Enter a length and width to calculate a rectangle's area, perimeter, and diagonal.
How to Use the Rectangle Calculator
Enter the length and width. Area, perimeter, and the diagonal length all update immediately.
area = length × width
perimeter = 2 × (length + width)
diagonal = √(length² + width²)The diagonal comes straight from the Pythagorean theorem — a rectangle's diagonal always splits it into two right triangles, with the length and width as legs. That's a handy check on-site: if a doorframe or foundation is really square-cornered, its two diagonals should measure equal, and this is the number to compare them against.