What Is an Exact Number?

There are exactly two apples in this image

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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.

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Helmenstine, Anne Marie, Ph.D. "What Is an Exact Number?" ThoughtCo, Aug. 25, 2020, thoughtco.com/exact-number-chemistry-definition-609327. Helmenstine, Anne Marie, Ph.D. (2020, August 25). What Is an Exact Number? Retrieved from https://www.thoughtco.com/exact-number-chemistry-definition-609327 Helmenstine, Anne Marie, Ph.D. "What Is an Exact Number?" ThoughtCo. https://www.thoughtco.com/exact-number-chemistry-definition-609327 (accessed September 21, 2021).