What Is an Exact Number?

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.

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.

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Helmenstine, Anne Marie, Ph.D. "What Is an Exact Number?" ThoughtCo, Mar. 15, 2017, thoughtco.com/exact-number-chemistry-definition-609327. Helmenstine, Anne Marie, Ph.D. (2017, March 15). 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 January 21, 2018).