Body mass index (BMI) is derived from Quetelet's equation. Federal guidelines define a healthy BMI range of 18.5 to 24.9. Unfortunately, few physicians can quickly convert these BMI recommendations into corresponding weights, because Quetelet's formula is tedious to compute. Two new linear equations (one metric, the other Avoirdupois) are introduced to allow easy calculation of the weight limits above which adults would be considered unhealthy. From these formulas, simple equations to calculate lower weight limits, obese weight limits, morbidly obese weight limits, and BMI follow. In fact, for any given height, it is possible to reproduce the entire BMI table, independent of Quetelet's formula. An isolated BMI value, under the current system, not only fails to convey a quantitative sense of obesity, but is also expressed in confusing units. It is proposed that the current BMI system be modified to yield a more pertinent BMI Prime system, which relates a patient's actual weight to his upper weight limit.