The Lewis formula and nomogram, published in widely distributed textbooks, are used to calculate power output from vertical jump-and- reach distance and body weight. Despite the fact that the method has never been supported by a refereed journal publication and the texts never revealed whether peak or average power was being estimated, the test has become increasingly used by physical educators, coaches and researchers. Theoretical analysis has raised questions about the formula's validity, In order to evaluate the test, Lewis formula derived power output was compared to peak power and average power generated by 17 male subjects jumping vertically from a computer-intefaced force plate. Lewis power, peak power and average power (mean ± SD) were 1,107 ± 144, 3,767 ± 686 and 1,325 ± 341 watt (W), respectively. The low standard deviation for Lewis power suggests the formula minimizes individual differences. Lewis power respectively correlated 0.83 and 0.72 with peak and average power but underestimated them by 70.1 ± 3.5 percent and 12.4 ± 18 percent. The following regression-derived equations whose respective r's were 0.88 and 0.73 better estimate peak and average power then does the Lewis formula and produce respective standard deviations of 603 watts, which more closely reflect actual inter-subject variability: Peak power (W) = 61.9 · jump height (cm) + 36.0 body mass (kg) + 1,822. Average power (W) = 21.2 · jump height (cm) + 23.0 body mass (kg) − 1,393. The Lewis formula does not provide accurate estimates of either peak or average power produced by the muscles. While the force-platform technique remains the methos of choice for precise jumping-power determiations in the laboratory, the above equaltions can be used in conjunction with other tests to screen applicants for athletic teams and physically demanding jobs, and to monitor progress among participants in physical training programs.