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BMI Calculator

Calculate your Body Mass Index (BMI) with visual gauge, BMI Prime, Ponderal Index, Trefethen formula, and Asian-adjusted thresholds. Supports metric and imperial units.

Metric measurements kg cm

How to Calculate BMI

BMI is a simple ratio of weight to height squared. It's used worldwide as a quick screening tool for weight categories.

BMI Formula

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²

Example: A person weighing 70 kg at 1.75 m tall: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 (Normal weight)

BMI Categories (WHO)

  • Severe Thinness: BMI < 16
  • Moderate Thinness: BMI 16-17
  • Mild Thinness: BMI 17-18.5
  • Normal weight: BMI 18.5-24.9 — associated with lowest health risks
  • Overweight: BMI 25-29.9 — increased risk for cardiovascular disease
  • Obese Class I: BMI 30-34.9
  • Obese Class II: BMI 35-39.9
  • Obese Class III: BMI ≥ 40 — highest health risk category

Additional BMI Metrics

BMI Prime

BMI Prime is the ratio of your BMI to the upper limit of the normal range (25). A value below 1.0 indicates a healthy BMI. For example, a BMI of 22.5 gives a BMI Prime of 0.90. This metric makes it easy to see how far above or below the normal threshold you are as a simple decimal.

Ponderal Index

The Ponderal Index (PI) divides weight by the cube of height (kg/m³) instead of the square. This makes it more reliable for people at the extremes of height — very tall or very short individuals get skewed results from standard BMI, but the Ponderal Index compensates for this. A normal PI range is roughly 11-15 kg/m³.

New BMI (Trefethen Formula)

Oxford mathematician Nick Trefethen proposed a revised BMI formula: 1.3 × weight (kg) / height (m)^2.5. Standard BMI systematically underestimates fatness in short people and overestimates it in tall people. The Trefethen formula corrects this height bias, producing more consistent results across the full range of adult heights.

Asian-Adjusted BMI Thresholds

Research shows that Asian populations — including South Asian, Chinese, Japanese, and Southeast Asian groups — tend to accumulate more visceral fat and face higher metabolic risk at lower BMI values compared to European-descent populations. The WHO recommends adjusted cut-offs:

  • Normal: BMI 18.5-22.9 (vs 18.5-24.9)
  • Overweight: BMI 23-27.4 (vs 25-29.9)
  • Obese: BMI ≥ 27.5 (vs ≥ 30)

Enable the Asian-adjusted toggle in the calculator to see how these thresholds apply to your BMI.

Limitations of BMI

BMI is a population-level screening tool, not a diagnostic measure. It has several known limitations:

  • Muscle vs fat: Athletes and bodybuilders often have "overweight" BMIs despite low body fat
  • Age: Older adults tend to have more body fat at the same BMI as younger adults
  • Sex: Women typically carry more body fat than men at identical BMIs
  • Ethnicity: Health risks associated with BMI vary across ethnic groups. Some populations face higher risk at lower BMIs
  • Fat distribution: BMI doesn't indicate where fat is stored. Visceral (abdominal) fat is more dangerous than subcutaneous fat

For a more complete picture, consider using our Body Fat Calculator or TDEE Calculator alongside BMI.

Better Metrics to Use With BMI

BMI works best as a starting point, not a final answer. Combine it with:

  • Waist circumference: Men above 40 inches (102 cm) and women above 35 inches (88 cm) face higher metabolic risk, regardless of BMI
  • Waist-to-hip ratio: Divides waist by hip measurement. Above 0.90 (men) or 0.85 (women) indicates higher cardiovascular risk
  • Body fat percentage: Directly measures fat vs lean mass, eliminating the muscle-mass blind spot in BMI

No single number tells the full story. A combination of measurements gives the most reliable picture of health risk.

BMI for Children and Teens

Child and teen BMI is interpreted differently from adult BMI. Instead of fixed thresholds, pediatric BMI uses age- and sex-specific percentiles based on CDC growth charts. A child at the 85th percentile is considered overweight; 95th percentile is obese. This percentile approach accounts for the natural changes in body composition during growth and puberty — a BMI of 22 means something very different for a 10-year-old than for a 30-year-old.

BMI and Health Risk

Research consistently shows a U-shaped or J-shaped relationship between BMI and mortality. Both very low (<18.5) and very high (>30) BMI values are associated with increased health risks. The lowest mortality rates generally fall in the 22-25 BMI range. However, the "obesity paradox" — where moderately overweight individuals sometimes show better outcomes in certain conditions like heart failure — remains an active area of research. Context matters: a BMI of 27 in an active 35-year-old with good metabolic markers carries different risk than the same BMI in a sedentary 60-year-old with high blood pressure.

History of BMI

Belgian mathematician Adolphe Quetelet developed the BMI formula in the 1830s as a population-level statistical tool, not a clinical diagnostic. It became widely adopted by insurance companies in the mid-20th century to classify risk. The WHO formally standardized the current BMI categories in 1995. Despite its limitations, BMI remains widely used because it requires no equipment beyond a scale and tape measure, making it accessible for large-scale public health screening.

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Frequently Asked Questions

What is BMI?

Body Mass Index (BMI) is a screening tool that estimates body fat based on height and weight. It is calculated as weight (kg) divided by height (m) squared.

What is a healthy BMI range?

WHO categories: Severe Thinness < 16, Moderate Thinness 16-17, Mild Thinness 17-18.5, Normal 18.5-24.9, Overweight 25-29.9, Obese I 30-34.9, Obese II 35-39.9, Obese III ≥ 40.

Is BMI accurate for everyone?

BMI does not distinguish between muscle and fat. Athletes with high muscle mass may have a high BMI despite low body fat. It also does not account for age, sex, ethnicity, or fat distribution.

How do I calculate BMI?

Metric: BMI = weight (kg) / height (m)². Imperial: BMI = (weight (lbs) × 703) / height (inches)².

What BMI is considered obese?

A BMI of 30 or higher is classified as obese. Class I obesity is 30-34.9, Class II is 35-39.9, and Class III (severe) is 40+.

Should I use BMI or body fat percentage?

BMI is a quick screening tool. Body fat percentage (measured via calipers, DEXA, or our body fat calculator) gives a more accurate picture of body composition.

What is BMI Prime?

BMI Prime is the ratio of your BMI to the upper limit of normal (25). A BMI Prime below 1.0 means you are within the healthy range. A value of 1.2 means your BMI is 20% above the normal threshold.

What is the Trefethen (New) BMI?

The Trefethen formula (New BMI) corrects a known height bias in standard BMI. It uses the exponent 2.5 instead of 2, giving more accurate results for very tall and very short people.

Are BMI thresholds different for Asian populations?

Yes. WHO recommends lower cut-offs for Asian populations: Overweight at BMI 23 (vs 25) and Obese at BMI 27.5 (vs 30), because Asian populations tend to have higher health risks at lower BMI values.