How is a rectangle's diagonal calculated?

Using the Pythagorean theorem: diagonal = √(length² + width²).

Why is this formula useful practically?

It's used to check if a rectangular space has true 90° corners by comparing measured diagonals.

Does this work for a square too?

Yes — a square is a special rectangle, so the same formula applies.

Rectangle Diagonal Calculator - CalculatorLabs.net

Rectangle Diagonal Calculator

Every rectangle (and square, which is just a special rectangle) has a diagonal — a straight line connecting two opposite corners.

Length (l)

Width (w)

Instant Result

Diagonal Length -

Diagonal Angle -

What Is a Rectangle & Square Diagonal Calculator?

Every rectangle (and square, which is just a special rectangle) has a diagonal — a straight line connecting two opposite corners. Knowing the diagonal is handy for checking that a frame or foundation is truly square, sizing a TV or screen, or fitting furniture through a doorway. This calculator finds the diagonal length and the angle it makes with the base.

How to Use This Rectangle & Square Diagonal Calculator

Measure the length and width of the rectangle (for a square, both values are equal).
Enter both values into the fields above.
Click Calculate to get the diagonal length and its angle from the base.

Formula

Diagonal = √(l² + w²)
Diagonal Angle = arctan(w / l)

Worked Example

A door frame measures 90 cm wide and 200 cm tall. Diagonal = √(90² + 200²) ≈ 219.32 cm. Angle from the base = arctan(200/90) ≈ 65.77° — useful for checking the frame is perfectly square by comparing both corner-to-corner diagonals.