There is a strong business case for automotive interfaces to undergo usability testing throughout their product development life cycle. System Usability Scale (SUS) is a simple and standard measure of usability. To meet the timing needs for product development, usability testing needs to be performed in a quick, cost effective manner. Hence the required sample size of participants for a usability study is one of the critical factors. To determine an acceptable sample size, a Monte Carlo simulation using SUS scores from eleven different in-vehicle automotive interface usability studies was used to create 500,000 sub-samples of different sample sizes. The percentage of sub-samples with mean scores within the confidence interval of the population mean was calculated. At a sub-sample size of thirty five, 95% of the sub-samples have a mean SUS score within the 95% confidence interval of the population mean.