In a measurement context, any digit that is certain and necessary to define this measurement, or an uncertain digit evaluated by the person who is taking the measurement.

- The
**most significant digit**is the first digit of a number, if this digit is different from zero. - The
**least significant digit**is the one (generally the last one) which is the most difficult to determine. This digit usually depends on the degree of precision of the measuring device used, the physical conditions of the measurement, or the care taken when reading the data or for the work technique used. - The
**number of significant digits**in a measurement or a calculation result is determined based on the context of this measurement or calculation. It consists of all digits from 1 to 9 forming a decimal number. By convention, the digit zero is not considered significant if it is the first digit of the number under consideration. - In the context of a measurement, the
**certain digits**are the digits with values that are universally accepted. The quality and precision of the measuring tool allow us to determine the number of certain digits. **Necessary digits**are digits that are required when taking the measurement.

### Examples

- 1.257 includes 4 significant digits
- 5.02 includes 3 significant digits
- 9.08100 includes 6 significant digits
- 0.091 0 includes 3 significant digits
- 3.12 x 10³ is a number that includes 3 significant digits.