Significant Figures
The digits in a measured value that carry meaningful precision, comprising all certain digits plus one estimated digit.
Unit
Dimensionless
In depth
Leading zeros never count, captive zeros always do, and trailing zeros count only when a decimal point is present. In multiplication and division the answer takes the fewest significant figures of any input; in addition and subtraction it takes the fewest decimal places. Exact numbers and defined constants have unlimited precision.
Examples in the real world
0.00420 has three significant figures; 25.0 mL × 1.2 M gives 30 mol, reported to two significant figures.