In any measurement, the precision of the measuring tool is an essential factor. An ordinary ruler, for example, can measure length to the closest millimeter; a caliper, on the other hand, can measure length to the nearest 0.01 mm. As a result, the caliper is a more precise measurement tool because it can measure extremely minute changes in length. The measurements will be more accurate if the measuring tool is more precise.

It should be emphasized that when we represent measured values, the last digit has been calculated in some way by the person making the measurement. For example, a person measuring the length of a stick with a ruler notices that it appears to be between 13.2 cm and 13.3 cm, and so they must estimate the value of the last digit. The rule of significant figures is that the last digit written down in a measurement is the first digit with some uncertainty. Start with the first measured value on the left and count the number of digits through the last digit written on the right to determine the number of significant digits in a value. Significant figures represent the precision of the measuring tool used to measure a value.

The text is adopted from Openstax, University Physics Volume 1, Section: 1.6 Significant Figures.