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Perimeter calculator

Compute the perimeter of a rectangle, square, circle or triangle.

Related tools

All Geometry tools →
Regular polygon perimeter calculatorCompute the perimeter of a regular polygon from its number of sides and side length.Ellipse perimeter calculatorEstimate the perimeter of an ellipse from its two semi-axes (Ramanujan's formula).Aquarium volume calculatorCompute the water volume of a rectangular aquarium in litres and gallons.Area calculatorCompute the area of a rectangle, square, circle or triangle.Cone surface area calculatorCompute the total surface area and slant height of a cone from its radius and height.Cone volume calculatorCompute the volume, surface area and slant height of a cone, drawn to scale.Cylinder volume calculatorCompute the volume and surface area of a cylinder from its radius and height.Equilateral triangle calculatorCompute the area, height and perimeter of an equilateral triangle from its side.

Enter Shape, Width / side / radius / side a, Height / side b, Side c (triangle only) and the Perimeter calculator works out Perimeter straight away. For instance, with Shape = Rectangle, Width / side / radius / side a = 4, Height / side b = 3 and Side c (triangle only) = 5 it returns Perimeter = 14.

How to use it

  1. Enter your values: Shape, Width / side / radius / side a, Height / side b, Side c (triangle only).
  2. Read the result instantly: Perimeter.

Frequently asked questions

How does the Perimeter calculator work?

It takes Shape, Width / side / radius / side a, Height / side b and Side c (triangle only) and derives Perimeter from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

4 values: Shape, Width / side / radius / side a, Height / side b and Side c (triangle only). Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Shape = Rectangle, Width / side / radius / side a = 4, Height / side b = 3 and Side c (triangle only) = 5, the calculator returns Perimeter = 14. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

How much does the result change with different inputs?

It moves a lot. Using Shape = Square, Width / side / radius / side a = 8, Height / side b = 6 and Side c (triangle only) = 10 instead, Perimeter goes from 14 to 32 — which is why it is worth testing a few scenarios rather than trusting a single figure.

Which “Shape” option should I choose?

You can pick between « Rectangle », « Square », « Circle » and « Triangle ». Each one changes what the calculator works out, so switch and compare — the default is « Rectangle ».

What does it give for smaller values?

Scaled down to Shape = Rectangle, Width / side / radius / side a = 2, Height / side b = 2 and Side c (triangle only) = 3, Perimeter comes out at 8. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Homework and site work alike: how much paint a wall takes, how much water a tank holds, whether a corner is square — and, on the coordinate side, the slope between two points, the distance across a plan, the midpoint of a span.

What is the most common mistake?

Mixing units inside one shape. Entering a length in centimetres and a width in metres gives an area wrong by a factor of a hundred, and the result looks perfectly plausible.

What is the difference between the Perimeter calculator and the Regular polygon perimeter calculator?

Both return Perimeter. What differs is what they ask for: this one wants Shape and Width / side / radius / side a, the Regular polygon perimeter calculator wants Number of sides and Side length. Use whichever matches the numbers you already have.

Is there a tool for the next step?

Ellipse perimeter calculator is the closest one after this: Estimate the perimeter of an ellipse from its two semi-axes (Ramanujan's formula).

Further reading

All guides →
ExplainerThe Pythagorean Theorem ExplainedIn a right triangle, a² + b² = c². Here's what the formula means, a worked example, how to find any side, and where it's used in real life.How-toHow to Calculate VolumeVolume is the space inside a shape. Here's how to find it for boxes, cylinders and spheres, and why keeping your units consistent is the whole game.How-toHow to Calculate the Square Metres of a RoomFind a room's area by multiplying length by width. Here's the method for rectangular, L-shaped and round rooms, and why area drives your material orders.How-toCircle: Circumference and AreaCircumference is 2πr, area is πr². Here's how radius, diameter and π fit together, both formulas worked through, and how not to mix them up.How-toHow Much Substrate Does an Aquarium Need? Depth, Weight and Grain SizeThe depth follows the job: a bare display, a planted tank whose roots need room, an under-gravel plate. Then comes the number people are unprepared for — substrate is what puts a filled tank past what a piece of furniture can hold. Plus why fine sand compacts and coarse gravel traps waste.ExplainerHow Many Fish Can a Tank Hold? The Inch-per-Gallon Rule Is WrongEvery stocking calculator implements a length-per-volume rule, and waste output scales with body mass, not length. A fish twice as long makes nearly five times the ammonia. Run through the tool's five methods, the same tank comes back holding anywhere from 6 to 12 fish.