We report our investigations on measuring the energy transfer upconversion (ETU) parameter in various neodymiumdoped laser crystals (YAG, YVO4, GdVO4, KGW, and YLF) via the z-scan technique. Starting with a simple two-level macro-parameter spatially dependent rate equation model we obtain a good correlation for Nd:YAG at different concentrations and crystal temperatures, however the other crystals illustrate significant deviation between simulation and measurement. Currently we attribute this difference to additional ion-ion interactions in the respective samples, for which a more detailed model is currently being considered. Of the tested materials Nd:YAG appears to have the lowest ETU macro parameter, at around 0.35 x10-16 cm3/s for a 0.6 at.% doping concentration, compared with nominally thrice this for 0.5 at% Nd:YLF and almost an order of magnitude higher for the 0.5 at.% vanadates (YVO4 and GdVO4). These values are significant for determining additional heat load in the respective gain media, especially when trying to increase the output power/energy from lasers employing these crystals, typically achieved by increasing the pump and cavity mode size.