With age taken into account, the Body Mass Index (BMI) Calculator can be used to determine the BMI value and associated weight status. To convert units into US or metric units, select the “Other Units” option or the “Metric Units” tab for the International System of Units. Keep in mind that the calculator not only calculates BMI but also the Ponderal Index, which are both covered in detail below.
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BMI introduction
BMI is a metric used to estimate tissue mass that measures a person’s leanness or corpulence depending on their height and weight. It is frequently used to determine whether a person’s body weight is appropriate for their height. To be more precise, the BMI value is used to determine if an individual is underweight, normal weight, overweight, or obese based on which category the value falls into. These BMI ranges can be further classified into subcategories like severely underweight or very severely obese, and they vary depending on factors like age and area. Although BMI is not a perfect estimate of healthy body weight, it is a helpful tool for determining whether further testing or action is necessary because being overweight or underweight can have serious health consequences. To view the various BMI-based categories that the calculator uses, check the table below.
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:
- Vitamin deficiencies, malnourishment, and anemia (reduced capacity to transport blood vessels)
- Bone weakness brought on by osteoporosis increases the chance of breaking a bone.
- A decline in the immune system’s performance
- Problems with growth and development, especially in kids and teens
- Hormonal imbalances that might interfere with the menstrual cycle may cause reproductive problems for women. Additionally, women who are underweight are more likely to miscarry during the first trimester.
- Possible side effects following surgery
- In general, a higher risk of death in comparison to people with a healthy BMI
Being underweight might occasionally indicate an underlying illness or condition, such as anorexia nervosa, which carries risks of its own. If you believe you or someone you know is underweight, especially if the cause is not immediately apparent, speak with your doctor.
Limitations of BMI
Despite being a popular and practical measure of a healthy body weight, BMI has drawbacks. Body composition is not taken into consideration by BMI, which is merely an estimate. BMI should not be used as the only way to determine a person’s healthy body weight because there are many different body types and variations in the distribution of muscle, bone mass, and fat.
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 degree of activity are some of the variables that affect BMI. For instance, even though they are not overweight, an elderly person who is deemed to be at a healthy weight but leads a fully sedentary life may have a sizable amount of excess body fat. A younger individual with a higher muscle composition and the same BMI would be seen as healthy, whereas this would be deemed unhealthy. It’s possible that athletes, especially bodybuilders, who are deemed overweight because their muscle mass exceeds their fat mass, are actually at a healthy weight for their body composition. In general, the CDC states:
- 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:
Children and adolescents may be affected by the same variables that restrict the effectiveness of BMI in adults. Children’s body fat and BMI can also be influenced by height and sexual development. Compared to overweight children, whose BMI may be due to elevated levels of either fat or fat-free mass (all body components except for fat, which includes water, organs, muscle, etc.), obese children’s BMI is a stronger sign of extra body fat. The BMI discrepancy in slim children may also be caused by fat-free mass.
Nevertheless, for 90–95% of people, BMI is a good indicator of body fat and can be used in conjunction with other metrics to assist establish a person’s healthy 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:
| USC Units: |
| BMI = 703×mass (lbs)height2 (in) = 703×160702 = 23.0 |
| SI, Metric Units: |
| BMI = mass (kg)height2 (m) = 72.571.7782 = 23.0 |
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 BMIupper is 25 kg/m2.
The BMI prime formula is:
| BMI prime = | BMI 25 |
BMI prime is a dimensionless value since it is the ratio of two BMI values. A person is considered underweight if their BMI is less than 0.74, normal if it is between 0.74 and 1, overweight if it is greater than 1, and obese if it is greater than 1.2. A person’s weight classification according to their BMI prime is displayed in the table below:
| 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 |
BMI prime allows us to make a quick assessment of how much a person’s BMI differs from the upper limit of BMI that is considered normal. It also allows for comparisons between groups of people who have different upper BMI limits.
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 primary distinction between the PI and BMI is that the height in the formula (shown below) is cubed rather than squared. Although BMI can be a helpful tool for analyzing large populations, it is not a reliable indicator of an individual’s leanness or corpulence. While the BMI tends to reflect abnormally high or low body fat levels for those on the extremes of the height and weight spectrum, the PI is more reliable when used with very tall or short people, while sharing similar factors. The formula for calculating an individual’s PI using USC is shown below, once more using a 5’10”, 160-pound person as an example:
| USC Units: |
| PI = height (in)∛mass (lbs) = 70∛160 = 12.9 |
| SI, Metric Units: |
| PI = mass (kg)height3 (m) = 72.571.7783 = 12.9 |
