The square root of 2, also known as Pythagoras' constant, is the positive real number that, when multiplied by itself, gives the number 2.
Geometrically the square root of 2 is the length of a diagonal across a square with sides of one unit of length; this follows from the Pythagorean theorem. It was probably the first number known to be irrational. Its numerical value truncated to 65 decimal places is:
The quick approximation 99/70 for the square root of two is most frequently used. Despite having a denominator of only 70, it differs from the correct value by less than 1/10,000.