Several methods to determine porosity downhole are available on the market. This study compares nuclear measurement methods, in particular neutron porosity and gamma density measurement, to NMR measurement. Thus it is very important to have a good knowledge of influencing factors of the measurement principles. Furthermore one must be aware, that error is dependent on: the primary measurement, the correction that is applied, the conversion of the calibration measurand to porosity and the conversion of the downhole measurand to porosity with the aid of mineralogy and fluid type. All three measurement methods are subjected to diverse output data having diverse influencing factors. To perform an adequate comparison of the final porosity values gained, some limitations and estimates have to be taken into account. The error estimation of nuclear tools is performed by API calibration measurements. In order to convert the measurand into porosity and to include environmental effects to error consideration an error propagation is performed. Concerning NMR the error has been characterized based on simulations with respect to magnetic field behavior depending on temperature and frequency, and regarding the influence of inversion. Summarized, under certain restrictions, the total errors of all three methods compared are in a similar range. The main advantage of NMR is that it is not limited by mandatory input data, as for example matrix and fluid densities in gamma density measurement.