/*
================
SquareRootFloat
================
*/
float SquareRootFloat(float number) {
long i;
float x, y;
const float f = 1.5F;

x = number * 0.5F;
y = number;
i = * ( long * ) &y;
i = 0x5f3759df - ( i >> 1 );
y = * ( float * ) &i;
y = y * ( f - ( x * y * y ) );
y = y * ( f - ( x * y * y ) );
return number * y;
}

Original article from code maestro