The perimeter is the outer limit of a region that's bounded by line segments. The perimeter is the path around an area. The word comes from the Greek, where peri means "around" and meter means "measure."
The perimeter (P) equals the sum of all the sides (s).
P = s + s + s + s + ...
Square: Multiply one side by four.
Rectangle: Multiply the length by two, multiply the width by two, and then add the products.
Parallelogram: Multiply the length (long side) by two, multiply the short side by two, and then add the products.
Trapezoid: Add all four sides.
Triangle: Add all three sides.
Regular polygon: For regular polygons (polygons with equal sides), multiply the length of one side by the total number of sides. For example, multiply one side of a regular pentagon by five to determine the shape’s perimeter.
Irregular polygon: Add all the sides.
A circle doesn't have a perimeter - it has a circumference. The circumference, just like a perimeter, is the distance around the curved edge of a circle. The word comes from the Latin circumferentia, from circum ("around") and ferre ("to carry").
Circumference equals pi multiplied by the diameter.
Circumference = π × diameter
C = π × d
C = πd
C = 2πr