Perimeter Calculator – Find Perimeter of Any Shape Online
Calculate the perimeter of any 2D shape with our free online perimeter calculator. Covers rectangles, triangles, circles, polygons, and more with step-by-step solutions.
What Is Perimeter?
Perimeter measures the total distance around the outside of a 2D shape. Think of it as the length of fence needed to enclose a yard, or the trim required to frame a picture. Unlike area which measures the space inside, perimeter traces the boundary itself.
This perimeter calculator handles the shapes you'll encounter most often – from basic rectangles and triangles to circles (where perimeter is called circumference) and regular polygons. Enter your dimensions, get instant results, and see exactly how each formula works step by step.
Perimeter Formulas Reference
Rectangle
Square
Triangle
Circle (Circumference)
Trapezoid
Parallelogram
Rhombus
Kite
Regular Polygon
Shape-by-Shape Breakdown
Rectangle
Opposite sides are equal. Add length and width, then double it. A 12 ft by 8 ft room needs 40 ft of baseboard (2 × (12 + 8) = 40).
Square
All four sides equal. Multiply one side by 4. A square garden with 7-meter sides requires 28 meters of fencing.
Triangle
Add all three sides. Works for any triangle – equilateral, isosceles, or scalene. The triangle inequality must hold: any two sides combined must exceed the third.
Circle (Circumference)
For circles, perimeter is called circumference. Formula is 2πr. A circle with radius 6 has circumference about 37.7 units. π (pi) ≈ 3.14159.
Trapezoid
Four sides with one pair parallel. Add all four side lengths. Unlike area, you don't need the height – just the actual side measurements.
Parallelogram
Opposite sides are equal and parallel. Double the sum of adjacent sides. Think of it as a "pushed" rectangle – perimeter stays the same even when slanted.
Rhombus
All four sides equal (like a square that's been tilted). Same formula as square: 4 times one side. Diagonals cross at right angles but aren't needed for perimeter.
Kite
Two pairs of adjacent equal sides. Double the sum of the two different side lengths. Kites have one pair of equal angles where the unequal sides meet.
Regular Polygon
All sides and angles equal. Multiply the number of sides by one side length. A regular hexagon with 5-unit sides has perimeter 30 units.
Practical Examples
Frequently Asked Questions
What's the difference between perimeter and area?
Perimeter measures the boundary (linear units like feet or meters). Area measures the space inside (square units like sq ft or m²). A 4×4 square has perimeter 16 units and area 16 sq units – same number, completely different meaning.
Why is circle perimeter called circumference?
"Circumference" comes from Latin meaning "to carry around." It's the same concept as perimeter but specifically for circles. The formula uses π because the ratio of circumference to diameter is always π for any circle.
Can perimeter be used for irregular shapes?
Yes – add up all the side lengths. For irregular shapes without straight sides, you may need to measure or approximate. This calculator handles regular geometric shapes with defined formulas.
What units should I use?
Any consistent linear units work – inches, feet, centimeters, meters. The perimeter will be in the same units you entered. Don't mix units (e.g., don't enter length in feet and width in inches).
How do I find perimeter if I only know area?
For squares, take the square root of area to get side, then multiply by 4. For other shapes, you need more information. A rectangle with area 100 could be 10×10 (perimeter 40) or 5×20 (perimeter 50) – same area, different perimeters.
Other Free Tools
Area Calculator – Find Area of Any 2D Shape Online
Calculate the area of any 2D shape with our free online area calculator. Supports circle, rectangle, triangle, trapezoid, parallelogram, and more with formula explanations.
Circle Calculator – Find Radius, Diameter, Area & Circumference
Calculate any property of a circle instantly with our free online circle calculator. Enter radius, diameter, area, or circumference and find all other measurements with formulas shown.
Pythagorean Theorem Calculator – Find Any Side of a Right Triangle
Solve for any missing side of a right triangle using the Pythagorean theorem with our free online calculator. Enter two sides and instantly find the third with step-by-step working.
Triangle Solver – Solve Any Triangle SSS SAS ASA AAS
Solve any triangle completely using SSS, SAS, ASA, or AAS methods with our free online triangle solver. Find all missing sides, angles, and area with step-by-step trigonometric solutions.
Polygon Interior Angle Sum Calculator – Find Angle Sum of Polygon
Calculate the sum of interior angles of any polygon with our free online calculator. Enter the number of sides and instantly find the total interior angle sum and each angle for regular polygons.
Volume Calculator – Compute Volume of 3D Shapes Online
Calculate the volume of any 3D shape with our free online volume calculator. Supports cube, sphere, cylinder, cone, pyramid, and more with formula references and instant results.
Surface Area Calculator – Find Surface Area of Any 3D Shape
Calculate the surface area of any 3D geometric shape with our free online surface area calculator. Covers cube, sphere, cylinder, cone, pyramid, and prisms with full formula details.
Right Triangle Calculator – Solve Right Triangles Online
Solve any right triangle by entering two known values with our free online right triangle calculator. Find all sides, angles, area, and perimeter with clear step-by-step solutions.