# Is sample standard deviation a valid approximation of population standard deviation for z-scores?

Given a Normal distributions for the population and my sample, a population mean of mu, and a sample standard deviation of s, if I want to find the area under the population curve from x to infinity, is it valid to say that s = sigma (population standard deviation) here? In other words, can I say that z=(x-mu)/s=(x-mu)/sigma or would I maybe have to calculate this as a t-score and use the degrees of freedom in the sample to find the area under the curve?