This paper describes a case study where voltage sags indices are estimated using Monte Carlo approach combined with ATP (Alternative Transient Program) and short-circuit calculation program. Voltage sag magnitude and frequency are used to evaluate the correlation between both programs. The results indicate that time-domain simulation and short-circuit calculation gives similar voltage sag indices. Considering the high correlation between the results, short-circuit calculation programs are preferable over the time-domain simulation tools as the modelling for time-domain simulation is more complex, time consuming, and rarely covers the whole network.