NewByte has been developing flight simulation tools for the PC platform since '88. Here is a free tool to calculate standard atmospheric conditions (ISA) at a given geometric or pressure altitude and ambient temperature. At those conditions, it is possible to convert between Mach number, Calibrated air speed (CAS) and True air speed (TAS). Geometric altitude is the true altitude above sea level while pressure altitude is the expected reading on an aircraft altimeter set to standard QNH.