BMI Calculator Screening index
Enter height and weight to get your BMI with the full WHO eight-level classification — plus two context indices (BMI Prime and the Ponderal index) that most BMI pages skip.
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BMI is the most-used weight screening index in the world, and the most criticized — both reputations are earned. It was popularized by Keys and colleagues in 1972 as a population-level weight-height index, and adopted by the WHO as the standard classification for epidemiological use. Its strengths: two measurements, no equipment, enormous reference datasets behind every threshold. Its weaknesses are structural, and this page is honest about them instead of hiding them behind a green checkmark.
What BMI cannot do is distinguish fat from lean mass. A muscular athlete and a sedentary person of the same height and weight get the same BMI and different health profiles; older adults can carry risky fat at a "normal" BMI because of muscle loss. That is why this page pairs the index with its two standard companions. BMI Prime (BMI ÷ 25) expresses the result as a fraction of the overweight threshold — 1.0 is the line, 0.92 is 8% below it. The Ponderal index (weight ÷ height³) re-weights the formula for human shape and is less dominated by height, which matters at the tall and short extremes.
For an individual, BMI is a screening signal, not a diagnosis — and the single best home complement is a composition estimate. Our body fat calculator runs the US Navy tape method to tell you what share of that weight is actually fat, and if your BMI sits above the healthy band, the calorie deficit calculator shows what a sustainable deficit looks like in day-to-day numbers.
BMI = weight ÷ height², with height in meters. Worked example: 80 kg at 1.80 m → 80 ÷ 3.24 = 24.7. BMI Prime divides that by 25 → 0.99. The Ponderal index divides weight by height³ instead → 80 ÷ 5.832 = 13.7 kg/m³.
The WHO classification this page displays has eight bands: below 16.0 severe thinness; 16.0–16.99 moderate thinness; 17.0–18.49 mild thinness; 18.5–24.99 normal range; 25.0–29.99 overweight (pre-obese); 30.0–34.99 obese class I; 35.0–39.99 obese class II; 40.0 and above obese class III. The boundaries are exact cut points applied to a continuous quantity, which is why two people 0.1 apart can sit in different bands with nearly identical bodies.
Internal math uses full precision and only the display rounds — the 0.1 you see is a label on an unrounded number, and the healthy-weight range is computed from unrounded band edges, then rounded once for display. No step of the pipeline silently rewrites your input; every echo field shows exactly what entered the formula.
Frequently asked questions
Is BMI accurate for muscular people?
It misclassifies them upward, and predictably so: muscle is denser than fat, so a lean, muscular person can land in "overweight" at a low body fat percentage. This is a known structural limitation of weight-height indices, not a defect of this calculator. If you lift weights or carry above-average muscle, the body fat calculator will describe your composition better than any index built on weight and height alone.
What is BMI Prime and why show it?
BMI Prime is BMI divided by 25 — the overweight threshold — so it reads as a simple ratio: 0.92 means 8% below the overweight line, 1.12 means 12% above it. It is useful for comparing across heights and for tracking change toward or away from the threshold, because a fixed difference in BMI Prime means the same thing for a 1.60 m and a 1.90 m person.
What is the Ponderal index?
Weight divided by height cubed (kg/m³) instead of squared. Humans do not scale with the square of height, so the cubic form drifts less at the height extremes — tall people get slightly less penalized and short people slightly less excused than BMI does. Adult reference values cluster around 11–15 kg/m³. It is a companion index, not a replacement: the WHO dataset and all population research run on BMI.
Can I use this BMI calculator for my child?
No. Children and teens are classified on age- and sex-specific percentile charts (the CDC and WHO growth standards), not on adult fixed cutoffs, so an adult BMI calculator will misread every pediatric input. This site does not offer a children's mode — the honest answer for a child's BMI is the pediatric growth chart your clinician uses.
My BMI is normal but I still carry a lot of belly fat. Is that possible?
Yes — the pattern is called normal-weight obesity, and it is precisely the case where a weight-height index is blind. Waist circumference and body fat percentage catch what BMI misses; a waist above roughly 102 cm in men or 88 cm in women (risk-threshold values used in metabolic screening) is a signal worth discussing with a clinician regardless of BMI. The tape-measure method on our body fat page is a reasonable home starting point.
Which WHO category should I actually aim for?
The 18.5–24.9 normal range is the population-level reference band, and within it, risk curves from large cohorts are comparatively flat. But the categories are screening bands, not personal targets — where within (or slightly outside) the band your health sits depends on composition, waist size, blood markers, and fitness, none of which BMI sees. Use the number as one input, then get the composition and deficit numbers to go with it.
References & sources
Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis. 1972;25:329–343.
Supports: The origin of BMI as the standard weight-height index (the "Quetelet index", popularized by Keys) and its intended population-level use.
WHO. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series 854. Geneva, 1995.
Supports: The BMI cutoffs behind the eight-level classification shown on this page (including the thinness grades).
WHO. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva, 2000.
Supports: The overweight and obese class I–III thresholds used here.
Gallagher D, et al. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr. 2000;72:694–701.
Supports: Why BMI and body fat tell different stories for muscular and older individuals — the cross-reference for the body fat page.