Rectangle Calculator
Area, perimeter and diagonal from length and width.
The formulas
Area = length × width. Perimeter = 2(length + width). The diagonal comes from the Pythagorean theorem: √(l² + w²). An 8 by 5 rectangle has area 40, perimeter 26 and a diagonal of about 9.43.
The diagonal check
Carpenters and framers verify a rectangle is square by measuring both diagonals: they're equal exactly when the corners are right angles. For a quick site check, the 3-4-5 triangle does the same job on one corner.