Perimeter of a Square Calculator P = 4a. Side Perimeter 20 Step-by-step with your numbers: 1. Values used: 2. Side = 5 3. Perimeter = 20 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! Square perimeter. FAQ Diagonal? d = a√2. Related calculators Square Calculator Find a square's area, perimeter and diagonal. Perimeter Calculator Rectangle perimeter. Perimeter of a Rectangle Calculator P = 2(L+W).