Add all side lengths of the polygon (regular or irregular).
For a rectangle: perimeter = 2 × (length + width).
For a square: perimeter = 4 × side length.
For a triangle: perimeter = side1 + side2 + side3.
For a parallelogram: perimeter = 2 × (base + side).
For a rhombus: perimeter = 4 × side length.
For a trapezoid: perimeter = top base + bottom base + leg1 + leg2.
For a circle: circumference = 2 × π × radius or π × diameter.
Use the given side lengths (or calculate missing sides) before adding.