Get Fit Lab - For Your Health and Fitness

Body mass index (BMI) is a quick, easy and useful tool to estimate healthy weight.

While not actually measuring the percentage of body fat, it is used to calculate whether a person is underweight, a healthy weight, overweight or obese for their height. It allows for natural variations in body shape, giving a healthy weight range for a particular height.

The calculation divides the adult’s weight in kilograms by their height in meters squared.

BMI Calculator

Numerous tools are available for calculating BMI. But we prefer this tool from the NHS and can calculate BMI for children.

NHS Healthy Weight Calculator

Note: Most widely available BMI calculators are for adults only.

BMI Categories

• BMI 16.5 or below: Severely underweight
• BMI 16.5-18.4: Underweight
• BMI 18.5-24.9: Healthy weight
• BMI 25-29.9: Overweight
• BMI 30-34.9: Obesity
• BMI 35 or higher: Extremely obese

Children’s BMI Results

For children, BMI is interpreted differently.

While the calculation is the same as for adults, the age and sex of the child are also taken into account. Instead of set ranges (as shown in the BMI Categories above), children’s BMI uses percentiles to assign whether a BMI is within a healthy range according to height, weight, age and sex.

By comparing the values to other children of the same age and sex, an overweight BMI means it is higher than average for a child of their age and sex.

Limitations of BMI

BMI is a simplistic calculation dependent on weight and height only. Therefore it does not take into account many factors, such as, muscle and bone mass. While widely used and respected, the medical community generally acknowledges these shortcomings.

Individuals who may be given an inaccurate BMI are athletes, children, the elderly, the infirm and naturally endomorphic or ectomorphic people ( people who do not have a medium frame).

According to their BMI, children and the elderly tend to be “underweight” because of the differences in bone density. To factor in the proportion of bone to total weight, their BMI should be adjusted downward or the use of a special scale for determining BMI (as described in Children’s BMI Results above).

Athletes tend to be “overweight” or “obese” according to their BMI. Because muscle weighs more than fat, a modified version is sometimes used to adjust the ranges upward. Alternatively, there are better methods for determining the body composition of athletes. For instance, body fat can be calculated using techniques, such skinfold measurements or underwater weighing.

Other BMI inaccuracies can occur. For instance, a tall person with a small frame and excess fat around the waist may show a normal BMI. Therefore, if you have any questions about your results, consult your doctor who may want to employ another measuring device to double-check your findings.