Perimeter Calculator
Choose a shape and enter its dimensions to get the perimeter, with the formula it used shown underneath.
Perimeter is the total distance around the edge of a two-dimensional shape. For a rectangle it is 2 ร (length + width); for a square, 4 ร side; for a triangle, the sum of the three sides; and for a circle the perimeter is the circumference, 2 ร ฯ ร radius.
How to Use This Calculator
Pick the shape you are measuring from the dropdown. The input fields change to match: a rectangle asks for length and width, a square asks for one side, a triangle asks for its three sides, and a circle asks for the radius.
Enter your measurements in any single unit โ inches, feet, centimeters โ and the perimeter comes back in the same unit. Mixing units in one calculation will give a meaningless result, so convert everything to one unit first.
How the Calculation Works
Perimeter is just the total length of the boundary. For straight-sided shapes you add up the sides โ a rectangle has two lengths and two widths, so 2 ร (length + width), and a square's four equal sides give 4 ร side.
A circle has no straight sides, so its perimeter is called the circumference and equals 2 ร ฯ ร radius, where ฯ is about 3.14159. This calculator uses the full precision of ฯ rather than a rounded value.
rectangle 2(l + w) ยท square 4s ยท triangle a + b + c ยท circle 2ฯr
Worked Example
Take a rectangle 5 units long and 3 units wide. Add the length and width to get 8, then double it: 2 ร 8 = 16 units.
For a circle with a radius of 5 units, the circumference is 2 ร ฯ ร 5 = 31.42 units. The same idea applies to every shape โ total up the boundary.
What the Result Means
Perimeter is the distance you would travel walking once around the shape's edge, which is exactly what you need for fencing a yard, framing a picture, or trimming an edge with tape or molding.
Don't confuse it with area. Perimeter measures the boundary length in ordinary units; area measures the space inside in square units. Two shapes can share a perimeter and have very different areas.
Assumptions and Limitations
- All measurements must be in the same unit; the result is in that unit
- Triangle side lengths must satisfy the triangle inequality to form a real triangle
- Circle calculations use the full precision of ฯ, with the result rounded for display
- For irregular shapes, split them into these basic shapes and add the outer edges
When to Use a Perimeter Calculator
Students, DIYers, and gardeners use this to find the distance around a rectangle, square, triangle, or circle. Reach for it when buying fencing or edging, framing a picture, trimming a room, or checking a geometry answer. It gives the total boundary length from a shape's dimensions, so you order the right amount of material the first time.
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Open calculator โFrequently Asked Questions
What is the formula for the perimeter of a rectangle?
Perimeter equals 2 ร (length + width). You add one length and one width, then double the total because a rectangle has two of each side.
Is a circle's perimeter the same as its circumference?
Yes. The perimeter of a circle is called its circumference, and it equals 2 ร ฯ ร radius, or equivalently ฯ ร diameter.
What is the difference between perimeter and area?
Perimeter is the distance around the edge of a shape, measured in ordinary units. Area is the space inside it, measured in square units. They answer different questions and are not interchangeable.
How do I find the perimeter of an irregular shape?
Break it into rectangles, triangles and other simple shapes, then add the lengths of only the outer edges. Do not count the internal edges where the pieces join.
Can I mix units, like feet and inches?
No. Convert every measurement to the same unit first. Mixing feet and inches in one calculation gives a result that is not meaningful.
Looking for a different calculator?
CalculatorPlus has free tools across finance, construction, math, health and more โ each one showing the formula and a worked example.