What Is an Exact Number? Definition and Examples

Exact Numbers, Significant Figures, and Uncertainty

There are exactly 2 apples in this image.
There are exactly 2 apples in this image. ballyscanlon, Getty Images

 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 example number cannot be simplified or reduced.

Examples of Exact Numbers

For example, these are exact numbers:

  • Number 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.