Perimeter Calculator
Calculate the perimeter of rectangles, triangles, circles, and regular polygons.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator finds the perimeter of four shape types: rectangles, triangles, circles, and regular polygons.
- Select a shape. Choose from rectangle, triangle, circle, or regular polygon.
- Enter the dimensions. Each shape requires different inputs. Rectangles need length and width. Triangles need all three side lengths. Circles need the radius. Regular polygons need the number of sides and the length of one side.
- Read the result. The perimeter appears with the formula used. For circles, this value is the circumference.
Perimeter is measured in the same units as your input dimensions (e.g., meters in, meters out). Results update live as you type.
About Perimeter Calculations
The perimeter is the total distance around the outside boundary of a two-dimensional shape. For polygons, it is the sum of all side lengths. For circles, it is called the circumference and equals 2 * pi * radius. Perimeter is used in fencing (how much material to enclose an area), framing (border length for a picture), running tracks (distance per lap), and construction (foundation dimensions). Unlike area, perimeter measures linear distance, not surface coverage. Two shapes can have the same area but very different perimeters, and vice versa.
Frequently Asked Questions
What is the perimeter of a rectangle?
The perimeter of a rectangle is P = 2 * (length + width). For a rectangle with length 10 and width 5, the perimeter is 2 * (10 + 5) = 30 units.
What is a regular polygon?
A regular polygon has all sides equal in length and all angles equal. Examples include equilateral triangles (3 sides), squares (4 sides), regular pentagons (5 sides), and regular hexagons (6 sides). The perimeter is simply the number of sides times the side length.
How is the perimeter of a circle different from its area?
The perimeter (circumference) of a circle is the distance around it: C = 2*pi*r, measured in linear units. The area is the space inside it: A = pi*r^2, measured in square units. They use different formulas and have different units.