Perimeter is the basic math concept that you will use frequently when you study geometry. Sometimes, you will solve word problems that involve perimeter, and other times you may be asked to find the perimeter of the figures. Perimeter is measured using units of length. Listed below are the valid units for perimeter along with their common abbreviations:

- millimeter (mm)
- centimeter (cm)
- meter (m)
- kilometer (km)
- inch (in)
- foot (ft)
- yard (yd)
- mile (mi)

**Perimeter of a rectangle:**

A rectangle with length ℓ and width w has perimeter P given by P=2ℓ+2w.

**Perimeter of a square:**

A square with sides of length ℓ has perimeter P given by P=4ℓ.