📏 Perimeter: Calculator

Calculate the perimeter of a rectangle, square, circle, triangle, or regular polygon.

units
units
PERIMETER
0
units (same unit as your inputs)
Select a shape and enter its dimensions.

Perimeter Formulas for Common Shapes

Perimeter is the total distance around the outside edge of a two-dimensional shape. This calculator covers the five most commonly needed shapes in school math and everyday projects, from simple rectangles to regular polygons with any number of sides.

Rectangle: P = 2(L + W)
Square: P = 4s
Circle (circumference): C = 2πr
Triangle: P = a + b + c
Regular Polygon: P = n × s  (n = number of sides, s = side length)

Perimeter vs. Circumference vs. Area

Perimeter and circumference measure the same concept — the distance around a shape's edge — but "circumference" is the specific term used for circles. Don't confuse either with area, which measures the space enclosed inside the shape rather than the distance around its boundary.

The Triangle Inequality Check

Not every set of three numbers can form a real triangle. For three side lengths to form a valid triangle, the sum of any two sides must be greater than the third side (the triangle inequality theorem) — this calculator flags side combinations that don't satisfy that rule.

📎 Related Tools: Area Calculator Circle Calculator Triangle Calculator Volume Calculator

❓ Frequently Asked Questions

What is the difference between perimeter and area?
Perimeter measures the total distance around the outside edge of a shape, while area measures the amount of two-dimensional space enclosed inside it. They use different units (linear units vs. square units) and different formulas.
Is circumference the same as perimeter?
Yes, conceptually — circumference is simply the specific term used for the perimeter of a circle, calculated as 2 × π × radius.
Can any three numbers form a triangle's sides?
No. The triangle inequality theorem requires that the sum of any two sides must be strictly greater than the third side. If this isn't true, the three lengths can't form a real triangle, which this calculator flags automatically.
How do I find the perimeter of an irregular polygon?
For an irregular polygon (where sides have different lengths), simply add up the length of every individual side. This calculator's 'Regular Polygon' mode assumes all sides and angles are equal; for irregular shapes, you'd sum each side manually.
What units does this calculator use?
This calculator is unit-agnostic — enter your measurements in any consistent unit (inches, feet, meters, cm), and the result will be in that same unit.

🏆 About This Tool — Accuracy & Trust

🔒 Data Privacy: All calculations run entirely within your browser using JavaScript. Nothing you enter here is ever transmitted to our servers, stored, sold, or shared.

📐 Accuracy Note: This tool uses standard, widely published formulas and guidelines. Results are estimates for informational purposes; for financial, medical, or engineering decisions, consult a licensed professional.

📅 Last Updated: September 2026.