Significant Figures: The Rules That Actually Matter

Significant figures show how precisely a value is known. Getting them right keeps your answer honest — neither claiming more precision than your data supports nor throwing precision away.

Counting significant figures

• All non-zero digits are significant.
• Zeros between non-zero digits are significant: 1002 has four.
• Leading zeros are never significant: 0.0045 has two.
• Trailing zeros count only with a decimal point: 1500 has two, but 1500. and 1.500 × 10³ have four.
• In scientific notation, every digit in the mantissa is significant.

Rounding in calculations

There are two separate rules, and which one you use depends on the operation:

• Multiplication and division: the answer keeps the same number of significant figures as the input with the fewest. 4.56 × 1.4 = 6.4 (two sig figs).
• Addition and subtraction: the answer keeps the same number of decimal places as the input with the fewest. 12.11 + 1.1 = 13.2 (one decimal place).

Round only at the end. Carry extra digits through intermediate steps to avoid rounding error building up.

Exact numbers

Counting numbers and defined conversions (like 1000 mL = 1 L, or "2 mol" from a balanced equation) are exact — treat them as having infinite significant figures, so they never limit the answer.

Count sig figs and round any value on the Significant Figures Calculator.