For printing you should ask a printer.
For people who are receiving it from the Internet you will not know which paper they are using but this would not be relevant anyway because only a few will staple them anyway.
Your question isn't entirely clear. Paper thickness is generally measured rather than calculated because different grades of paper have different densities, but there are some charts I've seen that purport to give thicknesses for various weights of different paper types. You would get more accurate results, I think, by checking with the paper manufacturer or using a caliper to measure a stack of 10 sheets.
By web publication do you mean something published on the the Internet, or something printed on a web-fed press? My guess is the latter as it makes more sense for needing to calculate the "creep" factor.