Perimeter Calculator — Circle, Rectangle, Triangle, Polygon
Choose a 2-D shape — circle, square, rectangle, general triangle, equilateral triangle or regular polygon — enter the dimensions and get the perimeter instantly with the formula and a labelled diagram.
Shape
2(l + w) = 2(8 + 5)
- 1
Length + Width
8 + 5 = 13 - 2
Perimeter = 2(l + w)
2 × 13 = 26
How does this calculator work?
Perimeter = total outer boundary length. Rectangle: 2(l + w). Circle: 2πr. Square: 4s. Triangle: a + b + c. Regular n-gon: n × s. Select the shape, enter its dimensions, and the calculator applies the correct formula and shows a labelled diagram.
Formula
How this is calculated
The perimeter is the total length of the boundary of a flat shape. For straight-sided shapes it is simply the sum of all the side lengths. A circle's boundary (its circumference) is calculated as 2πr, where r is the radius and π ≈ 3.14159. A square has four equal sides so P = 4s; a rectangle has two pairs of equal sides giving P = 2(l + w). A general triangle has three possibly unequal sides a + b + c, while an equilateral triangle has three equal sides: P = 3s.
A regular polygon with n sides, each of length s, has perimeter P = n × s. For a triangle, the three sides must satisfy the triangle inequality: each side must be strictly less than the sum of the other two — otherwise no real triangle exists with those measurements.
Perimeter differs from area, which measures the space inside the boundary. Both are useful in different contexts: perimeter for fencing, framing or trimming around an edge; area for flooring, painting or filling a region.
Frequently asked questions
They refer to the same concept — the total boundary length — but circumference is the specific term for circles. Perimeter is the general term for any closed 2-D shape. The circumference of a circle is 2πr.
For three lengths to form a valid triangle, each side must be less than the sum of the other two. If any side equals or exceeds that sum, the three segments can't close into a triangle — they either lie flat or leave a gap.
Add up the lengths of all its sides. This calculator covers regular polygons (all sides equal) and general triangles. For other irregular shapes, measure or look up each side length and sum them manually.
Also known as
TG we-Calculate Editorial Team. (2026). Perimeter Calculator — Circle, Rectangle, Triangle, Polygon [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/perimeter-calculator
TG we-Calculate Editorial Team. "Perimeter Calculator — Circle, Rectangle, Triangle, Polygon." TG we-Calculate. 2026. https://we-calculate.com/calculator/perimeter-calculator.
TG we-Calculate Editorial Team, "Perimeter Calculator — Circle, Rectangle, Triangle, Polygon," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/perimeter-calculator
@misc{wecalculate_perimeter_calculator, title = {Perimeter Calculator — Circle, Rectangle, Triangle, Polygon}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/perimeter-calculator}}, year = {2026}, note = {TG we-Calculate} }
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