An "exact number" is a value that is known with complete certainty.

Examples of exact numbers are counted numbers of objects or certain unit conversions. For example, there are exactly 3 feet in 1 yard. There are exactly 12 eggs in a dozen. A class may contain exactly 25 students.

An exact number is considered to have an infinite number of significant figures. It does not limit the number of significant figures in a calculation. It does not contribute to uncertainty in a calculation.