If measured with an ordinary student's ruler, the smallest unit is millimeters. When reading a book, what is above millimeter is accurate, and what is below millimeter is estimated by naked eyes. For example, the measurement result is 28.6 mm, and the final estimate is 0.6 mm But if centimeters are needed, it is expressed as 2.86 centimeters.

More accurately, when measuring the length, it should be accurate to one place behind the smallest measuring unit of the measuring tool.