The most common way to determine the normal weight for age and height is by calculating body mass index, states the Centers for Disease Control and Prevention. There are a number BMI calculators available online, including one on the Medical News Today website.
Body mass index is calculated by dividing a person's weight in kilograms by the square of his height in meters. A BMI between 18.5 and 24.9 for adults over 20 years of age is considered normal, according to the CDC. The differences in body fat increase among children and teens; therefore, age and sex are stronger factors for calculating BMI in people under 20. Although BMI can be used as an acceptable guide, many experts criticize the formula as flawed. A BMI does not take a person's bone density or muscle mass into account. It does not measure fat directly or diagnose impending health risks associated with weight.
A number of other recommended methods for determining an ideal weight are available, as listed on Medical News Today. Waist-to-hip ratio is calculated by dividing the lowest waist circumference by the widest part of the hips. Waist-to-height ratio charts optimal girth based on height. Most gyms and doctors' offices offer a more scientific approach for calculating body fat percentage, using techniques such as dual-energy X-ray absorptiometry and bioelectrical impedance analysis.