Walk around the boundary
Perimeter is the total length of the outside edge. Add every side once, using the same unit for each measurement. Do not add an internal diagonal.
Rectangle example
A rectangle 8 cm long and 3 cm wide has perimeter 8 + 3 + 8 + 3 = 22 cm. The shortcut is 2 × (length + width).
Check the unit
Perimeter is a length, so the answer is in cm, m or another length unit. Square centimeters belong to area, which measures the space inside.