How it works
Significant figures are the digits that carry meaning about a measurement's precision. The rules: all non-zero digits count; zeros between non-zero digits count; leading zeros never count; trailing zeros count only when there is a decimal point.
So 0.004560 has 4 significant figures (4, 5, 6 and the final 0), 1200 has 2, and 1200. (with a decimal point) has 4. In scientific notation only the digits of the coefficient count: 6.02 × 1023 has 3.
Formula
Count from the first non-zero digit. Include trailing zeros only if a decimal point is shown.
Questions people ask
How many significant figures does 100 have?
It is ambiguous: at least 1 and up to 3. Written as 100. or 1.00 × 10², it has 3. Without the decimal point most textbooks count 1.
Do leading zeros count?
No. In 0.0025 the zeros only place the decimal point, so it has 2 significant figures.
How many sig figs should an answer have?
For multiplication and division, use the fewest significant figures of any input. For addition and subtraction, use the fewest decimal places of any input.
Do exact numbers limit significant figures?
No. Counted quantities (12 eggs) and defined constants (100 cm in 1 m) are exact and have unlimited significant figures.