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. Most exact numbers are integers, but it's possible for a value to have a decimal point. An exact number cannot be simplified or reduced.
Examples of Exact Numbers
For example, these are exact numbers:
- Number of ounces in a pound
- Number of feet in a mile
- Number of centimeters in a meter
- Any counted number, such as the number of apples in a bag
Exact Numbers and Uncertainty
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.
While counted numbers are exact, any measured value contains inherent uncertainty. The uncertainty comes from the limit of the measuring device and the skill of the person performing the measurement.