BMI Calculator

What Is Body Mass Index (BMI)?

Body Mass Index (BMI) 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 is a screening tool — not a direct measure of body fat. It was developed in the 19th century by Adolphe Quetelet and has since become the standard method for population-level weight classification. Despite its limitations, BMI remains the most commonly used metric for assessing weight-related health risks because it is inexpensive, easy to calculate, and correlates reasonably well with body fat for the average person.

Who Is This Calculator For?

This BMI calculator is suitable for adults aged 20 and older, as well as children and teens aged 2-20. It can be used by individuals assessing their own weight status, healthcare professionals screening patients, fitness enthusiasts tracking body composition, and researchers conducting population-level studies. Note that BMI interpretation differs for children, pregnant women, athletes, and elderly individuals.

How the BMI Calculation Works

BMI is calculated by dividing a person's weight by the square of their height. The formula yields a value in kg/m² that correlates with body fatness for most people. This calculator supports both metric and US customary units and automatically converts between them.

Metric (SI) formula: BMI = weight (kg) / height² (m)

US Customary formula: BMI = 703 × weight (lbs) / height² (in)

For example, a person weighing 70 kg with a height of 1.75 m has a BMI of 70 / (1.75 × 1.75) = 22.9, which falls in the normal range.

How to Use This Calculator

1. Choose your unit system — Metric (kg/cm) or US (lbs/feet/inches) using the tabs at the top.
2. Select your gender — Used for pediatric BMI interpretation and certain reference ranges.
3. Enter your age — For children and teens (2-20), BMI is interpreted using CDC percentile charts.
4. Enter your weight — in kilograms or pounds depending on your unit selection.
5. Enter your height — in centimeters or feet/inches. For US units, both feet and inches fields are required.
6. Click "Calculate" — Your BMI, BMI Prime, Ponderal Index, healthy BMI range, and healthy weight range will be displayed instantly.
7. Review your results — Compare your BMI against the WHO classification table and consult a healthcare professional if you have concerns.

BMI Table for Adults (WHO Classification)

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 Categories for Asians and Other Ethnic Groups

Research shows that Asian populations have a higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels. Many Asian health organizations recommend lower BMI thresholds:

Category	Asian BMI Range (kg/m2)
Underweight	< 18.5
Normal	18.5 - 23
Overweight	23 - 27.5
Obese	≥ 27.5

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 based on percentiles. After calculating BMI, it is plotted on CDC growth charts to determine the percentile relative to peers of the same age and gender.

Category	Percentile Range
Underweight	< 5%
Healthy Weight	5% - 85%
At Risk of Overweight	85% - 95%
Overweight	> 95%

BMI Prime

BMI Prime is the ratio of your BMI to the upper limit of the normal BMI range (25 kg/m²). A BMI Prime of 1.0 means your BMI is exactly 25. Values below 1.0 indicate a BMI below normal, and values above 1.0 indicate a BMI above normal.

BMI Prime Range	Category
< 0.74	Underweight
0.74 - 1.00	Normal
1.00 - 1.20	Overweight
> 1.20	Obese

BMI Prime provides a single decimal that immediately communicates how far you are from the normal threshold. A BMI Prime of 1.15 means you are 15% above the upper limit of normal weight.

Ponderal Index

The Ponderal Index (Rohrer's Index) divides weight by the cube of height: Ponderal Index = weight (kg) / height³ (m). This index may be more accurate for very tall or very short individuals because it better accounts for body proportions. The normal range is approximately 11-14 kg/m³ for adults.

Risks Associated with Being Overweight

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

• High blood pressure
• Higher levels of LDL cholesterol, lower levels of HDL cholesterol, and high levels of triglycerides
• Type II diabetes
• Coronary heart disease
• Stroke
• Gallbladder disease
• Osteoarthritis
• Sleep apnea and breathing problems
• Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
• Mental illnesses such as clinical depression and anxiety

Risks Associated with Being Underweight

Being underweight has its own associated risks:

• Malnutrition, vitamin deficiencies, anemia
• Osteoporosis (bone weakness, increasing fracture risk)
• Decreased immune function
• Growth and development issues, particularly in children and teenagers
• Reproductive issues for women due to hormonal imbalances
• Potential complications as a result of surgery

Applications of BMI

• Clinical screening: Doctors use BMI as an initial screening tool for weight-related health issues.
• Epidemiological research: Public health researchers track obesity trends across populations using BMI.
• Insurance underwriting: Life and health insurers may use BMI as one factor in determining premiums.
• Fitness and wellness: Personal trainers use BMI as a baseline alongside body composition analysis.
• Surgical assessment: BMI is used to assess surgical risk and eligibility for procedures like bariatric surgery.

Limitations of BMI

Although BMI is a widely used and useful indicator of healthy body weight, it does have limitations. BMI 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. For personalized calorie and nutrition recommendations based on your measurements, try our Calorie Calculator.

• Muscle vs. fat: Athletes with high muscle mass may have a high BMI without being overweight. Muscle is denser than fat, so a muscular person may weigh more without excess body fat.
• Age: Older adults tend to have more body fat than younger adults with the same BMI. BMI does not account for age-related changes in body composition.
• Gender: Women naturally have a higher body fat percentage than men at the same BMI, but the same thresholds are used for both genders.
• Ethnicity: BMI cutoffs may not be appropriate for all ethnic groups. Asian populations may have higher health risks at lower BMI values.
• Pregnancy: BMI formulas are not accurate during pregnancy due to fetal weight and fluid.
• Fat distribution: BMI does not distinguish between visceral fat (around organs, more harmful) and subcutaneous fat (under the skin, less harmful).

For a more complete assessment, consider using our Body Fat Calculator alongside your BMI measurement.

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