How to Calculate the Diagonal

Calculate diagonal lengths with the Pythagorean Theorem.

Calculating the shortest distance between two corners will find you looking for the diagonal. Use the Pythagorean Theorem to determine this distance from one corner of a square or rectangle to another. This theory states that the square root of the sum of the squares of the two legs equals the length of the diagonal. Put this theory into practice to find the required length whether you're looking for the diagonal of a television screen, of a window or wall, or a rectangle for a math project.

Things You'll Need

• Length of rectangle or square
• Width of rectangle or square
• Calculator

Instructions

1. Multiply the width by itself to find that side squared. For instance, in a rectangle with a width of 3 inches, you would have 3 x 3 = 9.

2. Repeat squaring with the length. In the example, suppose that the rectangle with a width of 3 inches has a length of 4 inches: 4 x 4 = 16.

3. Add the squared width and length to each other. For the example: 9 + 16 = 25

4. Take the square root of this sum to find the length of the diagonal. In the example:

   √25 = 5 inches.

References

• National Council of Teachers of Mathematics: Corner to Corner Lesson
• Centre for Innovation in Mathematics Teaching: Calculating the Hypotenuse