Rounding vs. Estimating: Is there really a difference?

By Sharri Zachary, RME Mathematics Coordinator

Rounding is a familiar estimation strategy because the numbers in a given problem are changed to make computation easier for the problem solver. For rounding “to be useful in estimation, [it] should be flexible and well understood conceptually” (Van De Walle, Karp, Bay-Williams, 2013). For problems that involve the operations, students can generally round as a strategy for estimating their answer. However, when presented with a problem that involves measurement, perhaps, there is in fact a difference in rounding and estimating.

Consider this example:
     What is the approximate length of this line segment in inches?

By inspection, a student may try to visualize an inch ruler or use the tip of their pointer finger to estimate the length of the line segment to be about 3 inches long.

When given an inch ruler, a student may measure the length of the line segment and find that the precise length is 2 ¾ inches but recognize that they are not required to provide an exact answer. Knowing this, they can take their exact measure and round their answer to the nearest inch, providing an answer of about 3 inches long.

A solution to this question can be offered using estimation that is based on prior knowledge (and experience) or rounding the precise answer to the nearest inch.

There are other forms of estimation, such as compatible numbers, that can be used to solve problems. Often, it has been observed that students are told that rounding and estimating are one in the same. However, a measurement example sheds some light on this situation. Perhaps, there is a difference.

Van De Walle, J.A., Karp, K.S., & Bay-Williams, J.M. (2013). Elementary and middle school mathematics: Teaching developmentally (8th ed.). Upper Saddle River, NJ: Pearson Education, Inc.