The equation used to compute the UCL on the mean when the n data are assumed to be normally distributed is the usual formula (Gilbert 1987, Equation 11.6, page 139): $$UCL = \bar{x}+t_{1-\alpha, n-1}\frac{s}{\sqrt{n}}$$
Where:
\(\bar{x}\) is the sample mean, \(s\) is the sample standard deviation and \(n\) is the number of data points (see the Summary Data Sub-page for these values),
\(t_{1-\alpha, n-1}\) is the \(100(1-\alpha)\) percentile of the t distribution with n-1 degrees of freedom (Gilbert 1987, Table A2, page 255).
Provided courtesy of ProUCL.
Gilbert, R.O. 1987. Statistical Methods for Environmental Pollution Monitoring, Wiley, NY.
ProUCL. 2004. ProUCL Version 3.0 User Guide April 2004. Available for download from http://www.epa.gov/nerlesd1/tsc/tsc.htm