BMI Calculator

BMI Calculator: The Body Mass Index (BMI) Calculator can be used to calculate BMI value and corresponding weight status while taking age into consideration. Use the “Metric Units” tab for the International System of Units or the “Other Units” tab to convert units into either US or metric units. Note that the calculator also computes the Ponderal Index in addition to BMI, both of which are discussed below in detail.

BMI introduction

BMI measures a person’s leanness or corpulence by considering their height and weight. Its purpose is to quantify tissue mass and serve as a general indicator of whether someone has a healthy body weight for their height. We use the value calculated from BMI to classify individuals as underweight, normal weight, overweight, or obese, depending on where their value falls within specific ranges. These BMI ranges can vary based on factors like region and age and can even include subcategories like severely underweight or very severely obese. Both being overweight and underweight can significantly impact one’s health. Therefore, although BMI isn’t a perfect measure of healthy body weight, it does provide a useful indication of whether further testing or action is necessary. Please consult the table below for the various BMI categories used by the calculator.

BMI table for adults

This is the World Health Organization’s (WHO) recommended body weight based on BMI values for adults. It is used for both men and women, age 20 or older.

Classification	BMI range – kg/m2
Severe Thinness	< 16
Moderate Thinness	16 – 17
Mild Thinness	17 – 18.5
Normal	18.5 – 25
Overweight	25 – 30
Obese Class I	30 – 35
Obese Class II	35 – 40
Obese Class III	> 40

BMI chart for adults

This is a graph of BMI categories based on the World Health Organization data. The dashed lines represent subdivisions within a major categorization.

BMI table for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) recommends BMI categorization for children and teens between age 2 and 20.

Category	Percentile Range
Underweight	<5%
Healthy weight	5% – 85%
At risk of overweight	85% – 95%
Overweight	>95%

BMI chart for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) BMI-for-age percentiles growth charts.Chart for boys
Chart for girls

Risks associated with being overweight

Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks, according to the Centers for Disease Control and Prevention (CDC):

• High blood pressure
• Higher levels of LDL cholesterol, which is widely considered “bad cholesterol,” lower levels of HDL cholesterol, considered to be good 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 illnesses such as clinical depression, anxiety, and others
• Body pains and difficulty with certain physical functions
• Generally, an increased risk of mortality compared to those with a healthy BMI

As can be seen from the list above, there are numerous negative, in some cases fatal, outcomes that may result from being overweight. Generally, a person should try to maintain a BMI below 25 kg/m², but ideally should consult their doctor to determine whether or not they need to make any changes to their lifestyle in order to be healthier.

Risks associated with being underweight

Being underweight has its own associated risks, listed 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
• Possible reproductive issues for women due to hormonal imbalances that can disrupt the menstrual cycle. Underweight women also have a higher chance of miscarriage 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 some cases, being underweight can be a sign of some underlying condition or disease such as anorexia nervosa, which has its own risks. Consult your doctor if you think you or someone you know is underweight, particularly if the reason for being underweight does not seem obvious.

Limitations of BMI

Although BMI is a widely used and useful indicator of healthy body weight, it does have its limitations. BMI is only an estimate that cannot take body composition into account. Due to a wide variety of body types as well as distribution of muscle, bone mass, and fat, BMI should be considered along with other measurements rather than being used as the sole method for determining a person’s healthy body weight.

In adults:

BMI cannot be fully accurate because it is a measure of excess body weight, rather than excess body fat.

Age, sex, ethnicity, muscle mass, body fat, and activity level, among other factors, play a significant role in influencing BMI. To illustrate, let’s take the example of an older individual who falls within the healthy weight range but leads a completely sedentary lifestyle. Despite their lower weight, they might have a substantial amount of excess body fat, which is considered unhealthy. In contrast, a younger person with a higher proportion of muscle mass at the same BMI would be deemed healthy.

For athletes, especially bodybuilders, who may register as overweight on the BMI scale due to the fact that muscle weighs more than fat, their weight may actually align with a healthy body composition. This underscores the importance of considering more than just the BMI number when assessing an individual’s health and fitness level. It’s crucial to take into account these additional factors to gain a comprehensive understanding of a person’s well-being.

Generally, according to the CDC:

• Older adults tend to have more body fat than younger adults with the same BMI.
• Women tend to have more body fat than men for an equivalent BMI.
• Muscular individuals and highly trained athletes may have higher BMIs due to large muscle mass.

In children and adolescents:

The same factors that limit the efficacy of BMI for adults can also apply to children and adolescents. Additionally, height and level of sexual maturation can influence BMI and body fat among children. BMI is a better indicator of excess body fat for obese children than it is for overweight children, whose BMI could be a result of increased levels of either fat or fat-free mass (all body components except for fat, which includes water, organs, muscle, etc.). In thin children, the difference in BMI can also be due to fat-free mass.

That being said, BMI is fairly indicative of body fat for 90-95% of the population, and can effectively be used along with other measures to help determine an individual’s healthy body weight.

BMI formula

Below are the equations used for calculating BMI in the International System of Units (SI) and the US customary system (USC) using a 5’10”, 160-pound individual as an example:

The Body Mass Index (BMI) is a measurement calculated using a person’s weight and height. The formula for the BMI is:

BMI = Weight (kg) / [Height (m)]^2

It’s important to note that the BMI is a general, simplified measure and does not take into account factors such as muscle mass, bone density, or distribution of fat. Therefore, while it can provide a rough estimate of a person’s body fat, it may not accurately reflect an individual’s overall health or fitness level.

BMI Prime

BMI prime is the ratio of a person’s measured BMI to the upper limit of BMI that is considered “normal,” by institutions such as the WHO and the CDC. Though it may differ in some countries, such as those in Asia, this upper limit, which will be referred to as BMI_upper is 25 kg/m².

The BMI prime formula is:

BMI prime =

BMI/25

Since BMI prime is a ratio of two BMI values, BMI prime is a dimensionless value. A person who has a BMI prime less than 0.74 is classified as underweight; from 0.74 to 1 is classified as normal; greater than 1 is classified as overweight; and greater than 1.2 is classified as obese. The table below shows a person’s weight classification based on their BMI prime:

Classification	BMI	BMI Prime
Severe Thinness	< 16	< 0.64
Moderate Thinness	16 – 17	0.64 – 0.68
Mild Thinness	17 – 18.5	0.68 – 0.74
Normal	18.5 – 25	0.74 – 1
Overweight	25 – 30	1 – 1.2
Obese Class I	30 – 35	1.2- 1.4
Obese Class II	35 – 40	1.4 – 1.6
Obese Class III	> 40	> 1.6

Ponderal Index

The Ponderal Index (PI) is similar to BMI in that it measures the leanness or corpulence of a person based on their height and weight. The main difference between the PI and BMI is the cubing rather than squaring of the height in the formula (provided below). While BMI can be a useful tool when considering large populations, it is not reliable for determining leanness or corpulence in individuals. Although the PI suffers from similar considerations, the PI is more reliable for use with very tall or short individuals, while BMI tends to record uncharacteristically high or low body fat levels for those on the extreme ends of the height and weight spectrum. Below is the equation for computing the PI of an individual using USC, again using a 5’10”, 160-pound individual as an example:

The Ponderal Index (PI), also known as the Rohrer’s index, is a similar measurement to the Body Mass Index (BMI), but it uses the cubic root of the person’s height instead of the square. The formula for the Ponderal Index is:

PI = Weight (kg) / [Height (m)]^3

Please note, just like the BMI, the Ponderal Index is a simple mathematical calculation and it may not fully reflect a person’s health or physical fitness. This index does not take into account factors like muscle mass, bone density, and overall body composition. Therefore, it should be used as a general guide rather than a definitive assessment of an individual’s health.