BMI Calculator
Calculate your Body Mass Index (BMI) to determine if you have a healthy body weight for your height.
What is BMI?
BMI (Body Mass Index) is a measurement of a person's leanness or corpulence based on their height and weight, and is intended to quantify tissue mass. It is widely used as a general indicator of whether a person has a healthy body weight for their height. Specifically, the value obtained from the calculation of BMI is used to categorize whether a person is underweight, normal weight, overweight, or obese depending on what range the value falls between.
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 Formula
The Body Mass Index is calculated using specific mathematical formulas. 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:
BMI Prime
BMI Prime represents 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.
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 |
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 main difference between the PI and BMI is the cubing rather than squaring of the height in the formula. 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:
Risks Associated with Being Overweight
Carrying excess weight can significantly elevate your risk of developing numerous serious medical conditions and diseases. According to research from the Centers for Disease Control and Prevention (CDC), individuals with a BMI above the healthy range face increased susceptibility to the following health concerns:
- •High blood pressure - Elevated blood pressure that can strain your cardiovascular system
- •Cholesterol imbalances - Higher levels of LDL cholesterol (widely considered "bad cholesterol"), lower levels of HDL cholesterol (considered to be good cholesterol in moderation), and elevated triglyceride levels
- •Type II diabetes - Increased risk of developing insulin resistance and diabetes
- •Coronary heart disease - Conditions affecting the heart's blood vessels
- •Stroke - Higher likelihood of experiencing cerebrovascular events
- •Gallbladder disease - Increased risk of gallstones and related complications
- •Osteoarthritis - A type of joint disease caused by breakdown of joint cartilage, leading to pain and stiffness
- •Sleep apnea and breathing problems - Disrupted sleep patterns and respiratory difficulties
- •Certain cancers - Increased risk of various cancers including endometrial, breast, colon, kidney, gallbladder, and liver cancer
- •Low quality of life - Reduced physical and mental well-being
- •Mental health concerns - Mental illnesses such as clinical depression, anxiety, and other psychological conditions
- •Physical limitations - Body pains and difficulty with certain physical functions
- •Increased mortality risk - Generally, an increased risk of mortality compared to those with a healthy BMI
Important Note: As evident from the comprehensive list above, there are numerous negative, and 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
While much attention is given to the risks of being overweight, being underweight also presents significant health challenges. Individuals with a BMI below the healthy range may face the following health risks:
- •Nutritional deficiencies - Malnutrition, vitamin deficiencies, and anemia (lowered ability to carry blood vessels)
- •Osteoporosis - A disease that causes bone weakness, significantly increasing the risk of breaking a bone
- •Weakened immune system - A decrease in immune function, making the body more susceptible to infections and illnesses
- •Developmental concerns - Growth and development issues, particularly in children and teenagers
- •Reproductive health issues - 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
- •Surgical complications - Potential complications as a result of surgery
- •Increased mortality risk - Generally, an increased risk of mortality compared to those with a healthy BMI
When to Seek Help: 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
While BMI serves as a valuable and widely utilized tool for assessing healthy body weight, it's important to recognize that it has certain limitations. BMI provides only an estimate and cannot account for individual body composition variations. Given the diverse range of body types and the complex distribution of muscle, bone mass, and fat throughout the body, BMI should be viewed as one component of a comprehensive health assessment rather than the definitive measure of a person's healthy body weight.
Limitations in Adults:
BMI cannot be fully accurate because it measures excess body weight rather than excess body fat directly. The calculation is further influenced by numerous factors including age, sex, ethnicity, muscle mass, body fat percentage, and activity level, among others.
Consider this example: An older person who falls within a "healthy weight" range according to BMI, but leads a completely sedentary lifestyle, may actually have significant amounts of excess body fat despite not being heavy. This scenario would be considered unhealthy. Conversely, a younger person with a higher muscle composition who has the same BMI would likely be considered healthy, as muscle tissue weighs more than fat tissue.
In athletes, particularly bodybuilders who might be classified as overweight based on BMI due to muscle being heavier than fat, it's entirely possible that they are actually at a healthy weight for their body composition. 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.
Limitations in Children and Adolescents:
The same factors that limit the effectiveness of BMI for adults also apply to children and adolescents. Additionally, height and level of sexual maturation can significantly influence BMI and body fat measurements among younger individuals.
BMI serves as a better indicator of excess body fat for obese children than it does 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, differences in BMI can also be attributed to variations in fat-free mass rather than body fat percentage alone.
Key Takeaway: Despite these limitations, 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. It's best used as part of a comprehensive health evaluation that may include other assessments like body fat percentage, waist circumference, and overall health markers.