An experimental investigation supported by numerical simulations to determine the performance parameters of dual booster mirror box type Soar cookers having three different length to width ratios, has been carried out in this paper. In addition, the most optimal tilt angles for each of the two booster mirrors, for each day of the year, have been determined for tracking free operation. The numerical investigation has been carried out for a 25 latitude location for all days of the year to determine the best power collection capability of a dual booster mirror solar cooker during 6 h of the most desirable cooking period, ranging from 3 h before to 3 h after the solar noon. Three solar cookers selected for this investigation have length to width ratios of 1.33, 2.66 and 3.99, respectively. Tracking free experiments are conducted for three days to compare the simultaneous performance of each cooker with proportionate water load. Finally, the analysis of the experimental results has been carried out, including the determination of the first and second figures of merit, cooking power, exergy efficiency and the quality factor, in each case. Results indicate that tracking free performance of fully loaded box type solar cookers with two booster mirrors inclined at appropriate angles, becomes optimal with an aspect ratio of 2.66. (C) 2015 Elsevier Ltd. All rights reserved.