Math

Instant, private, and free

Diagonal of a Rectangle Calculator.

Calculate a rectangle's diagonal using the Pythagorean theorem.

On-device calculationNo signup

Set your values

Results update as you type.

02 / ANSWER

Width

Height

Result

diagonal: 5.000000

Use Cases

Home Improvement and DIY Projects

Determine the diagonal measurement of a rectangular space or object to ensure proper fit, such as for furniture, screens, or construction layouts.

Example: Check if a 60-inch wide and 40-inch high TV will fit in a cabinet opening.

Design and Engineering

Quickly find the diagonal length for technical drawings, packaging, or structural components where diagonal measurements are critical.

Example: Calculate the diagonal of a rectangular metal plate for drilling holes.

Frequently Asked Questions

How do I calculate the diagonal of a rectangle?: Enter the width and height of the rectangle in the same units. The calculator uses the Pythagorean theorem: diagonal = √(width² + height²). It computes the result instantly.
What units can I use for width and height?: You can use any unit of measurement (e.g., inches, feet, centimeters, meters) as long as both width and height are in the same unit. The diagonal will be in that same unit.
Why is the diagonal of a rectangle calculated using the Pythagorean theorem?: A rectangle's diagonal forms a right triangle with the width and height as the two legs. The Pythagorean theorem states that the square of the hypotenuse (diagonal) equals the sum of the squares of the other two sides.

Tips & Common Mistakes

Tips

Ensure width and height are in the same unit before calculating.
For very large or small numbers, use decimal values for more precise results.
Double-check your inputs: a small error in width or height can significantly affect the diagonal.
Use the diagonal to verify if an object fits through a doorway or opening.

Common Mistakes to Avoid

Mixing units (e.g., width in inches and height in feet) without converting first.
Forgetting to square the width and height before adding them.
Using the perimeter or area formula instead of the Pythagorean theorem.

Last updated: August 13, 2026

Use Cases

Home Improvement and DIY Projects

Determine the diagonal measurement of a rectangular space or object to ensure proper fit, such as for furniture, screens, or construction layouts.

Example: Check if a 60-inch wide and 40-inch high TV will fit in a cabinet opening.

Design and Engineering

Quickly find the diagonal length for technical drawings, packaging, or structural components where diagonal measurements are critical.

Example: Calculate the diagonal of a rectangular metal plate for drilling holes.

Frequently Asked Questions

How do I calculate the diagonal of a rectangle?

Enter the width and height of the rectangle in the same units. The calculator uses the Pythagorean theorem: diagonal = √(width² + height²). It computes the result instantly.

What units can I use for width and height?

You can use any unit of measurement (e.g., inches, feet, centimeters, meters) as long as both width and height are in the same unit. The diagonal will be in that same unit.

Why is the diagonal of a rectangle calculated using the Pythagorean theorem?

A rectangle's diagonal forms a right triangle with the width and height as the two legs. The Pythagorean theorem states that the square of the hypotenuse (diagonal) equals the sum of the squares of the other two sides.

Tips & Common Mistakes

Tips

Ensure width and height are in the same unit before calculating.
For very large or small numbers, use decimal values for more precise results.
Double-check your inputs: a small error in width or height can significantly affect the diagonal.
Use the diagonal to verify if an object fits through a doorway or opening.

Common Mistakes to Avoid

Mixing units (e.g., width in inches and height in feet) without converting first.
Forgetting to square the width and height before adding them.
Using the perimeter or area formula instead of the Pythagorean theorem.

Use Cases

Home Improvement and DIY Projects

Determine the diagonal measurement of a rectangular space or object to ensure proper fit, such as for furniture, screens, or construction layouts.

Example: Check if a 60-inch wide and 40-inch high TV will fit in a cabinet opening.

Design and Engineering

Quickly find the diagonal length for technical drawings, packaging, or structural components where diagonal measurements are critical.

Example: Calculate the diagonal of a rectangular metal plate for drilling holes.

Frequently Asked Questions

How do I calculate the diagonal of a rectangle?

Enter the width and height of the rectangle in the same units. The calculator uses the Pythagorean theorem: diagonal = √(width² + height²). It computes the result instantly.

What units can I use for width and height?

You can use any unit of measurement (e.g., inches, feet, centimeters, meters) as long as both width and height are in the same unit. The diagonal will be in that same unit.

Why is the diagonal of a rectangle calculated using the Pythagorean theorem?

Tips & Common Mistakes

Tips

Ensure width and height are in the same unit before calculating.
For very large or small numbers, use decimal values for more precise results.
Double-check your inputs: a small error in width or height can significantly affect the diagonal.
Use the diagonal to verify if an object fits through a doorway or opening.

Common Mistakes to Avoid

Mixing units (e.g., width in inches and height in feet) without converting first.
Forgetting to square the width and height before adding them.
Using the perimeter or area formula instead of the Pythagorean theorem.