A critical analysis of the methodologies for evaluating the x-ray half value layer was conducted. Experimental data were fitted by using MATLAB curve fitting toolbox, to an exponential function. Half value layer and its uncertainties were determined directly from the parameter of the fitted function. Goodness of the fit was high in all cases. Uncertainties were less than 10%, usually about 6%. For the first time it was hypothesized and verified the dependence of the half value layer determination, from the number of the measured data. These discrepancies achieved sometimes 25%. The necessity of improving the standard procedures for half value layer determination was confirmed.