Perimeter Calculator

This perimeter calculator finds the perimeter of a square, rectangle, triangle, circle, semicircle, ellipse, equilateral triangle and regular polygon, showing the exact formula and step-by-step working for every shape. Whether you are measuring a garden fence, a picture frame, a running track or solving a geometry homework problem, you enter the side lengths or radius and get an instant, accurate result. The tool also doubles as a circumference calculator for circular shapes, so one screen covers every common boundary-length question.

What Is Perimeter?

Perimeter is the total length of the outer boundary of a closed two-dimensional shape. For any straight-sided figure you simply add the lengths of all its sides; for curved shapes such as a circle you use a dedicated formula. The result is always expressed in a single length unit — centimetres, metres, feet or kilometres — never a square unit, which is what separates perimeter from area. In everyday language perimeter answers the question "how far is it around the edge?", which is exactly what you need when buying fencing, edging, trim or border material.

Perimeter Formulas for All Shapes

The table below lists every perimeter formula the calculator uses, with the inputs each shape needs and a worked example. Keep your measurements in the same unit and the result will be correct every time.

How to Find the Perimeter of a Rectangle

To find the perimeter of a rectangle, add the length and the width then multiply by two: P = 2 × (a + b). The reason you multiply by two is that a rectangle has two pairs of equal sides. For an 8 cm × 5 cm rectangle the perimeter is 2 × (8 + 5) = 26 cm. This is the everyday formula for working out how much fencing, skirting board or picture-frame moulding you need around a rectangular space, and the rectangle perimeter calculator above does it instantly.

How to Find the Perimeter of a Square

A square has four equal sides, so finding the perimeter of a square is the simplest case of all: multiply one side by four, P = 4 × a. A square with a 6 cm side has a perimeter of 24 cm. Because every side is identical you only ever need a single measurement, which makes the square the fastest shape to calculate.

How to Find the Perimeter of a Triangle

To find the perimeter of a triangle, add the three side lengths together: P = a + b + c. Sides of 5, 7 and 9 cm give a perimeter of 21 cm. The same logic extends to any polygon — a quadrilateral, pentagon or hexagon — because the perimeter of any straight-sided figure is just the sum of all its sides. For a four-sided shape (how to find the perimeter of a quadrilateral) you simply add all four edges, and the triangle perimeter calculator tab handles the three-sided case for you.

Circle Circumference (Perimeter of a Circle)

The perimeter of a circle has a special name: the circumference. To find the circumference of a circle, use P = 2 × π × r, or if you only know the diameter, P = π × d. A circle with a radius of 7 cm has a circumference of 2 × π × 7 ≈ 43.98 cm. Used this way the tool works as a full circle circumference calculator: enter the radius, read the boundary length, and use it for belt lengths, pipe wrapping, wheel travel or any round component.

Perimeter vs Circumference vs Area

These three terms are often confused. Perimeter and circumference measure the same thing — the distance around the edge — but "circumference" is reserved for circles and other curved shapes, while "perimeter" is used for straight-sided polygons. Area, by contrast, measures the space inside the boundary and is given in square units. So a rectangle has both a perimeter (measured in cm) and an area (measured in cm²); knowing one does not tell you the other.

Common Uses of a Perimeter Calculator

Knowing how to find perimeter length quickly is useful far beyond the classroom. Typical real-world applications include:

• Fencing & borders: Calculate how much fencing material you need around a garden, field or yard.
• Interior design: Find the length of skirting board, cornice, trim or LED strip running along a room's perimeter.
• Maths & school: Solve geometry problems for square, rectangle, triangle, circle and polygon shapes in seconds.
• Engineering: Work out belt length, wire length or pipe circumference for circular and oval components.
• Crafts & framing: Measure ribbon, edging or framing material needed around an object.

For every shape the calculator displays the formula it used and the full substitution, so you not only get the answer but also see exactly how it was reached. Consistent units are the single most important step — mixing centimetres and metres is the most common source of error. For more worked examples, see the frequently asked questions below.