The following formula is involved in the calculation of Zscores when the population mean and standard deviation are known:

Standard normal score (Zscore):
where x is the raw score to be standardized, μ is the population mean, and σ is the population standard deviation.

The following references can be used to cite this zscore calculator:
 Soper, D.S. (2015). Zscore Calculator [Software]. Available from http://www.danielsoper.com/statcalc 
 Cohen, J., Cohen, P., West, S.G., and Aiken, L.S. (2003). Applied Multiple Regression/Correlation Analysis for the Behavioral Sciences (3rd edition). Mahwah, NJ: Lawrence Earlbaum Associates. 