The errors of calculation of the availability may bring as a consequence mistakes in the decision making in operational systems management. For this reason, the objective of this paper is to evaluate the efficacy of the current equations for the calculation of the availability of series and parallel systems. For that purpose, the non-manufactured production calculated by these equations was compared with the real non-manufactured production which is a function of the down time of each component. These calculations constituted the deterministic model used in the simulations through Monte Carlo method, for which the statistical software R was used. The results were evaluated by the Kolmogorov Smirnov normality test with the modification of Lilliefors. The comparison of means to evaluate the efficacy of the current equations were performed using the t statistic; for this the data were previously approximated to the normal distribution by the central limit theorem.