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.
The ANFP adds that health care professionals must assess dietary changes, activity levels, behavioral changes, and the kinds of herbs or medicines an individual may be ingesting if weight loss is unexpected. Any unplanned loss of weight greater than 5 percent within a 30-day period is considered severe, as of 2014.
The National Heart, Lung and Blood Institute points out that, besides one's weight-loss percentage, the BMI, or body mass index, can assist in projecting one's risk for disease, especially if he is currently overweight. BMIs can be obtained by the use of online calculators and are based on one's weight and height.