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Body Mass Index is a quick way to relate someone's weight to their height. It doesn't measure body fat directly, but because most people's weight rises roughly in proportion to the square of their height, BMI turns out to be a useful, easy-to-calculate screening number. Doctors, insurers, and public health agencies use it as a first-pass signal of whether someone's weight falls in a range associated with higher health risk — not as a diagnosis on its own.
The World Health Organization defines the following bands for adults aged 20 and over, applied the same way to men and women:
| Classification | BMI range (kg/m²) |
|---|---|
| Severe thinness | below 16 |
| Moderate thinness | 16 – 17 |
| Mild thinness | 17 – 18.5 |
| Normal weight | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese, Class I | 30 – 35 |
| Obese, Class II | 35 – 40 |
| Obese, Class III | above 40 |
Kids are still growing, so a fixed BMI scale doesn't make sense for them. Instead, the CDC plots a child's BMI against thousands of other children of the same age and sex and reports where they land as a percentile:
| Category | Percentile |
|---|---|
| Underweight | below 5th percentile |
| Healthy weight | 5th – 85th percentile |
| At risk of overweight | 85th – 95th percentile |
| Overweight | above 95th percentile |
This calculator reports a raw BMI value for any age, but the fixed adult categories and the related stats below (healthy weight range, BMI Prime, Ponderal Index) only apply once growth has leveled off, so we show them for ages 20 and up. For a child or teen, compare the number against a pediatric growth chart or ask a pediatrician rather than reading it against the adult table.
Carrying excess weight for your height is statistically linked to a longer list of health problems, including:
None of this means a single BMI reading determines someone's fate — but a BMI persistently above 25 is a reasonable cue to talk with a doctor about whether lifestyle changes would help.
Being underweight carries its own set of risks:
A persistently low BMI — especially when it's unexplained — is worth discussing with a doctor, since it can sometimes point to an underlying condition such as an eating disorder or another illness.
BMI is built from just two numbers, height and weight, so it can't distinguish muscle from fat. A heavily muscled athlete and a sedentary person of the same height and weight will get the identical BMI, even though their actual body composition — and health risk — can be very different. A few general patterns worth knowing:
Despite these caveats, BMI correlates reasonably well with body fat for the large majority of people, which is exactly why it remains the standard first screening tool — it just shouldn't be the only number a clinician looks at.
The metric version divides weight in kilograms by height in meters, squared:
The US customary version uses pounds and inches, with a 703 conversion factor folded in:
For example, someone who is 5′10″ (70 in) and weighs 160 lb comes out to 703 × 160 ÷ 70² ≈ 23.0 kg/m² — the same result you'd get converting that same person to 1.778 m and 72.6 kg and using the metric formula.
BMI Prime simply expresses your BMI as a fraction of the upper edge of the "normal" range (25 kg/m²):
Because it's a ratio rather than an absolute number, BMI Prime makes it easy to see at a glance how far someone sits from the normal cutoff, and to compare that distance across people or groups even if their reference cutoff differs slightly by country. A BMI Prime under 0.74 lines up with underweight, 0.74–1 with normal, 1–1.2 with overweight, and above 1.2 with obesity.
The Ponderal Index answers the same basic question as BMI — how heavy is this person relative to their height — but cubes the height instead of squaring it:
Cubing the height makes the Ponderal Index noticeably more reliable than BMI for people at the extremes of the height range, where BMI tends to overstate leanness in very tall people and overstate corpulence in very short people. It's used less often day-to-day, but it's a useful cross-check alongside BMI.