Is 20 a perfect square number? A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 20 is about 4.472. Thus, the square root of 20 is not an integer, and therefore 20 is not a square number.
A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 21 is about 4.583. Thus, the square root of 21 is not an integer, and therefore 21 is not a square number.
Therefore neither one is a perfect square.