Rectangle Calculator

Rectangle Calculator: Compute Area, Perimeter, and Diagonal

A rectangle is a four-sided polygon with opposite sides equal and parallel, and all interior angles measuring 90 degrees. Our rectangle calculator lets you instantly compute the area, perimeter, and diagonal length of any rectangle. Simply enter the two side lengths, and all properties are calculated in real time.

What is a Rectangle?

A rectangle is a quadrilateral with four right angles. It is one of the most fundamental shapes in geometry and appears everywhere in daily life — from sheets of paper and computer screens to rooms, buildings, and land plots. The longer side is typically called the length, and the shorter side is called the width. In this calculator, we refer to them as side a and side b.

Key Properties of a Rectangle

Area (A): The space enclosed by the rectangle, calculated as A = a × b.
Perimeter (P): The total distance around the rectangle, calculated as P = 2a + 2b.
Diagonal (d): The line connecting opposite corners, calculated using the Pythagorean theorem: d = √(a² + b²).

How to Use This Calculator

Enter the length of side a in the input field.
Enter the length of side b in the input field.
The area, perimeter, and diagonal are computed instantly as you type.

Applications of Rectangle Calculations

Rectangle calculations are essential in numerous real-world scenarios. Architects and construction workers calculate areas for flooring, tiling, and painting projects. Interior designers measure perimeters for baseboards and crown molding. Engineers compute diagonals for structural bracing and frame construction. Real estate professionals determine square footage for property valuation. Anyone planning a garden, installing a fence, or framing a picture will benefit from quick rectangle calculations.

Also check: Square Calculator, Area Calculator, Volume Calculator, Surface Area Calculator, Rectangular Prism Calculator, and Triangle Calculator.

Frequently Asked Questions

How do I find the area of a rectangle if I only know the perimeter and one side?

If you know the perimeter (P) and one side (a), you can find the other side using b = (P - 2a) / 2. Then calculate the area as A = a × b. For example, if P = 26 and a = 5, then b = (26 - 10) / 2 = 8, and the area is 5 × 8 = 40 square units.

What is the diagonal of a rectangle and how is it calculated?

The diagonal of a rectangle is the straight line connecting two opposite corners. It divides the rectangle into two congruent right triangles. Using the Pythagorean theorem, the diagonal length d is calculated as d = √(a² + b²), where a and b are the side lengths.

What is the difference between a rectangle and a square?

A square is a special type of rectangle where all four sides are equal in length. While every square is a rectangle (it has four right angles and opposite sides parallel), not every rectangle is a square. In a rectangle, the side lengths can be different, while a square requires all sides to be equal.

Can a rectangle have sides of equal length?

Yes, when a rectangle has equal side lengths (a = b), it becomes a square. All square properties apply, and the formulas still work correctly. The area becomes A = a², and the diagonal becomes d = a√2.

What units should I use for rectangle calculations?

You can use any unit of length as long as you are consistent. If you enter side lengths in meters, the area will be in square meters, the perimeter in meters, and the diagonal in meters. Common units include millimeters, centimeters, meters, inches, feet, and yards.

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