Diagonal of a Rectangle Calculator d = √(L²+W²). Length Width Diagonal 10 Step-by-step with your numbers: 1. Values used: 2. Length = 8 3. Width = 6 4. Diagonal = 10 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! Diagonal of a rectangle. FAQ Square? d = a√2. Related calculators Rectangle Area Calculator Find the area and perimeter of a rectangle. Pythagorean Theorem Calculator Find the hypotenuse of a right triangle from the two legs. Diagonal of a Square Calculator d = a√2.