The following formula is involved in the calculation of confidence intervals for a population mean when the population standard deviation is not known:

Population mean confidence interval (when the population standard deviation is unknown):
where x is the sample mean, t is a value from the tdistribution, α is the desired confidence level, v is the degrees of freedom, s is the sample standard deviation, and n is the sample size.

The following references can be used to cite this confidence interval calculator for the population mean:
 Soper, D.S. (2015). Confidence Interval Calculator for the Population Mean [Software]. Available from http://www.danielsoper.com/statcalc 
 Keller, G. (2011). Statistics for Management and Economics (9th edition). Mason, OH: SouthWestern Cengage. 