what is a diagonal
How to Find the Measurement of the Diagonal Inside a Rectangle. Three Methods: Using the Length and the Width Using the Area and Perimeter Using the Area and Relational Side Lengths Community Q&A. A diagonal is a straight line that connects one corner of a rectangle to the opposite corner. A rectangle has two diagonals, and each is the same length. If you know side lengths of the rectangle, you can easily find the length of the diagonal using the Pythagorean Theorem, since a diagonal divides a rectangle into two right triangles.