Identify the shape and list the side lengths
Add all outer side lengths to get the perimeter
For a rectangle: use (P = 2(L + W))
For a square: use (P = 4s)
For a triangle: use (P = a + b + c)
For a parallelogram: use (P = 2(b + h)) or (P = 2(a + b))
For a trapezoid: use (P = a + b + c + d)
For a circle: use (P = 2pi r) or (P = pi d)
For a regular polygon with side length (s) and (n) sides: use (P = ns)
For an irregular polygon: add every boundary side length in order
For composite shapes: add the lengths of the outer edges only (exclude internal shared edges)