The perimeter of a square, rectangle, triangle, circle, regular polygon or ellipse — with the formula shown and the triangle inequality checked.
perimeter
How to use this tool
Square, rectangle, triangle, circle, regular polygon or ellipse.
Only the fields for your shape are used; the rest are ignored.
The result plus the exact formula used, so you can check it by hand.
If the three sides cannot form a triangle, you get an explanation instead of a nonsense answer.
Formula & worked example
A 3-4-5 triangle has perimeter 12. A circle of radius 3 has perimeter 2π × 3 ≈ 18.8496. An ellipse with axes 5 and 3 has perimeter ≈ 25.527 by Ramanujan’s approximation.
Limitations — read this before you rely on it
Ellipse perimeters have no exact elementary formula — Ramanujan’s approximation is used and is accurate to well under 1% for ordinary ellipses. All lengths must share one unit; the answer is in that unit.
Frequently asked questions
The total distance around the outside of a shape — the length of fence you would need to enclose it.
The circumference: 2πr, or π × diameter.
If one side is longer than the other two combined, no triangle exists (the triangle inequality). Shorten the longest side.
No — there is no simple exact formula for an ellipse perimeter. Ramanujan’s approximation is extremely close for normal shapes.
No — convert everything to one unit first, or the answer will be wrong. The result is in whatever unit you entered.