This sample size calculator helps you to determine number of the sampling/observation required for certain measurement according to the requirements.
When it comes to statistics, information is inferred usually about any given population by performing studies on a finite set of individuals coming from the population. This is called population sampling and it works on the assumption that characteristics shown by the sample represent those of entire population. Here it's assumed that there exists some population for which a proportion p in that population can be distinguished from the rest 1-p in one way or another. For instance, this p proportion of population may have brown colored hair whereas the rest 1-p population has blond, red, black, etc. So, for estimating p from that population n individuals are taken as sample and sample proportion p̂ is calculated for the sampled individuals having brown hair. To be unfortunate, unless there is sampling done on entire population, this estimate proportion p̂ will not likely be equal to true value p because p̂ can have sampling noise depending on individuals being sampled. The sampling statistics, however, may be used in calculation of confidence intervals that indicate how close these estimates are to the actual value.