Maybe I should clarify my point then. Taylor series is the way pretty much every system (calculator, computer, cell phone, etc) computes a square root. You can look into the c stadard library and find the the sqrt funtion is implemented with a taylor series. This is because it is computationally efficient and provides a predictable level of accuracy. The estimation algorithm that every other person on this thread has mentioned is really only used by people wanting to do a manual computation, or as an exercise in an introductory computer science class.