BMI Calculator: Body Mass Index
Calculate your Body Mass Index with full WHO category breakdown, BMI Prime, healthy weight range, and a target weight table for your height.
⚖️ What is Body Mass Index (BMI)?
Body Mass Index (BMI) is a numerical measure of body weight relative to height, calculated by dividing a person's weight in kilograms by their height in metres squared. The formula, BMI = kg / m2, was developed by Belgian mathematician Adolphe Quetelet in the 1830s and later adopted by the World Health Organization (WHO) as the standard screening tool for identifying underweight, overweight, and obesity in adult populations. Unlike clinical measurements such as DEXA scans or hydrostatic weighing, BMI requires only two measurements and can be computed in seconds, which is why it has become the dominant metric in public health research and routine medical screening worldwide.
BMI is used across a wide range of real-world applications. Physicians use it to identify patients at elevated risk for type 2 diabetes, cardiovascular disease, hypertension, sleep apnea, and certain cancers. Insurance companies reference BMI in underwriting decisions. Public health researchers use it to track obesity trends across populations. Personal trainers and dietitians use it as a starting point when designing weight management programs. Individuals use it to set realistic weight-loss targets and understand where they stand relative to clinical thresholds. This calculator covers two modes: computing your current BMI with a full seven-category WHO breakdown, and a Target Weight mode that shows you the exact weight range for every BMI category at your height.
The WHO's full classification system has eight categories rather than the four typically shown (Underweight, Normal, Overweight, Obese). The complete system distinguishes between Severe Thinness (BMI below 16), Moderate Thinness (16 to 16.9), and Mild Thinness (17 to 18.4) at the low end, and between Obese Class I (30 to 34.9), Obese Class II (35 to 39.9), and Obese Class III (40 and above, also called morbid obesity) at the high end. This granularity matters clinically because the health risks associated with Class III obesity are substantially higher than those of Class I obesity. This calculator uses the full eight-category WHO scale.
BMI has well-known limitations. It does not distinguish between fat mass and lean mass, so athletes with high muscle mass often register as overweight despite low body fat. It does not account for fat distribution, and abdominal fat carries higher cardiovascular risk than fat deposited elsewhere. It also does not adjust for age-related changes in body composition. For a more complete picture of health, combine your BMI result with a waist circumference measurement and, where possible, a body fat percentage estimate using the Body Fat Calculator on this site.
📐 BMI Formula
The imperial conversion factor 703 converts the lb/in2 ratio to the same scale as the metric kg/m2 formula. The Ponderal Index uses the cube of height rather than the square, which reduces the systematic overestimation of BMI for taller individuals and underestimation for shorter individuals. The WHO defines BMI thresholds without reference to sex or ethnicity, although research consistently shows that Asian populations have higher body fat and metabolic risk at lower BMI values, leading to a recommended Asian threshold of 23.0 for overweight (per the WHO Expert Consultation 2004).
📖 How to Use This Calculator
Using BMI Calculator and Target Weight Modes
💡 Example Calculations
Example 1 -- Normal Weight Adult
Person: 170 cm tall, 70 kg
Example 2 -- Overweight Classification
Person: 175 cm tall, 90 kg
Example 3 -- Obese Class I
Person: 165 cm tall, 92 kg
Example 4 -- Imperial Units (US)
Person: 5 ft 10 in tall, 185 lb
❓ Frequently Asked Questions
🔗 Related Calculators
What is Body Mass Index and how is it calculated?
Body Mass Index (BMI) is a numerical value derived from a person's weight and height. The formula is weight in kilograms divided by height in metres squared (kg/m2). It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as a population-level obesity screening tool. It does not directly measure body fat but correlates reasonably well with health outcomes across large populations.
What are the WHO BMI categories for adults?
The WHO defines 8 BMI categories: Severe Thinness (under 16), Moderate Thinness (16.0 to 16.9), Mild Thinness (17.0 to 18.4), Normal Weight (18.5 to 24.9), Overweight (25.0 to 29.9), Obese Class I (30.0 to 34.9), Obese Class II (35.0 to 39.9), and Obese Class III (40 and above). Most calculators only show four categories; this one shows all eight as defined by the full WHO classification.
What is a healthy BMI for adults?
A BMI between 18.5 and 24.9 is classified as Normal Weight according to the WHO. Values below 18.5 indicate varying degrees of thinness and increased risk of nutritional deficiency. Values from 25 to 29.9 are overweight, and 30 or above is obese. For Asian populations, researchers recommend lower thresholds: overweight begins at 23.0 and obese at 27.5 due to higher body fat at equivalent BMI values.
What is BMI Prime and why does it matter?
BMI Prime is the ratio of your BMI to the upper limit of the Normal Weight range (25.0). A BMI Prime below 1.0 means you are in or below the Normal Weight range. A value above 1.0 means you are overweight or obese. BMI Prime of 1.2 means your BMI is 20% above the upper normal limit. It is a simple normalised score that allows direct comparison across different populations and studies.
What is the Ponderal Index and how does it differ from BMI?
The Ponderal Index is weight divided by height cubed (kg/m3). It is less sensitive to height extremes than BMI because it scales with the cube of height rather than the square. Very tall and very short people may get misleading BMI scores because body weight scales approximately with the cube of height in three-dimensional bodies. The Ponderal Index corrects for this scaling issue. A healthy Ponderal Index range is approximately 11 to 15 kg/m3.
Is BMI accurate for everyone?
No. BMI has documented limitations. Athletes and bodybuilders with high muscle mass are often classified as overweight or obese despite low body fat. Elderly people may have a normal BMI but high body fat due to muscle loss (sarcopenia). People with narrow frames may carry more body fat than BMI suggests. Women generally carry more body fat than men at the same BMI. Despite these limitations, BMI remains a useful first-pass screening tool in clinical settings.
What BMI is considered obese?
A BMI of 30.0 or above is classified as obese according to the WHO. The three classes of obesity are: Class I (BMI 30.0 to 34.9), associated with moderately increased health risk; Class II (35.0 to 39.9), associated with severe health risk; and Class III (40.0 and above), also called morbid obesity, associated with very severe health risk for conditions such as type 2 diabetes, cardiovascular disease, and sleep apnea.
How does BMI differ for men and women?
The BMI formula and WHO classification ranges are the same for men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI value because of differences in body composition. A woman with BMI 22 typically has about 5 to 6 percentage points more body fat than a man with BMI 22. This means BMI slightly underestimates metabolic risk in women and overestimates it in very muscular men.
How much weight do I need to lose to lower my BMI by one point?
Subtract your target BMI from your current BMI to get the number of BMI points to lose. Then multiply that by your height in metres squared. For example, at 170 cm: height squared = 1.70 x 1.70 = 2.89 m2. To drop 3 BMI points, you need to lose 3 x 2.89 = 8.67 kg. The Target Weight mode in this calculator shows the exact weight range for every BMI category at your height.