Rectangle Area, Perimeter and Diagonal Solver

About this calculator

From a rectangle's length and width, this calculator gives all three core measurements at once: area = length × width, perimeter = 2(length + width), and diagonal = √(length² + width²) by the Pythagorean theorem.

It's a genuine three-in-one for anyone framing a room, cutting plywood or fabric, or checking a shipping box — the diagonal in particular is what carpenters use to square up a rectangular frame (equal diagonals confirm true 90° corners), while area and perimeter cover material and trim quantities.