**Practice Problem 7**

Calculate the length in inches of a piece of wood 1.245 feet long.

**Solution**

This problem is easy to set up.

It is not as easy to decide how many significant figures the final answer should have. The original measurement (1.254 feet) has four significant figures, but there seem to be only two significant figures in the number of inches in a foot. Thus, it might seem that the answer should contain only two significant figures.

We can clear up this confusion by remembering that only measurements involve error or uncertainty. Many unit factors arebased on definitions. For example, 1 foot is defined as exactly 12 inches. Unit factors based on definitions have an infinite number of significant figures. The answer to this problem therefore contains four significant figures.