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.

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.

BMI Calculator: Body Mass Index

Q: 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.

Q: 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.

Q: 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.

Q: 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.

Q: 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.

Calculate your Body Mass Index with full WHO category breakdown, BMI Prime, healthy weight range, and a target weight table for your height.

Height170 cm

100 cm230 cm

Weight70 kg

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Weight154 lb

66 lb440 lb

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WHO Category

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BMI Prime

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Ponderal Index

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Asia-Pacific Category

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Healthy Weight Range

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Distance from Healthy

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Your Height170 cm

100 cm230 cm

Your Height5 ft

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⚖️ 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

BMI = w_kg ÷ h_m²

BMI (Imperial) = (w_lb ÷ h_in²) × 703

BMI Prime = BMI ÷ 25

Ponderal Index = w_kg ÷ h_m³

w_kg = body weight in kilograms

h_m = height in metres (e.g., 170 cm = 1.70 m)

w_lb = body weight in pounds

h_in = height in inches (e.g., 5 ft 7 in = 67 inches)

BMI Prime = ratio to upper normal limit of 25; values below 1.0 are normal weight

Ponderal Index = weight per cubic metre; normal range approximately 11 to 15 kg/m3

Example: 70 kg, 170 cm: BMI = 70 / (1.70 x 1.70) = 70 / 2.89 = 24.2 (Normal Weight)

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

Select your unit system -- Choose Metric (kg and cm) or Imperial (lb, ft, in) using the unit tabs.

Choose your mode -- BMI Calculator computes your BMI from weight and height. Target Weight shows a full weight-range table for all BMI categories at your height.

Enter height and weight -- In BMI Calculator mode, enter both your height and weight. In Target Weight mode, only your height is needed.

Read the results -- BMI Calculator mode shows BMI, WHO category, BMI Prime, Ponderal Index, Asia-Pacific category, healthy weight range, and how far your weight is from the healthy range.

💡 Example Calculations

Example 1 -- Normal Weight Adult

Person: 170 cm tall, 70 kg

Height in metres: 170 cm = 1.70 m; height squared = 1.70 x 1.70 = 2.89 m2

BMI = 70 / 2.89 = 24.2

WHO category: Normal Weight (18.5 to 24.9); BMI Prime = 24.2 / 25 = 0.97 (below 1.0 = normal); Healthy range at this height: 53.5 kg to 71.9 kg

BMI = 24.2 (Normal Weight) • BMI Prime = 0.97

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Example 2 -- Overweight Classification

Person: 175 cm tall, 90 kg

Height squared = 1.75 x 1.75 = 3.0625 m2

BMI = 90 / 3.0625 = 29.4

WHO category: Overweight (25.0 to 29.9); Healthy range at 175 cm: 56.7 kg to 76.3 kg. Weight above healthy range: 90 - 76.3 = 13.7 kg

BMI = 29.4 (Overweight) • 13.7 kg above healthy range

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Example 3 -- Obese Class I

Person: 165 cm tall, 92 kg

Height squared = 1.65 x 1.65 = 2.7225 m2

BMI = 92 / 2.7225 = 33.8

WHO category: Obese Class I (30.0 to 34.9); BMI Prime = 33.8 / 25 = 1.35 (35% above the upper normal limit)

BMI = 33.8 (Obese Class I) • BMI Prime = 1.35

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Example 4 -- Imperial Units (US)

Person: 5 ft 10 in tall, 185 lb

Total height in inches: (5 x 12) + 10 = 70 inches; height squared = 70 x 70 = 4,900 in2

BMI = (185 / 4,900) x 703 = 0.03776 x 703 = 26.5

WHO category: Overweight (25.0 to 29.9); Healthy weight at this height: 129 lb to 174 lb

BMI = 26.5 (Overweight)

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❓ Frequently Asked Questions

What is Body Mass Index and how is it calculated?+

Body Mass Index (BMI) is a number derived from your weight and height using the formula BMI = weight in kg divided by height in metres squared. It was created by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as the primary population-level obesity screening metric. It requires only two inputs and correlates reasonably well with direct measures of body fat in large populations, making it widely used in clinical practice and public health research.

What are all 8 WHO BMI categories for adults?+

The full WHO BMI classification has 8 categories: Severe Thinness (BMI below 16.0), 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 (40.0 and above). Most popular calculators only show 4 categories. The three thinness categories and three obesity classes carry distinct clinical implications for treatment and monitoring.

What is a healthy BMI for adults?+

A BMI between 18.5 and 24.9 is classified as Normal Weight by the WHO and is associated with the lowest risk for weight-related health conditions. For Asian populations, researchers recommend lower thresholds: overweight begins at 23.0 and obesity at 27.5, reflecting higher metabolic risk at lower BMI values. Optimal BMI may also vary by age, as older adults with BMI slightly above 25 sometimes show better health outcomes than those at the lower end of normal.

What is BMI Prime and how do I interpret it?+

BMI Prime is your BMI divided by 25 (the upper limit of the Normal Weight range). A BMI Prime below 1.0 means you are in the Normal Weight range or below. A value of exactly 1.0 means your BMI is exactly 25 (the boundary of overweight). A value of 1.2 means your BMI is 20% above the upper normal limit. BMI Prime provides a normalised score that allows consistent interpretation across studies and populations without needing to remember the absolute BMI thresholds.

What is the Ponderal Index and when is it more accurate than BMI?+

The Ponderal Index is weight in kg divided by height in metres cubed (kg/m3). Human body weight scales approximately with the cube of height in three-dimensional space, so the square in the BMI formula creates a systematic bias: BMI tends to overestimate leanness in tall people and overestimate heaviness in short people. The Ponderal Index corrects for this. A healthy Ponderal Index range is approximately 11 to 15 kg/m3. At 170 cm and 70 kg, the Ponderal Index is 70 / (1.70 x 1.70 x 1.70) = 14.2 kg/m3.

Is BMI accurate for athletes and muscular people?+

No. BMI does not distinguish between fat mass and muscle mass. Athletes and bodybuilders often register as Overweight or even Obese Class I on the BMI scale despite having very low body fat percentages. A 180 cm athlete weighing 95 kg would have BMI 29.3 (Overweight) even if their body fat percentage is 10 to 12%. For athletes, body fat percentage measurement (via DEXA, hydrostatic weighing, or the US Navy circumference method) is a far more informative metric than BMI.

What BMI is considered obese and what are the obesity classes?+

Any BMI at or above 30.0 is classified as obese. The three classes are: Class I (30.0 to 34.9, moderately increased risk), Class II (35.0 to 39.9, severely increased risk), and Class III (40.0 and above, very severely increased risk, also called morbid obesity). Class III obesity is associated with substantially higher rates of type 2 diabetes, cardiovascular disease, sleep apnea, and reduced life expectancy compared to Classes I and II.

How does BMI differ for men and women?+

The BMI formula and WHO cutoffs are identical for men and women. However, at the same BMI, women typically carry 5 to 6 percentage points more body fat than men due to differences in reproductive biology and body composition. This means BMI slightly underestimates health risk in women (more fat at same BMI) and can overestimate it in heavily muscled men (more muscle at same BMI). The BMI Calculator for Men and BMI Calculator for Women on this site add sex-specific body fat estimation alongside standard BMI.

How much weight do I need to lose to lower my BMI by 1 point?+

To drop your BMI by 1 point, multiply 1 by your height in metres squared. For example, at 170 cm: height squared = 1.70 x 1.70 = 2.89 m2. You need to lose 2.89 kg to lower your BMI by 1 point. To drop 5 BMI points from 30 to 25, you would need to lose 5 x 2.89 = 14.45 kg. The Target Weight mode in this calculator shows the exact weight range for every BMI category at any given height.

Does BMI apply to people over 65?+

The standard WHO BMI categories apply to adults of all ages, including those over 65. However, older adults experience age-related muscle loss (sarcopenia), so a person aged 70 with BMI 23 may have a higher body fat percentage than a 30-year-old with the same BMI. Some geriatric medicine guidelines suggest a slightly higher healthy range for older adults (BMI 22 to 27) to account for this shift in body composition and the protective effect of modest fat reserves against illness-related weight loss.

What is the Asia-Pacific BMI classification?+

The WHO Expert Consultation on BMI and Asian populations (2004) recommended lower thresholds for South and East Asian people because they carry greater body fat and have higher metabolic risk at lower BMI values than Caucasian populations. The Asia-Pacific classification defines Normal Weight as 18.5 to 22.9, overweight as 23.0 to 27.4, and obese as 27.5 and above. This is why a South Asian person with BMI 24 might be classified as Normal by the global WHO scale but Overweight under the Asia-Pacific scale. This calculator displays both classifications.

Can I have a normal BMI and still be at health risk?+

Yes. This is called normal-weight obesity or metabolically obese normal weight (MONW). A person with BMI 22 can have high body fat, low muscle mass, insulin resistance, and elevated cardiovascular risk if they are sedentary and have accumulated abdominal fat. Waist circumference is a better predictor of metabolic risk than BMI alone. Health risk is elevated when waist circumference exceeds 80 cm in women or 94 cm in men, regardless of BMI category. Combine BMI with waist measurement and physical activity level for a fuller health picture.

🔗 Related Calculators

📌 Quick Tips

💡BMI is a population screening tool, not a clinical diagnosis. Combine it with waist circumference and body fat percentage for a complete picture of your health.

💡Athletes and bodybuilders often register as overweight or obese on the BMI scale due to high muscle mass. If you lift weights regularly, use the Body Fat Calculator alongside BMI.

💡South Asian and East Asian populations face higher metabolic risk at lower BMI values. Researchers suggest an overweight threshold of 23.0 for Asian populations rather than the WHO standard of 25.0.

💡For children and teenagers (ages 2 to 19), adult BMI categories do not apply. Use the BMI Calculator for Kids or BMI Calculator for Teens, which use CDC age- and sex-specific percentile charts.

💡To lower your BMI by 1 point, you need to reduce your weight by about (height in metres squared) kilograms. For a 170 cm person, that is approximately 2.89 kg of weight loss.