results honest about the quality of the underlying measurements.