In the Warm Up section you thought of informal ways to measure an object. One common informal measurement strategy is to count the number of body parts that make up a length. For example, you may have described the thickness of the tree trunk in terms of hand spans or the perimeter of a town in terms of the number of strides needed to walk all the way around. This informal style of measurement is useful when you don't have a measuring tool readily available. Relating SI or imperial measures to common objects will allow you to quickly estimate a measurement. Such objects of known length are called referents.

Referent

An object of known size that can be used to estimate measurements.


An example of a referent is a hand. Historically, hands have been used to measure the heights of horses. The breadth of a human hand is about four inches and a typical healthy horse is usually 15 to 17 hands in height.

Referents are rarely 100% accurate, but they can provide you with a pretty good estimation. They are very useful when a quick estimate is required and no measuring instrument is available. In the following Investigation, you will create referents for common measurements.