Perimeter and Circumference of 2D Shapes
The perimeter is the total distance around the outline of a flat shape. Pick one of nine common 2D shapes, type in its dimensions, and the perimeter appears the moment you enter a value — with the exact formula shown so you can follow the math. For a circle, that boundary distance has its own name: the circumference.
Shapes You Can Measure
Circle & Ellipse
Quadrilaterals
Triangle & Polygon
How to Calculate Perimeter
Select the Shape
Choose the shape you want to measure. The input fields change to ask only for the dimensions that shape needs.
Enter the Dimensions
Type in the side lengths, radius, or semi-axes the shape requires. Fill in every field for a complete result.
Read the Result Instantly
The perimeter is computed as you type, with the formula and the substituted values shown so you can check each step.
Set Your Unit
Pick mm, cm, m, in, or ft from the dropdown. The result is labelled in that same linear unit.
Perimeter Formulas by Shape
Each shape uses a standard formula. The calculator applies them for you, but here is the full reference for checking your own work by hand.
| Shape | Formula | Inputs Needed |
|---|---|---|
| Circle | P = 2 × π × r | radius r |
| Square | P = 4 × a | side a |
| Rectangle | P = 2 × (l + w) | length l, width w |
| Triangle | P = a + b + c | sides a, b, c |
| Trapezoid | P = a + b + s₁ + s₂ | bases a, b, sides s₁, s₂ |
| Parallelogram | P = 2 × (a + b) | sides a, b |
| Rhombus | P = 4 × a | side a |
| Ellipse | P ≈ π × (3(a+b) − √((3a+b)(a+3b))) | semi-axes a, b |
| Regular Polygon | P = n × a | sides n, side length a |
Calculator Features
Nine 2D Shapes
Circle, square, rectangle, triangle, trapezoid, parallelogram, rhombus, ellipse, and regular polygon — all covered.
Real-Time Results
The perimeter updates the instant you change a value — no submit button, no waiting.
Formula & Steps Shown
Every result displays its formula with your numbers substituted in, so you can see exactly how it was worked out.
Five Unit Labels
Label your result in mm, cm, m, in, or ft — the same linear unit you enter your dimensions in.
Frequently Asked Questions
What is the difference between perimeter and circumference?
Circumference is simply the perimeter of a circle. Both measure the total length around the outside of a shape; "circumference" is the word reserved for a round boundary like a circle or ellipse, while "perimeter" is the general term for any 2D shape.
How do I find the perimeter of a shape?
For a polygon, add the lengths of every side. Regular shapes have shortcuts — P = 4 × a for a square, P = 2 × (l + w) for a rectangle, and P = 2 × π × r for a circle's circumference. This tool applies the right formula automatically once you pick the shape.
How do you calculate the circumference of a circle?
Multiply the radius by 2 and by π: P = 2 × π × r. A circle with radius 5, for example, has a circumference of 2 × π × 5 = 10π ≈ 31.42. Enter the radius and the tool returns the circumference instantly.
How is the ellipse perimeter calculated?
An ellipse has no simple exact perimeter formula, so the tool uses Ramanujan's approximation: P ≈ π × (3(a+b) − √((3a+b)(a+3b))). It is extremely accurate for everyday ellipses.
What unit is the perimeter given in?
Perimeter is a length, so it stays in the linear unit you selected — mm, cm, m, in, or ft — the same unit you used for the dimensions. The unit is a label, so enter every value in one consistent unit.
Do I need every side of a triangle?
Yes. The perimeter of a triangle is the sum of all three sides (a + b + c), so each side length must be entered for a correct total.
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