Perimeter Calculator
Find the perimeter (or circumference) of any 2D shape — with the exact formula, substitution and step-by-step working shown.
Perimeter calculator
Step by step calculations
Choose a shape, enter measurements and press Calculate to see the step by step formula and working.
Perimeter Calculator Explained
Core concepts, classroom applications, and plain-English explanations for all audiences.
When would you actually use this?
You want to fence a garden that's 12 m long and 8 m wide. How much fencing do you buy? Or you're laying skirting board around a rectangular room — perimeter tells you exactly how many metres to order. Every time you measure around the outside of something, you're calculating perimeter.
Worked example
"A picture frame is needed for a print that is 42 cm wide and 59 cm tall. How much frame edging is required?"
Perimeter = 2 × (width + height)
= 2 × (42 + 59)
= 2 × 101 = 202 cm
Always add 10–15% extra for cuts and joins when buying materials.
Perimeter vs area
- → Perimeter is the distance around the outside — used for fencing, edging, framing.
- → Area is the space inside — used for carpets, paint, turf.
Circle tip
The perimeter of a circle is called its circumference: C = 2πr. A circular pond with radius 3 m has circumference 2 × π × 3 ≈ 18.85 m.
Perimeter — the essentials
What is perimeter?
The total distance around the outside of a shape. It is measured in linear units (cm, m, km). For a rectangle, P = 2l + 2w because opposite sides are equal — you travel along each side once as you go all the way around.
Why π for circles?
The circumference C = πd = 2πr. π is the ratio of circumference to diameter for ANY circle — always the same, no matter the size. π ≈ 3.14159… It is irrational: its decimal expansion never terminates or repeats.
Regular polygons
All sides equal, all angles equal. Interior angle = (n − 2) × 180 ÷ n. The more sides, the closer the shape approaches a circle. At infinity, a regular polygon becomes a circle (P → C = 2πr).
Perimeter in real life
Fencing a garden, measuring picture-frame lengths, marking out a running track. To tile around a pool you need the perimeter. Area tells you how much paint; perimeter tells you how much skirting board.