Body Mass Index with 7-tier scale, healthy weight range, body fat estimate, BMI Prime, and Ponderal Index — all in one place.
Unit system
Uses the Deurenberg equation: %BF = 1.20×BMI + 0.23×age − 10.8×sex − 5.4. Accurate within ~4% for adults aged 18–65.
Unit system
Unit system
kg
Log multiple BMI readings and track your trend over time. Data is kept in memory for this session.
Unit system
ℹ️ For imperial use: weight in lbs, height in inches (e.g. 5'9" = 69 inches)
BMI Categories — WHO Classification
The World Health Organization defines seven BMI tiers, not just the four commonly cited. Understanding which tier you fall into gives a more precise picture of health risk than a simple "normal/overweight" binary.
BMI Range
Category
Health Risk
< 16.0
Severely Underweight
Very high — malnutrition, organ damage risk
16.0 – 18.4
Underweight
Elevated — micronutrient deficiency, bone density
18.5 – 24.9
Normal Weight
Lowest risk — optimal metabolic range
25.0 – 29.9
Overweight
Mildly elevated — metabolic syndrome risk begins
30.0 – 34.9
Obese Class I
Moderate — type 2 diabetes, cardiovascular risk
35.0 – 39.9
Obese Class II
High — sleep apnea, joint problems, comorbidities
≥ 40.0
Obese Class III (Severe)
Very high — significantly reduced life expectancy
What BMI Does and Doesn't Tell You
BMI is a population-level screening tool, not a diagnostic test. It correlates reasonably well with body fatness in large groups, but has known limitations at the individual level. Muscular individuals (bodybuilders, athletes) often register as "overweight" or even "obese" despite low body fat. Older adults may have normal BMI while carrying excess fat due to muscle loss (sarcopenia). Ethnic differences also exist: South Asian, East Asian, and other populations face metabolic risk at lower BMI thresholds than the standard WHO cutoffs.
For a complete health picture, BMI is best used alongside waist circumference (<94cm/37in for men, <80cm/31in for women is recommended), body fat percentage, blood markers (fasting glucose, lipids, blood pressure), and physical fitness assessments. This calculator provides BMI Prime and Ponderal Index as additional reference points.
BMI Prime & Ponderal Index
BMI Prime is your BMI divided by 25 (the upper limit of normal). A BMI Prime of exactly 1.0 = BMI of 25. Values below 1.0 are normal; above 1.0 are overweight or obese. It makes it easy to see by what percentage you're above or below the upper limit of normal weight.
Ponderal Index (PI = kg/m³) is an alternative to BMI that some researchers argue scales better with height. It uses the cube of height rather than the square, making it more accurate for very tall or very short individuals. The healthy range for Ponderal Index is approximately 11–15 kg/m³.
Frequently Asked Questions
Is BMI accurate for muscular or athletic people?
No — BMI significantly overstates fatness for muscular individuals because muscle weighs more than fat per unit volume. A well-trained athlete with 10% body fat and significant muscle mass can register as "overweight" or "obese" on the BMI scale. For athletes, body fat percentage (via DEXA scan, hydrostatic weighing, or skinfold calipers) is a far better measure. The Ponderal Index has similar limitations. BMI is most useful as a rough population-level screening tool, not for individual athletic assessment.
What is a healthy BMI for women vs. men?
The standard BMI categories (18.5–24.9 = normal) apply to both sexes. However, women naturally carry 5–10% more body fat than men at the same BMI due to hormonal and physiological differences. A woman at BMI 22 and a man at BMI 22 have similar weight-for-height ratios but different body compositions. The Deurenberg body fat estimate in this calculator accounts for sex differences. Women also face different health thresholds for waist circumference (<80cm vs. <94cm for men).
What is BMI Prime and why does it matter?
BMI Prime is your BMI divided by 25. It reframes BMI as a ratio to the upper limit of normal weight. A BMI Prime of 0.92 means you're 8% below the upper limit of normal. A BMI Prime of 1.15 means you're 15% above normal. It's a more intuitive way to understand how far you are from the healthy weight range and how much weight change would bring you to BMI 25.
How is the body fat percentage estimated?
This calculator uses the Deurenberg equation: %BF = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 (where sex = 1 for male, 0 for female). It was validated in populations of European descent and is accurate within approximately 4 percentage points for most adults aged 18–65. It becomes less accurate for elderly individuals, very muscular people, and some ethnic groups. More precise methods include DEXA scanning, hydrostatic weighing, and Bod Pod air displacement.
What is a healthy body fat percentage?
Generally accepted healthy ranges are: Men 10–20%, Women 18–28%. Athletes typically fall lower: men 6–13%, women 14–20%. Essential fat (the minimum for basic organ function) is about 3% for men and 12% for women. Body fat above 25% for men and 32% for women is generally considered obese by body composition standards, even if BMI appears normal.
Do BMI thresholds differ by ethnicity?
Yes. Several health organizations now recommend lower cutoffs for Asian populations. The WHO's Asia-Pacific guidelines suggest overweight begins at BMI 23 and obesity at BMI 27.5 for East and Southeast Asian populations, as metabolic complications like type 2 diabetes appear at lower BMI values in these groups. South Asian, Chinese, Japanese, and Korean guidelines all use lower thresholds. The standard 25/30 cutoffs were primarily developed from European population data.