How to Find Perimeter of Any Shape

Learn how to find perimeter instantly with our free online calculator. Get accurate results for rectangles, circles, triangles, squares, regular polygons, ellipses, and more with step-by-step explanations.

Free Perimeter Calculator

Rectangle Perimeter

Enter dimensions to calculate perimeter

Circle Perimeter (Circumference)

OR
Enter radius or diameter to calculate circumference

Triangle Perimeter

Enter all three sides to calculate perimeter

Square Perimeter

Enter side length to calculate perimeter

Regular Polygon Perimeter

Enter number of sides and side length to calculate perimeter

Ellipse Perimeter

Enter both semi-axes to calculate perimeter

Perimeter Formulas for Different Shapes

Rectangle Perimeter Formula

P = 2(l + w)

Where P = perimeter, l = length, w = width

Circle Perimeter Formula

P = 2πr or P = πd

Where P = perimeter (circumference), r = radius, d = diameter

Note: Enter either radius or diameter - the calculator will auto-convert and show both values.

Triangle Perimeter Formula

P = a + b + c

Where P = perimeter, a, b, c = lengths of the three sides

Square Perimeter Formula

P = 4s

Where P = perimeter, s = side length

Regular Polygon Perimeter Formula

P = n × s

Where P = perimeter, n = number of sides, s = side length

Ellipse Perimeter Formula

P ≈ π[3(a+b) - √((3a+b)(a+3b))]

Where P = perimeter, a = semi-major axis, b = semi-minor axis (Ramanujan's approximation)

How to Find Perimeter: Step-by-Step Guide

What is Perimeter?

Perimeter is the distance around the outside of a two-dimensional shape. Learning how to find perimeter is essential for calculating the total length of all the sides of a polygon or the circumference of a circle.

General Steps to Find Perimeter

  1. Identify the shape you're working with
  2. Measure or identify all the side lengths
  3. Apply the appropriate formula
  4. Calculate and include units

Common Mistakes to Avoid

  • Forgetting to include all sides
  • Using area formulas instead of perimeter
  • Not including units in your answer
  • Confusing radius and diameter for circles

Real-World Applications

  • Fencing a yard or garden
  • Installing trim around rooms
  • Calculating material needed for borders
  • Determining running track distances

Perimeter Calculation Examples

Rectangle Example

Problem: Find the perimeter of a rectangle with length 8 cm and width 5 cm.

Solution: P = 2(l + w) = 2(8 + 5) = 2(13) = 26 cm

Circle Example

Problem 1: Find the perimeter of a circle with radius 7 meters.

Solution: P = 2πr = 2 × π × 7 ≈ 43.98 meters

Problem 2: Find the perimeter of a circle with diameter 10 cm.

Solution: P = πd = π × 10 ≈ 31.42 cm

Triangle Example

Problem: Find the perimeter of a triangle with sides 3 ft, 4 ft, and 5 ft.

Solution: P = a + b + c = 3 + 4 + 5 = 12 ft

Square Example

Problem: Find the perimeter of a square with side length 6 inches.

Solution: P = 4s = 4 × 6 = 24 inches

Regular Polygon Example

Problem: Find the perimeter of a regular hexagon with side length 4 cm.

Solution: P = n × s = 6 × 4 = 24 cm

Ellipse Example

Problem: Find the perimeter of an ellipse with semi-major axis 5 units and semi-minor axis 3 units.

Solution: P ≈ π[3(5+3) - √((15+3)(5+9))] ≈ 25.53 units