Perimeter Calculator
Find the perimeter of 13 shapes, from rectangles, triangles and circles to sectors, ellipses, trapezoids and regular polygons, with the area too.
Perimeter is the total length around the edge of a flat shape: the fencing around a yard, the trim around a window. Pick one of 13 shapes, enter its measurements in a single unit, and the calculator adds up the sides (or the curved edge). It also gives the area whenever the measurements you entered are enough to fix it.
How to use this tool
- Choose the shape. The list shows what a, b, c and d stand for with that shape.
- Enter the measurements, all in the same unit. Boxes the shape does not use are ignored.
- Pick the unit so the answer is labelled; it does not change the numbers.
- Read the perimeter, the formula with your numbers filled in, and the area where available.
Formula
Rectangle P = 2(a + b) · Triangle P = a + b + c · Circle C = 2πr · Regular polygon P = n × a
A semicircle is the curved half (πr) plus the diameter (2r); a sector is the arc (πrθ/180) plus two radii. A rhombus is found from its diagonals, since each side is √(a² + b²)/2. An ellipse perimeter has no simple closed formula, so it is computed with the arithmetic–geometric mean series, which is accurate to every digit shown.
Example
A rectangular garden bed is 8 m long and 5 m wide. P = 2(8 + 5) = 26 m of edging, and the area is 8 × 5 = 40 m². A triangular plot with sides of 3, 4 and 5 m has P = 3 + 4 + 5 = 12 m, and Heron’s formula gives its area as 6 m².
Frequently asked questions
What is the difference between perimeter and circumference?
Circumference is the name for the perimeter of a circle (2πr, or π times the diameter). For every other shape the word perimeter is used.
Why is there no area for a parallelogram or a kite?
Side lengths alone do not fix those shapes: the same sides can lean at different angles and enclose different areas. You also need a height, an angle or the diagonals. A trapezoid is different: once you say which two sides are parallel, the four sides fix the height, so the area is shown.
How do I find the perimeter of an L-shaped room?
Add the lengths of all the outside walls. For any L shape the total equals the perimeter of the rectangle that encloses it, 2 × (overall length + overall width), so two missing wall lengths do not matter.
Do any three lengths make a triangle?
Only if each side is shorter than the other two combined (the triangle inequality). The calculator warns you when they are not.
Last updated: 2026-10-30