Significant digits or significant figures are the number of meaningful or important digits. Those that effect the accuracy of the number.

Rules

• All non-zero numbers count as a significant digit.
• All zeros between two non-zero numbers count as a significant digit.
• All zeros that are on the right of a decimal point and also to the left of a non-zero digit is never significant. 0.034 has only 2 significant digits.
• All zeros after the decimal point count as significant digits. 20.00 has 4 significant digits.
• All zeros right of the last non-zero number are only significant when in a measurement.