The distance around a shape
Perimeter is the total length of the boundary of a two-dimensional shape — the distance you would walk if you traced its outline. For a circle this boundary has a special name, the circumference. Perimeter is what you need for fencing a yard, framing a picture, edging a garden bed, or measuring skirting board, because those jobs follow the edge rather than filling the space inside.
How to use the perimeter calculator
Select a shape — circle, rectangle, triangle, or square — and enter the relevant sides or radius. The calculator adds up the boundary for you and returns the total length in your chosen unit.
The formulas
A circle's circumference is 2 × π × r (equivalently π × diameter). A rectangle is 2 × (length + width), because it has two of each side. A triangle is simply a + b + c, the sum of its three sides, and a square is 4 × side.
Perimeter versus area
It is easy to confuse the two: perimeter is a length (metres), while area is a surface (square metres). Fencing is priced by perimeter; turf is priced by area. Interestingly, for a fixed perimeter the shape that encloses the most area is the circle, which is why bubbles and water droplets are round — nature minimises the boundary for the space enclosed.