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Rectangle Calculator

How do I find the area of a rectangle?

How it is calculated

For a rectangle with width w and height h (both in the same unit, above zero): area = w × h, perimeter = 2 × (w + h) and diagonal = √(w² + h²), which is the Pythagorean theorem applied to the right angle at a corner. Area comes out in square units of whatever you entered, while perimeter and diagonal stay in the plain unit. Everything is rounded to two decimals. Zero or negative sides give no result.

Worked example

A rectangle 8 wide and 6 high. Area = 8 × 6 = 48.00. Perimeter = 2 × (8 + 6) = 28.00. Diagonal = √(8² + 6²) = √(64 + 36) = √100 = 10.00.

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Common mistakes

  • Mixing units, for example width in metres and height in centimetres; convert both to one unit first.
  • Reading the area as a length: 48.00 is in square units, not metres or centimetres.
  • Confusing perimeter with area when buying material: fencing needs perimeter, flooring needs area.
  • Forgetting that the diagonal is the straight line between opposite corners, not the sum of two sides.
  • Entering a square's side only once; both width and height are required.

About this calculator

This calculator takes a rectangle's width and height and computes its area (width × height), perimeter (2 × (width + height)), and diagonal length using the Pythagorean theorem (√(width² + height²)) — the three measurements most commonly needed for a rectangular shape.

It's used for practical, everyday sizing problems: figuring out how much flooring, paint, fencing or fabric is needed for a rectangular room or plot, checking whether a piece of furniture or a TV will fit through a doorway using the diagonal, and by students working through geometry homework that asks for area, perimeter and diagonal together.

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