Free BMI Calculator

What is BMI?

Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI was originally called the "Quetelet Index" and has since become the most widely used screening measure for weight-related health risks.

BMI provides a quick way to estimate whether a person's weight falls within a healthy range relative to their height. It is used by healthcare professionals worldwide, from routine doctor's visits to large-scale epidemiological studies.

While BMI is not a direct measure of body fat, research has shown a strong correlation between BMI and more direct measures of body fatness such as underwater weighing and dual-energy X-ray absorptiometry (DEXA scans). The World Health Organization (WHO), Centers for Disease Control (CDC), and National Institutes of Health (NIH) all use BMI as a key indicator in their obesity-related health guidelines.

How to Calculate BMI

BMI is calculated using a simple formula that relates weight to height:

Example (Metric): A person weighing 70 kg with a height of 175 cm (1.75 m):

BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9

Example (Imperial): A person weighing 154 lbs with a height of 5'9" (69 inches):

BMI = (154 × 703) / (69 × 69) = 108,262 / 4,761 = 22.7

A newer formula proposed by mathematician Nick Trefethen (University of Oxford) in 2013 adjusts for height more accurately:

This updated formula reduces the systematic underestimation of BMI for short people and overestimation for tall people that exists in the traditional formula.

BMI Categories (WHO Classification)

The World Health Organization classifies adult BMI into the following categories:

BMI and Ethnicity

Research has shown that the relationship between BMI and body fat — and the health risks associated with different BMI levels — varies significantly across ethnic groups. This is one of the most important limitations of standard BMI categories.

Asian populations: The WHO Western Pacific Region recommends lower BMI cutoffs for Asian populations because studies show higher health risks at lower BMI values. For Asian individuals, overweight is defined as BMI ≥ 23 (vs. 25 for the standard) and obesity as BMI ≥ 27.5 (vs. 30).

Black populations: Some research suggests that Black individuals may have a lower body fat percentage at a given BMI compared to White individuals, partly due to higher bone mineral density and lean mass. However, the clinical significance of adjusting cutoffs remains debated.

Pacific Islander populations: Polynesian and Micronesian populations tend to have more lean body mass at a given BMI, and some researchers have suggested higher BMI cutoffs may be more appropriate.

Our calculator includes the option to select "Asian (WHO WPRO)" cutoffs, which use the evidence-based adjusted thresholds. We encourage all users to discuss their BMI results with a healthcare professional who can consider their full clinical picture.

What is BMI Prime?

BMI Prime is a dimensionless ratio that compares your BMI to the upper limit of the "normal" BMI range (25.0 for standard WHO cutoffs, or 23.0 for Asian cutoffs). It was proposed to simplify international comparison.

The formula is straightforward:

A BMI Prime of less than 1.0 means you are within or below the normal weight range. A value above 1.0 indicates you are above the upper limit of normal. For example:

• BMI Prime of 0.74 = 26% below the upper limit of normal
• BMI Prime of 1.00 = exactly at the upper limit of normal (BMI 25)
• BMI Prime of 1.20 = 20% above the upper limit of normal

BMI Prime is particularly useful for quickly understanding how far someone is from a healthy range, and for comparing across populations with different BMI cutoffs.

Limitations of BMI

While BMI is a useful population-level screening tool, it has several well-documented limitations that are important to understand:

• Muscle vs. Fat: BMI cannot distinguish between muscle mass and fat mass. Athletes and bodybuilders often have a high BMI despite having low body fat and excellent fitness. A muscular person at 5'10" and 200 lbs would have a BMI of 28.7 (overweight), even if their body fat is below 15%.
• Fat Distribution: BMI does not indicate where fat is stored. Abdominal (visceral) fat is more strongly associated with cardiovascular disease and type 2 diabetes than subcutaneous fat. Waist circumference and waist-to-hip ratio are better indicators of fat distribution.
• Age: Older adults tend to have more body fat than younger adults at the same BMI. Additionally, age-related loss of muscle mass (sarcopenia) can keep BMI in a "normal" range even when body fat percentage is elevated.
• Sex: Women naturally have more body fat than men. At the same BMI, women typically have about 10% more body fat than men.
• Height: The traditional BMI formula can underestimate BMI for very short people and overestimate it for very tall people (which is partly addressed by the Trefethen formula).
• Children: For children and teens (ages 2-20), BMI must be interpreted using age- and sex-specific percentile charts (CDC growth charts) rather than absolute categories.

For a more comprehensive health assessment, consider using BMI alongside body fat percentage, waist circumference, blood pressure, cholesterol levels, and other clinical indicators.

BMI for Children and Teens

For children and adolescents aged 2 to 20, BMI is calculated the same way (weight / height²), but the result is interpreted differently. Instead of fixed categories, pediatric BMI is compared to CDC growth chart percentiles that account for age and sex.

This percentile-based system is necessary because body composition changes significantly during growth and development. A BMI of 18 is perfectly normal for a 10-year-old but would be considered underweight for an adult.

Our calculator supports pediatric BMI interpretation for ages 2-20 using approximate CDC growth chart thresholds. For precise percentile values, consult your child's pediatrician.

Health Risks by BMI Category

Underweight (BMI < 18.5): Being underweight is associated with malnutrition, vitamin deficiencies, anemia, weakened immune function, osteoporosis, and increased risk of complications from surgery. Severely underweight individuals may experience heart irregularities and fertility issues.

Normal Weight (BMI 18.5–24.9): This range is associated with the lowest overall health risk. However, it is possible to be "metabolically unhealthy" at a normal BMI — for example, having high visceral fat, poor fitness, or elevated blood sugar. Overall health depends on many factors beyond weight alone.

Overweight (BMI 25–29.9): Overweight is associated with increased risk of type 2 diabetes, hypertension, cardiovascular disease, sleep apnea, and certain cancers. However, being slightly overweight (BMI 25-27) with good fitness may carry minimal additional risk — this is sometimes called the "obesity paradox."

Obese (BMI ≥ 30): Obesity significantly increases the risk of type 2 diabetes, heart disease, stroke, certain cancers (breast, colon, kidney), osteoarthritis, fatty liver disease, sleep disorders, and mental health issues. The risk increases progressively through Obese Class I, II, and III.

According to the WHO, worldwide obesity has nearly tripled since 1975. In 2016, more than 1.9 billion adults were overweight, of whom 650 million were obese. Understanding and monitoring your BMI is one step toward managing weight-related health risks.

References

1. World Health Organization. "Body mass index - BMI." WHO, 2024.
2. CDC. "About Adult BMI." Centers for Disease Control and Prevention, 2024.
3. Trefethen, N. "BMI: New Calculation Proposed." BMJ, 2013.
4. WHO Expert Consultation. "Appropriate body-mass index for Asian populations." The Lancet, 2004.
5. Deurenberg P, Weststrate JA, Seidell JC. "Body mass index as a measure of body fatness." Br J Nutr, 1991.
6. National Heart, Lung, and Blood Institute. "Classification of Overweight and Obesity by BMI." NHLBI, 2024.