BMI Calculator

Use our Body Mass Index (BMI) calculator to easily determine your BMI and evaluate your current weight status based on your age. By default, the calculator uses the metric system under the "Metric Units" tab. However, you can effortlessly switch to imperial measurements on the "US Units" tab or utilize the unit converter on the "Other Units" tab. As an added feature, this tool automatically calculates your Ponderal Index alongside your BMI.

BMI Overview

The Body Mass Index (BMI) is a widely used health metric that estimates an individual’s body fat and lean tissue based on their weight and height. It helps assess the proportionality of your weight-to-height ratio, providing valuable insights into your overall body composition and determining whether your weight falls within a healthy range.

Calculating your BMI reveals whether you are classified as underweight, a healthy weight, overweight, or obese. These primary BMI ranges are often broken down into specific subgroups, such as severe thinness or Class III obesity. Keep in mind that ideal BMI categories can vary slightly depending on factors like age and demographic region.

Carrying excess weight or experiencing malnutrition can have severe consequences for your long-term health. While BMI is not a perfect measurement of overall well-being, it serves as a highly useful screening tool to indicate whether lifestyle changes, further medical testing, or professional interventions are necessary. Review the tables below for a detailed breakdown of BMI classifications and formulas.

Adult BMI table

The following body weight classification is recommended by the World Health Organization (WHO). It is based on standard BMI values for adults (ages 18 and older).

Adult BMI charts

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

BMI table for children and teenagers aged 2-20

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

BMI chart for children and teenagers aged 2-20

The CDC provides specialized growth charts that track the rise in BMI in relation to age percentiles.

Boys’ Chart

Girls’ Chart

Risk factors of being overweight

Obesity significantly increases the risk of developing serious, long-term health conditions. The Centers for Disease Control and Prevention highlights the following risk factors associated with being overweight:

• Increased LDL cholesterol (“bad cholesterol”) levels, lower HDL cholesterol (“good cholesterol”) levels, and elevated triglycerides;
• High blood pressure (hypertension);
• Type II diabetes;
• Coronary artery disease (CAD);
• Gallbladder disease and dysfunction;
• Stroke;
• Breathing issues, including sleep apnea;
• Osteoarthritis (a degenerative joint disease caused by the breakdown of joint cartilage);
• Poorer overall quality of life;
• Increased risk of certain cancers (including endometrial, colon, breast, liver, gallbladder, and kidney cancer);
• Mental health disorders such as clinical depression and anxiety;
• Chronic body aches, joint pain, and physical limitations that hinder daily responsibilities;
• A significantly higher mortality rate compared to individuals with a healthy BMI.

Carrying excess weight can have adverse, sometimes devastating, effects on the body. Because of this, medical professionals generally recommend maintaining a BMI below 25 kg/m², which indicates a healthy weight. If your BMI falls outside the normal range, it is advisable to consult a physician to determine if dietary or lifestyle changes are necessary to improve your health.

The risks of being underweight

Being underweight also carries substantial health risks, including:

• Malnutrition, anemia, and essential vitamin deficiencies, which can impair blood transport and oxygen delivery;
• Osteoporosis, a condition characterized by bone mass loss and an increased susceptibility to fractures;
• Immune system dysfunction and a higher risk of illness;
• Stunted growth and developmental delays, particularly in children and adolescents;
• Increased risk of complications following surgery;
• Reproductive health issues in women due to hormonal imbalances, which can cause irregular menstrual cycles. (Note: While underweight women face fertility issues, overweight and obese women face a higher risk of miscarriage during the first trimester);
• A relatively higher overall mortality rate.

Unexpected weight loss may be a symptom of a more serious underlying medical condition or an eating disorder, such as anorexia nervosa. If you or someone you know is struggling to maintain a healthy weight for unknown reasons, it is crucial to consult a healthcare provider.

BMI Limitations

Despite being widely used as a standard screening tool for a healthy body weight, the body mass index (BMI) has several notable limitations. Most importantly, it does not account for an individual's complete body composition. Because body types, fat distribution, muscle mass, and bone density vary vastly from person to person, BMI should ideally be evaluated alongside other health indicators.

In adults:

Online BMI calculators offer a general estimate based solely on height and weight, failing to factor in variables that affect true health accuracy. Because BMI cannot differentiate between muscle weight and fat weight, it is not a direct measurement of body fat percentage. Furthermore, elements such as age, gender, muscle mass, and physical activity levels can heavily skew BMI readings and their clinical interpretation.

For example, consider an older adult who leads a highly sedentary lifestyle. They may carry a significant amount of excess body fat without being technically overweight on the scale. By objective medical standards, this person's health may be at risk, yet their BMI could still fall within the "normal" range.

The reverse is true for bodybuilders and high-level athletes. Muscle tissue is significantly denser and heavier than fat, despite taking up less physical volume. As a result, many muscular individuals will register near the upper limits of "normal" or even fall into the "overweight" categories. In reality, they are often in peak physical health. A highly fit person can look visibly lean while carrying substantial, heavy muscle mass.

According to data from the CDC:

• At identical BMI levels, women generally possess a higher body fat percentage than men;
• Older adults tend to carry more body fat than younger individuals with the exact same BMI;
• Intensely trained athletes frequently have a higher BMI due to their increased muscle mass, not excess fat.

In adolescents and children:

The same variables that limit the accuracy of BMI in adults also apply to teenagers and children. Factors like growth spurts, height, and stages of sexual development can significantly influence a child's body fat percentage and BMI.

Among obese children, BMI serves as a stronger predictor of excess body fat compared to children who are merely overweight. Their elevated BMI could stem from either high fat mass or high fat-free mass (which includes muscle, water, bone, and organs). In lean children, variations in BMI are often attributed to differences in fat-free mass rather than body fat.

For the general population, BMI remains a helpful, quick-reference indicator of potential health risks linked to being underweight, overweight, or obese. However, due to its inherent limitations, it should not be the sole measurement of health. BMI is best used as one of several assessment tools, combined with comprehensive physical exams and personalized medical evaluations.

BMI Formula

Below are the standard equations used to calculate BMI in both the United States Customary System (USC) and the International System of Units (SI/Metric). The formulas 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) is another metric that evaluates a person’s adiposity (body fatness) in relation to their weight and height. The primary difference between BMI and the Ponderal Index is that the PI formula (below) cubes the height measurement, rather than squaring it.

While BMI is an incredibly useful tool for analyzing broad population data, it can sometimes fall short when evaluating individual leanness or obesity. The Ponderal Index is often considered a more reliable metric for individuals who are exceptionally short or exceptionally tall. Because BMI does not scale perfectly with height, it may falsely identify unusually high or low body fat levels in people with extreme height proportions.

Below is the equation used to calculate a person's Ponderal Index, again using a 5 feet 10 inches tall, 160-pound individual as the 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:

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