BMI Calculator

The Body Mass Index (BMI) Calculator can be used to determine the BMI value and accompanying weight status while taking age into account. Use the “Metric Units” tab for the International System of Units, or the “Other Units” tab to convert units to US or metric units. It should be noted that the calculator calculates both the Ponderal Index and the BMI, which are addressed in depth below.

BMI Introduction

BMI is a measurement of a person’s leanness or corpulence based on their height and weight, designed to assess tissue mass.  It is commonly used as a general indicator of whether a person’s body weight is appropriate for their height.  Specifically, the result derived from the BMI calculation is used to determine whether a person is underweight, normal weight, overweight, or obese, depending on which category the value falls into. These BMI values vary depending on geography and age, and are sometimes further classified as severely underweight or very severely obese.  Being overweight or underweight can have serious health consequences, therefore while BMI is an inaccurate measure of healthy body weight, it can help determine whether further testing or action is necessary.  The table below shows the different BMI categories used by the calculator.

BMI table for adults

The World Health Organization (WHO) recommends an adult’s body weight based on BMI readings. It is suitable for both men and women aged 20 and up.

BMI chart for adults

This graph depicts BMI classifications based on World Health Organization statistics. The dashed lines reflect subcategories of a major classification.

BMI table for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) recommends BMI categorization for children and adolescents aged 2 to 20.

BMI chart for children and teens, age 2-20

BMI-for-age percentile growth charts provided by the Centers for Disease Control and Prevention (CDC).

Risks associated with being overweight

Being overweight raises the risk of a variety of major diseases and health problems. According to the Centers for Disease Control and Prevention (CDC), the following dangers exist:

High blood pressure

High amounts of LDL cholesterol, which is usually regarded “bad cholesterol,” lower levels of HDL cholesterol, which is considered healthy cholesterol in moderation, and high levels of triglycerides.

Type II diabetes

Coronary heart disease

Stroke

Gallbladder disease

Osteoarthritis, a type of joint disease caused by breakdown of joint cartilage

Sleep apnea and breathing problems

Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)

Low quality of life

Mental disorders like clinical depression, anxiety, and others

Body discomfort and trouble performing specific physical duties

In general, a higher risk of mortality than those with a healthy BMI.

As can be seen from the list above, being overweight can lead to a variety of bad, and sometimes fatal, results. In general, a person should attempt to keep their BMI below 25 kg/m2, but they should also visit their doctor to see if they need to make any lifestyle adjustments to improve their health.

Risks associated with being underweight

Being underweight has its own related hazards, as noted below:

Malnutrition, vitamin deficiencies, anemia (lowered ability to carry blood vessels)

Osteoporosis, a disease that causes bone weakness, increasing the risk of breaking a bone

A decrease in immune function

Growth and development issues, particularly in children and teenagers

Female reproductive difficulties may arise as a result of hormonal abnormalities that disturb the menstrual cycle. Underweight women are also more likely to miscarry in the first trimester.

Potential complications as a result of surgery

Generally, an increased risk of mortality compared to those with a healthy BMI

In other situations, being underweight may indicate an underlying ailment or disease, such as anorexia nervosa, which carries its own set of hazards. Consult your doctor if you suspect you or someone you know is underweight, especially if the reason does not appear clear.

Limitations of BMI

Although BMI is a popular and effective predictor of healthy body weight, it does have some limits. BMI is merely an estimate and does not take body composition into account. Due to the broad range of body types and the distribution of muscle, bone mass, and fat, BMI should be used in conjunction with other metrics rather than as the single tool for establishing a person’s healthy body weight.

In adults:

BMI is not completely accurate because it measures extra body weight rather than excess body fat.  BMI is also determined by age, gender, ethnicity, muscle mass, body fat, and activity level, among other things.  For example, an elderly person who is deemed a healthy weight but is entirely sedentary in their everyday life may have considerable quantities of excess body fat despite not being overweight.  This would be regarded unhealthy, whereas a younger person with a higher muscle composition and the same BMI would be deemed healthy. Athletes, particularly bodybuilders, who are deemed overweight because muscle weighs more than fat, may actually be at a healthy weight for their body composition.  Generally, according to the CDC:

Older persons typically have greater body fat than younger adults with the same BMI.

Women tend to have more body fat than men for an equivalent BMI.

Muscular people and highly trained athletes may have higher BMIs due to their increased muscle mass.

In children and adolescents:

The same criteria that restrict the effectiveness of BMI in adults also apply to children and adolescents. Height and sexual maturation stage can also influence children’s BMI and body fat. BMI is a better predictor of excess body fat in obese children than it is in overweight children, whose BMI could be due to increased levels of either fat or fat-free mass (all body components except fat, which include water, organs, muscle, and so on). In slim children, the difference in BMI may also be related to fat-free mass.

That being said, BMI is a good predictor of body fat for 90-95% of the population and can be used in conjunction with other measurements to assist estimate an individual’s healthy weight.

BMI formula

The following formulae are used to calculate BMI in the International System of Units (SI) and the US Customary System (USC), using a 5’10”, 160-pound individual as an example:

BMI formula

The BMI prime is the ratio of a person’s measured BMI to the top range of BMI considered “normal” by organizations such as the WHO and the CDC. Though it may vary in some countries, such as those in Asia, the upper limit, referred to as BMIupper, is 25 kg/m2.

Because BMI prime is a ratio of two BMI values, it has no dimensions. A person with a BMI prime of less than 0.74 is considered underweight; 0.74 to 1 is considered normal; greater than 1 is considered overweight; and greater than 1.2 is considered obese. The following table displays a person’s weight classification depending on their BMI prime:

BMI prime allows us to quickly determine how much a person’s BMI deviates from the highest limit of BMI that is deemed normal. It also enables comparisons between groups of people with varying top BMI limitations.

Ponderal Index

The Ponderal Index (PI) is similar to BMI in that it assesses a person’s leanness or corpulence based on height and weight.  The key difference between the PI and BMI is that the height is cubed rather than squared in the formula (seen below).  While BMI can be useful when analyzing large groups, it is unreliable for evaluating leanness or corpulence in individuals. Although the PI and BMI have comparable considerations, the PI is more reliable for use with very tall or short persons, whereas BMI tends to report unusually high or low body fat levels for those at the extremes of the height and weight continuum.  The equation for computing an individual’s PI via USC is provided below, using a 5’10”, 160-pound individual as an example: