Everything runs locally. You choose a shape, enter its dimensions, and the perimeter is computed on the page - nothing is uploaded or stored, and it works offline once loaded. Ten shapes are covered: square, rectangle, triangle, circle, parallelogram, trapezoid, rhombus, regular polygon, ellipse and semicircle. The result updates as you type, and the formula panel shows both the symbolic formula and the same formula with your numbers filled in, so you can check your own working.
Free Perimeter Calculator
Runs entirely in your browser - no upload, no sign-up.
Shape & dimensions
Perimeter26Rectangle
FormulaP = 2 × (l + w)P = 2 × (8 + 5) = 26
What is the perimeter of a shape?
The perimeter is the total distance around the outside of a 2D shape - the sum of every side. For a rectangle it is 2 × (length + width); for a circle the same idea is called the circumference, 2 × π × radius. Pick your shape, type the measurements, and this free calculator returns the perimeter and shows the formula it used, all in your browser.
How to use
- 1Choose the shape. Select the shape you're measuring - square, rectangle, triangle, circle, polygon, ellipse and more - from the shape menu.
- 2Enter the dimensions. Type the side lengths, radius or semi-axes the shape asks for. Use the same unit for every field; the perimeter comes out in that same unit.
- 3Read the perimeter. The perimeter appears instantly with the formula and your numbers substituted in. Use Copy result to grab the value and the working.
Who it's for
- Students checking homework on the perimeter of a rectangle, triangle or circle and seeing the formula spelled out step by step.
- DIY and home projects measuring fencing, skirting board, edging or trim - work out how much material wraps around a room, garden bed or table.
- Teachers demonstrating how the perimeter changes as the sides of a shape change, with a live formula for each shape.
- Crafters and sewers estimating ribbon, fabric edging or picture-frame length around a square, circle or oval.
- Anyone comparing shapes - see how a circle's circumference stacks up against a square of the same width.
FAQ
Is the perimeter calculator free?
Yes - it's 100% free, with no sign-up and no limits. The whole calculation runs in your browser and nothing you enter is uploaded.
How do you calculate the perimeter of a rectangle?
Add the length and the width and double the result: P = 2 × (length + width). A 8 by 5 rectangle has a perimeter of 2 × (8 + 5) = 26. For a square, all four sides are equal, so P = 4 × side.
What is the perimeter of a circle?
The distance around a circle is its circumference: C = 2 × π × radius, or equivalently π × diameter. For a radius of 5 that's about 31.42. Because π is irrational the answer never ends exactly, so it's normally rounded.
How do I find the perimeter of a triangle?
Add the three side lengths together: P = a + b + c. It doesn't matter whether the triangle is right-angled, isosceles or scalene - the perimeter is always just the sum of its sides. If one side is longer than the other two combined the lengths can't form a real triangle, and the calculator warns you.
What units does the perimeter come out in?
The same unit you enter. The calculator is unit-agnostic: if you type the sides in centimetres the perimeter is in centimetres, if in feet then feet. Just keep every field in the same unit - don't mix metres and inches in one shape.
How is the perimeter of an ellipse calculated?
An ellipse has no exact formula using elementary functions, so this tool uses Ramanujan's second approximation from the two semi-axes a and b: P ≈ π[3(a + b) − √((3a + b)(a + 3b))]. It is accurate to well under 0.01% for most ovals, far better than the rough π(a + b) estimate.
What is the difference between perimeter and area?
Perimeter is the distance around the edge of a shape, measured in a length unit (cm, m, ft). Area is the space inside it, measured in square units (cm², m²). This calculator finds the perimeter; two shapes can have the same perimeter but very different areas.