Diagonal of a Square Calculator d = a√2. Side Diagonal 7.071 Step-by-step with your numbers: 1. Values used: 2. Side = 5 3. Diagonal = 7.071 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! Square diagonal. FAQ From diagonal to side? a = d/√2. Related calculators Square Calculator Find a square's area, perimeter and diagonal. Pythagorean Theorem Calculator Find the hypotenuse of a right triangle from the two legs.