Find the area, perimeter, and diagonal of a rectangle. Enter the length and width.
Usage Tip
For a fixed perimeter, area is largest when the rectangle is closest to a square.
diagonal = √(length² + width²)
The diagonal splits it into two right triangles, so the Pythagorean theorem gives its length.
The diagonal connects opposite corners.
A rectangle with equal sides is a square.
A rectangle has opposite sides equal and four right angles. The area is the length times the width, the perimeter is twice the sum of the length and width, and the diagonal comes from the Pythagorean theorem as the square root of the length squared plus the width squared.
Two rectangles can have the same perimeter and very different areas, and for a fixed perimeter the area is largest when the rectangle is a square. A practical way to check that a frame or foundation is truly rectangular is to measure both diagonals, which must be equal. Keep all measurements in the same unit so the area comes out in the matching square unit.