How to Find the Length of a Diagonal

Diagonals of a square or a rectangle can help you to find the area of a rectangle or square and give you the distance between two corners of a rectangle or square. However, you will need to calculate the actual length of the diagonal before you can use it. Because all corners of rectangles and squares are right angles, you can use the Pythagorean Theorem to find the diagonal.

Instructions

1. Measure the two adjacent sides (length and width) of the rectangle or square, "a" and "b," respectively. For example, assume the length of a is 7 and the width of b is 4.

2. Draw the diagonal in the rectangle from one corner to the corner opposite it. Give the diagonal the variable name "c."

3. Insert the variables into the Pythagorean Theorem, which is "a^2 + b^2 = c^2." With 7 and 4, 49 plus 16 equals 65.

4. Calculate the square root of the result to determine c. In this example, the square root of 65 is approximately 8.06.

References

• UG Athens: The Pythagorean Theorem
• Math is Fun: Quadrilaterals -- Square, Rectangle, Rhombus, Trapezoid, Parallelogram