BMI Calculator

You can use the Body Mass Index (BMI) calculator to compute your BMI value and the corresponding weight status based on your age. The calculator uses the metric system of units in the "Metric Units" tab by default. You can select the U.S. system of units on the “US Units” tab or use the unit converter on the “Other Units” tab. The Ponderal Index is computed together with the BMI.

BMI Overview

BMI measures an individual’s degree of fatness or leanness regarding weight and height. It can also provide information about the tissue mass in a person’s body. You can use it to assess the proportionality of the weight-to-height ratio.

The results of a BMI calculation determine whether a person is obese, overweight, average weight, or underweight. BMI ranges are sometimes broken down into subgroups, such as very underweight or seriously obese. These BMI ranges vary by factors such as region and age.

Obesity or malnutrition can have severe consequences on one’s health. And while BMI is an imperfect indicator of healthy body weight, it is a helpful indicator of whether additional testing or action is needed. For further information on BMI classifications and calculations, see the table below.

Adult BMI table

This body weight classification is recommended by the World Health Organization (WHO). It is based on BMI values for adults (18+).

Adult BMI charts

This graph shows the different BMI classifications based on statistics from the World Health Organization. The solid lines show major subdivisions, while the dashed lines represent minor subdivisions.

BMI table for children and teenagers aged 2-20

The CDC (Centers for Disease Control and Prevention) recommends using BMI to assess the health of children and adolescents aged two to twenty.

BMI chart for children and teenagers aged 2-20

The CDC has created graphics showing a rise in BMI regarding age percentile growth.

Boys’ Chart

Girls’ Chart

Risk factors of being overweight

Obesity raises the chance of significant health issues, and the Centers for Disease Control and Prevention lists the following risk factors:

• Increased LDL cholesterol (“bad cholesterol”) levels, lower HDL cholesterol (“good cholesterol”) levels, and higher levels of triglycerides;
• Elevated Blood pressure;
• Type II diabetes;
• Coronary artery disease (CAD);
• Gallbladder dysfunction;
• Stroke;
• Breathing issues and sleep apnea;
• Osteoarthritis (a joint disease caused by the destruction of articular cartilage);
• The poor overall quality of life;
• Certain forms of cancer (endometrial, colon, breast, liver, gallbladder, kidney, etc.);
• Mental disorders such as clinical depression, anxiety, and other;
• Body aches and pains, as well as difficulty completing physical responsibilities;
• Compared to individuals with healthy BMI, people with a much higher BMI demonstrate a significantly greater chance of mortality.

Being overweight may have several adverse and sometimes devastating effects on the body. Hence, it is essential to have a BMI score below 25 kg/m², which shows a healthy weight. However, it is advisable to consult a physician to determine if any lifestyle changes are necessary to maintain better health.

The risks of being underweight

Risk factors associated with being underweight include:

• Malnutrition, anemia, and vitamin deficiency are notable issues responsible for reduced blood transportation abilities;
• Osteoporotic conditions characterized by loss of bone mass and increased likelihood of fractures;
• Immune system dysfunction;
• Growth and developmental issues, notably in adolescents and children;
• Some complications after surgery;
• Women may have reproductive health issues because of hormone imbalances, which may cause interruptions in their menstrual cycles. Women who are overweight or obese are more likely to miscarry in the first trimester of pregnancy;
• A relatively higher mortality rate.

Weight loss may indicate a more serious underlying disease or illness with its risks, as seen in anorexia nervosa. If you suspect that you or your acquaintances are struggling with weight for unclear reasons, consult a doctor.

BMI Limitations

Despite its extensive use as a reliable predictor of healthy body weight, the body mass index (BMI) has several limitations. These include not considering the entire body makeup. The diversity of body types and the distribution of fat, muscle, and bone mass make it necessary to use BMI with other indicators.

In adults:

Online calculators provide a general estimate of BMI based on height and weight, but they don't account for specific factors that can influence its accuracy. BMI doesn't differentiate between weight from muscle and weight from fat, so it's not a direct measure of body fat. Additionally, various factors, such as age, gender, muscle mass, levels of exercise, and others, can influence BMI readings and their interpretations.

Let’s take the example of a passive older person who sits or lies down most of his life. He has a significant excess fat, but he is not overweight overall. In terms of objective health, this person may not be relatively healthy, but in terms of BMI, he might be normal.

The opposite is true of bodybuilders, who have powerful muscles. Muscle is heavier than fat, yet it takes up less volume. Regarding BMI, many bodybuilders may be either near the upper limit of normal weight or be considered overweight. At the same time, they will be in excellent health. A person can look more trim and still be heavier with a solid body mass.

CDC (Centers for Disease Control and Prevention) data shows that:

• At the same BMI level, women tend to have more body fat than men;
• Older people tend to have more body fat than younger people with the same BMI;
• Athletes undergoing intense training may have a higher BMI because of significant muscle mass.

In adolescents and children:

The same variables responsible for limiting the efficacy of BMI in adults also apply to adolescents and even children. Height and the level of sexual development may influence the BMI and body fat percentage.

In obese children, BMI is a stronger predictor of excess body fat than in overweight children. Their BMI can be caused by a high fat- or fat-free mass (all body components other than fat, such as water, organs, muscle, and so on). In lean children, the difference in BMI may also be due to fat-free mass.

For the general population, BMI can be a useful indicator for potential health risks associated with being underweight, overweight, or obese. However, due to its limitations, BMI should be used as one of several tools for assessing an individual's health and body weight, always in conjunction with other assessments and personalized health evaluations.

BMI Formula

Below are the equations used to calculate BMI in the International System of Units (SI) and the United States General System (USC). They use a person who is 5 feet 10 inches tall and weighs 160 pounds as an example.

USC Units:

$$BMI=703×\frac{mass(lbs)}{height^{2}(in)}=703×\frac{160}{70^{2}}=22.96\frac{kg}{m^{2}}$$

SI, Metric Units:

$$BMI={\frac{mass(kg)}{height^{2}(m)}=\frac{72.57}{1.78^{2}}=22.90\frac{kg}{m^{2}}}$$

Ponderal Index

The Ponderal Index (PI) examines a person’s adiposity proportionally to weight and height. The main difference between the BMI and the Ponderal Index is that the Ponderal Index formula (below) places measurements in the cube, not the square.

Although BMI can be a useful tool when studying large populations, it is not reliable for determining thinness or obesity in individuals. PI is more reliable when used on very short or tall people. The BMI is more likely to identify unusually high or low quantities of body fat in people with extreme weight and height measurements.

Below is the equation for calculating a person's Ponderal Index. It uses a 5 feet 10 inches tall, 160 pound person as an example:

USC Units:

$$PI={\frac{height(in)}{\sqrt[3]{mass(lbs)}}=\frac{70}{{\sqrt[3]{160}}}=12.89\frac{in}{\sqrt[3]{lbs}}}$$

SI, Metric Units:

$$PI={\frac{mass(kg)}{height^{3}(m)}=\frac{72.57}{1.78^{3}}}=12.87\frac{kg}{m^{3}}$$