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Rectangle Area Calculator

Get the area, perimeter and diagonal of any rectangle instantly

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Enter the length of the rectangle, for example 4.
  2. 2.Enter the width of the rectangle, for example 5.
  3. 3.Read the area (20 square units), perimeter and diagonal, which update instantly as you type.

About Rectangle Area Calculator

A rectangle is a four-sided shape with four right angles, and it is one of the most common shapes you will ever need to measure — rooms, floors, gardens, screens, canvases, tables and sheets of material are almost always rectangular. This rectangle area calculator takes the two numbers that define any rectangle, its length and its width, and instantly returns the area, the perimeter and the diagonal, all computed privately in your browser.

The area of a rectangle is the amount of flat space it covers, and the formula could not be simpler: area = length × width. If a room is 4 units long and 5 units wide, its area is 4 × 5 = 20 square units. Multiplying the two sides works because a rectangle can be split into a grid of unit squares — length tells you how many squares fit along one edge, width tells you how many rows there are, and multiplying counts them all. The answer is always in square units: if you measure in centimetres you get square centimetres (cm²), in metres you get square metres (m²), and so on.

The perimeter is the total distance around the outside of the rectangle. Because opposite sides are equal, you add one length and one width and double the result: perimeter = 2 × (length + width). For the same 4 by 5 rectangle that is 2 × (4 + 5) = 18 units. Perimeter is what you need when buying fencing for a garden, skirting board for a room, or a frame for a picture.

The diagonal joins two opposite corners and is found with the Pythagorean theorem, because the length, the width and the diagonal form a right-angled triangle: diagonal = √(length² + width²). A 3 by 4 rectangle has a diagonal of √(9 + 16) = √25 = 5, and a 6 by 8 rectangle has a diagonal of exactly 10. Diagonals matter for TV and monitor sizes, for checking that a frame is square, and for fitting long objects across a space.

A square is simply the special case where length and width are equal, so this tool also works as a square calculator — enter the same value twice and the area becomes side², the perimeter becomes 4 × side, and the diagonal becomes side × √2. The calculator flags that case for you.

Type a length and a width, add an optional unit label if you like, and read off the area, perimeter and diagonal at once. It is a fast, free way to size a room before buying flooring, plan a raised garden bed, estimate paint or fabric, or check homework — with every calculation running instantly on your device.

Frequently asked questions

How do I calculate the area of a rectangle?
Multiply the length by the width: area = length × width. For a rectangle that is 4 units long and 5 units wide, the area is 4 × 5 = 20 square units. The result is always in square units, so measuring in centimetres gives square centimetres (cm²).
What is the formula for the area of a rectangle?
The formula is area = length × width, often written A = l × w. It works because a rectangle divides into a grid of unit squares — length gives the squares along one edge and width gives the number of rows, so multiplying them counts every square inside.
How do I find the perimeter of a rectangle?
Add one length and one width, then double the total: perimeter = 2 × (length + width). For a 4 by 5 rectangle that is 2 × (4 + 5) = 18 units. Perimeter is the distance all the way around the outside, useful for fencing, framing or skirting.
How do I find the diagonal of a rectangle?
Use the Pythagorean theorem: diagonal = √(length² + width²), because the length, width and diagonal form a right-angled triangle. A 3 by 4 rectangle has a diagonal of √(9 + 16) = √25 = 5, and a 6 by 8 rectangle has a diagonal of exactly 10.
Is a square a rectangle?
Yes. A square is a special rectangle whose length and width are equal, so every square is a rectangle but not every rectangle is a square. For a square the area becomes side², the perimeter becomes 4 × side, and the diagonal becomes side × √2.

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