Rectangle Calculator

Free Online Rectangle Calculator

This rectangle calculator helps you find the area, perimeter, and diagonal of any rectangle instantly. Just enter the length and width, and the calculator solves for every other property, complete with a to-scale diagram and a full step-by-step solution showing exactly how each formula was applied.

Whether you're a student working through geometry homework, a teacher preparing worked examples, a contractor estimating flooring or wall material, or simply someone who needs a fast and accurate area of a rectangle calculator or perimeter of a rectangle calculator, this tool covers every common calculation you'll ever need for a rectangle.

A rectangle is a four-sided shape with two pairs of equal, parallel sides and four right angles. Unlike a square, its length and width can differ, which means two measurements — rather than one — are needed to fully describe the shape. This calculator takes both values and instantly works out the area, perimeter, and diagonal for you.

Rectangle Formulas Used

A rectangle is defined by its length (l) and width (w). Every other measurement can be derived from these two values using these standard geometry formulas:

• Area: A = l × w (the length multiplied by the width)
• Perimeter: P = 2(l + w) (twice the sum of the length and width)
• Diagonal: d = √(l² + w²) (from the Pythagorean theorem, since a diagonal splits the rectangle into two right triangles)

How to Use This Rectangle Calculator

Using the calculator only takes two steps. First, enter the length of the rectangle. Second, enter the width. The calculator instantly displays the area, perimeter, and diagonal, along with a scaled visual diagram and a detailed breakdown of the math behind every number.

This means you never need to manually apply the area, perimeter, or diagonal formula yourself — the calculator handles all the arithmetic for you. It's especially handy when you're planning material for a room, garden bed, or panel and want to double-check both the covered area and the surrounding border length at once.

Worked Example

Suppose you're tiling a rectangular room that measures 8 meters in length and 5 meters in width. Enter 8 as the length and 5 as the width, and the calculator immediately finds the area (A = l × w = 8 × 5 = 40 square meters), the perimeter (P = 2(l + w) = 2 × 13 = 26 meters), and the diagonal (d = √(8² + 5²) = √89 ≈ 9.43 meters).

That means you'd need enough tile to cover 40 square meters, roughly 26 meters of skirting board around the edges, and — if you were laying a diagonal support beam or checking the room is truly rectangular — the diagonal distance would be about 9.43 meters corner to corner.

Rectangle vs. Square: What's the Difference?

A rectangle and a square share the same basic properties — four sides, four right angles, and opposite sides that are equal and parallel. The key difference is that a square requires all four sides to be exactly equal, while a rectangle only requires opposite sides to match, so the length and width can be different. If your shape has all four sides equal, you can also use the Square Calculator, which only needs a single measurement to solve everything.

Where Rectangle Calculations Are Used

Rectangle geometry shows up constantly in real life and across many fields of study:

• Students and teachers use it for geometry and mensuration homework, exams, and lesson planning.
• Architects and engineers use rectangle measurements when designing rooms, walls, doors, windows, and structural panels.
• Home improvement and construction projects often need area or perimeter for flooring, fencing a rectangular plot, painting walls, or cutting sheet material.
• Interior designers calculate square footage of rugs, furniture layouts, and floor plans to plan spaces and estimate costs.
• Manufacturing, packaging, and printing rely on precise length, width, and diagonal calculations for boxes, screens, panels, and paper sizes.