The aim of this study was to develop a rapid indirect method to determine an individual's maximal strength or 1RM (one repetition maximum) in untrained subjects during half squat exercise. 105 physically active young subjects (87 men, 18 women) performed a submaximal and a maximal load test during half squat exercises on a Smith machine. In the submaximal test, subjects completed three repetitions with a load equivalent to body weight. The velocity and power of barbell displacement were recorded during the upward movement from 90° of knee flexion. All repetitions were performed at maximum velocity. In a subsequent 1-2 RM test, the 1RM for the exercise was calculated. The variables load and mean velocity (Vmean) were used to construct an adjusted 1RM prediction model, which was capable of estimating the 1RM with an accuracy of 58% (Fexp=72.82; 2; 102 df; p<0.001). Our results indicate a good correlation between the mean displacement velocity of a load equivalent to body weight and 1RM. This relationship enables a safe and fast estimation of 1RM values in half squat exercise [1RM= -61.93 + (121.92·Vmean) + (1.74·load)] and provides valuable information to untrained subjects who are starting resistance-training programs.