Overview
A rectangle has four sides and four right angles, with opposite sides equal. Its area is length times width, its perimeter is the sum of all four sides, and its diagonal follows from the Pythagorean theorem.
Formula
Area = L × W, perimeter = 2(L + W), diagonal = √(L² + W²).
How it works
Enter length and width
Use the same measurement unit for both dimensions.
Multiply for area
Area = length × width.
Add for perimeter
Perimeter = 2 × (length + width).
Compute the diagonal
Apply the Pythagorean theorem to length and width.
Worked examples
Example 1: 8 × 5
Area 40, perimeter 26, diagonal ≈ 9.4340.
Example 2: 10 × 10
A square with area 100, perimeter 40 and diagonal ≈ 14.1421.
Frequently asked questions
Is a square a rectangle?
Yes. A square is a special rectangle with all sides equal.
How do I find the width from the area?
Width = area ÷ length.
Why is the diagonal longer than each side?
Because it is the hypotenuse of a right triangle whose legs are the length and width.
This calculator is for educational and informational purposes only and is not a substitute for professional advice.