Online College Courses for Credit

3 Tutorials that teach Calculating Diagonals
Take your pick:
Calculating Diagonals

Calculating Diagonals

Author: Sophia Tutorial

This lesson applies the Pythagorean Theorem to calculate the diagonal length of a rectangle. 

See More

Try Our College Algebra Course. For FREE.

Sophia’s self-paced online courses are a great way to save time and money as you earn credits eligible for transfer to many different colleges and universities.*

Begin Free Trial
No credit card required

29 Sophia partners guarantee credit transfer.

310 Institutions have accepted or given pre-approval for credit transfer.

* The American Council on Education's College Credit Recommendation Service (ACE Credit®) has evaluated and recommended college credit for 27 of Sophia’s online courses. Many different colleges and universities consider ACE CREDIT recommendations in determining the applicability to their course and degree programs.


What's Covered

Calculating Diagonals

Calculating Diagonals

A diagonal connects two non-adjacent vertices in an enclosed shape.  Below is an example of a diagonal of a rectangle:

Notice that the diagonal of the rectangle connects two opposite corners.  It also creates two congruent triangles.  Congruent means of equal measure, so the two triangles are the same size, and take up the same amount of space. We should also point out that the triangles are right triangles, because one of their angles is a 90 degree angle (taken from the 90 degree angles of the rectangle).

Let's take a closer look at the rectangle and the two triangles that the diagonal created.  The sides of the rectangle correspond to the vertical and horizontal legs of the right triangle.  What about the diagonal?  We can refer to the diagonal as the hypotenuse of the right triangle.  (The hypotenuse is always opposite of the right angle). 

To calculate the length of the diagonal, we can use the Pythagorean Theorem to calculate the length of the hypotenuse.   The Pythagorean Theorem uses the side lengths of the other legs of the right triangle in order to find the length of the hypotenuse:


  • The Pythagorean Theorem
  • a squared plus b squared equals c squared
c equals square root of a squared plus b squared end root

So we if take the sum of the squares of the side lengths, this equals the square of the hypotenuse leg.  We'll just need to take the square root of the sum in order to express the length of the hypotenuse.  Let's look at an example:

We can substitute 3.5 ft and 8 ft into a and b, respectively, and apply the Pythagorean Theorem:

Another method to solving is to rewrite the Pythagorean Theorem, isolating c on one side of the equation.  Then, we can substitute a and b into the equation, and calculate the length of the diagonal.  This is shown in the example below:

Formulas to Know
Pythagorean Theorem

a squared plus b squared equals c squared

c equals square root of a squared plus b squared end root