Rectangle Area Calculator

A rectangle is a quadrilateral with four right angles. Knowing only its width and height, you can find both its area and perimeter. When the width and height are equal, it becomes a square.

Enter the width and height to calculate both the area and perimeter of a rectangle. This is the most common calculation for finding the area of a room or plot of land.

For example, a rectangle 8 cm wide and 5 cm tall has an area of 40 cm² and a perimeter of 26 cm.

Rectangle Area Calculator Calculation Method

S: area, L: perimeter, a: width, b: height

Area: 0

Perimeter: 0

About This Calculator

A rectangle is a four-sided shape with four right angles, and its area and perimeter follow from its width and height. It is the most common shape in construction, layout and design.

Enter the width and height to get the area, perimeter and diagonal.

A rectangle has area length × width and perimeter 2 × (length + width), and the two scale differently: doubling both sides quadruples the area but only doubles the perimeter. A 12 × 5 rectangle has area 60 and perimeter 34, and among all rectangles with that perimeter the square 8.5 × 8.5 holds the largest area.

Key Formula:Area = w × h · Perimeter = 2(w + h) · Diagonal = √(w² + h²)

Frequently Asked Questions

How do I find the diagonal of a rectangle?

Use the Pythagorean theorem: the diagonal is √(w² + h²). That is why a 40-inch TV measures 40 inches across the diagonal, not the width.

Is a square a rectangle?

Yes — a square is a rectangle with all four sides equal, just as it is also a rhombus and a parallelogram. The rectangle properties (four right angles, opposite sides equal) all still hold.

How do I calculate the diagonal of a rectangle?

Use Pythagoras: d = √(l² + w²). For 12 × 5 that is √169 = 13, and a 16:9 screen 1920 pixels wide has a diagonal of about 2203 pixels.

What is the difference between a rectangle and a parallelogram?

Every rectangle is a parallelogram, but a parallelogram becomes a rectangle only when all four angles are right angles. Both have area base × height, yet only the rectangle has equal angles.

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Sources & last update

2026-10-19