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Geometry · Quick Reference
Perimeter Cheat Sheet
In one line: perimeter measures the total distance around the outside edge of a shape — simply the sum of every side's length.
Key Ideas
1Perimeter Definition. The total distance around the boundary of a two-dimensional shape.
2Perimeter of a Rectangle. P = 2*(length + width), since opposite sides are equal.
3Perimeter of a Square. P = 4*side, since all four sides are equal.
4Perimeter of a Triangle. P = sum of all three side lengths.
5Perimeter of Any Polygon. Simply add up the lengths of every side, regardless of the shape's regularity.
Worked Examples
Find the perimeter of a rectangle with length 7 and width 3.
20 units
Find the perimeter of a square with side 6.
24 units
Find the perimeter of a triangle with sides 5, 7, and 9.
21 units
Formulas
Rectangle: P = 2*(length + width)
Square: P = 4*side
Triangle/general polygon: P = sum of all sides
Practice Yourself
Find the perimeter of a rectangle with length 10 and width 4.
28
Find the perimeter of a square with side 8.
32
Find the perimeter of a triangle with sides 4, 5, 6.
15