How Do You Calculate Weight-Loss Percentage?

To determine weight-loss percentage, the current weight is subtracted from the original weight. Then, that answer is multiplied by 100. That answer is divided by the original weight. For example, an individual who weighs 150 pounds and loses 15 pounds uses the following calculation: 150 pounds - 135 pounds = 15 x 100 = 1,500. 1,500 divided by 150, or the original weight, yields the answer of 10 percent.

While the weight-loss percentage is used for figuring the percentage of weight lost on a diet plan, it can also be used in cases where there is an unplanned loss of weight. According to the Association of Nutrition and Foodservice Professionals, an unplanned loss of weight factors in such criteria as the amount of weight lost, when weight loss began, and a person's recent health or medical history.