Pi is an example of a dimensionless number. A dimensionless number doesn’t imply any measurement units. Pi is just 3.14, not 3.14 miles nor pounds nor cents. In contrast, **c**, the speed of light, must be stated in terms of miles per second or meters per second or other equivalent measurement units. The symbol **c** is not a dimensionless number.

Many dimensionless numbers are constants, like pi and **e** (2.718…). But many dimensionless numbers are not constants, for example, decibels (sound level) and pH (degree of acidity). The pH level can range from 0 to 14. We don’t say a “pH of 3 volts” or “3 dollars.” We just say that the pH is 3, period.

Dimensionless numbers are also called “dimensionless quantities,” “quantities of dimension one,” “bare numbers,” and “bare quantities.”