What Is BMI?
Body Mass Index (BMI) is a number derived from your weight and height that places you on a scale from underweight through obese. Belgian mathematician Adolphe Quetelet developed the underlying ratio in the 1830s as a sociological measure of the "average man." American physiologist Ancel Keys re-evaluated it in 1972, named it BMI, and concluded it was the best simple proxy for body fatness among available alternatives.
The World Health Organization adopted BMI for global use in 1995, setting the thresholds still used today: 18.5 for underweight, 25 for overweight, and 30 for obese. Its main advantage is simplicity, you need only a scale and a tape measure, and the calculation takes ten seconds.
The BMI Formula
There are two versions of the formula depending on your unit system:
Metric Formula (kg and cm)
Convert your height from centimetres to metres by dividing by 100 before squaring. A person who is 170 cm tall has a height of 1.70 m.
Imperial Formula (lb and inches)
The factor 703 converts the lb/in² unit to the same numeric scale as kg/m². Without it, a 150 lb, 5′8″ person would get a BMI around 0.032 instead of the correct 22.8.
Step-by-Step Worked Examples
Example 1, Metric: 75 kg, 175 cm
Example 2, Imperial: 160 lb, 5 ft 6 in
Example 3, Finding healthy weight range for 1.68 m
WHO BMI Weight Categories
The WHO defines four adult BMI categories using the same thresholds for both men and women (Asian-Pacific populations use lower thresholds, see below):
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Malnutrition, bone loss, immune deficiency |
| 18.5 – 24.9 | Normal weight | Lowest all-cause mortality (WHO) |
| 25.0 – 29.9 | Overweight | Elevated metabolic and cardiovascular risk |
| 30.0 – 34.9 | Obese Class I | Significantly elevated risk |
| 35.0 – 39.9 | Obese Class II | Severely elevated risk |
| 40.0 and above | Obese Class III | Extremely elevated risk (morbid obesity) |
Asian BMI Cutoffs
Large studies in Japan, China, South Korea, and India consistently found that South and East Asian populations develop insulin resistance, type 2 diabetes, and cardiovascular disease at lower BMI values than European populations. The WHO 2004 Expert Consultation recommended lower thresholds for these groups:
- ›Underweight: below 18.5 (same)
- ›Normal weight: 18.5 – 22.9 (vs 24.9 standard)
- ›Overweight: 23.0 – 27.4 (vs 29.9 standard)
- ›Obese: 27.5 and above (vs 30.0 standard)
The likely reason is that Asian populations carry a higher percentage of visceral (abdominal) fat at any given BMI, and visceral fat is more metabolically harmful than subcutaneous fat.
BMI Prime and Ponderal Index
Two derived metrics give a more intuitive picture than raw BMI:
BMI Prime divides your BMI by 25 (the upper boundary of Normal). A BMI Prime of exactly 1.0 means your BMI is precisely at the top of the normal range. A value of 1.15 means you are 15% above Normal, much easier to interpret than "your BMI is 28.75."
Ponderal Index divides weight by height cubed rather than squared. This is theoretically more accurate for very tall or very short individuals, since body volume scales with height³.
Key Limitations of BMI
BMI is a screening tool, not a diagnosis. It systematically misclassifies several groups:
- ›Athletes and bodybuilders, dense muscle mass inflates BMI into the "overweight" range despite low body fat. An elite sprinter can have BMI 27 with body fat under 8%.
- ›Older adults, muscle loss with age means the same BMI can represent more body fat in a 70-year-old than a 30-year-old.
- ›Sex differences, women carry 8–10% more essential body fat than men at the same BMI; the same thresholds apply to both.
- ›Height extremes, BMI overestimates fatness in very tall people and underestimates it in very short people.
- ›Fat distribution, BMI ignores where fat is stored. Abdominal (visceral) fat is far more metabolically harmful than fat elsewhere; waist circumference or waist-to-height ratio captures this better.
How BMI Is Used in Medicine
Despite its limitations, BMI drives real clinical and policy decisions:
- ›Bariatric surgery, most guidelines require BMI ≥ 40, or BMI ≥ 35 with obesity-related comorbidities.
- ›Anti-obesity medications, licensed at BMI ≥ 30, or ≥ 27 with comorbidity.
- ›Gestational weight gain, the Institute of Medicine sets pregnancy weight targets by pre-pregnancy BMI category.
- ›Population surveillance, the WHO tracks global obesity trends using self-reported or measured BMI.
Quick Reference
Imperial shortcut: divide your weight in lb by your height in inches twice, then multiply by 703.
Use our free BMI Calculator to get your BMI, BMI Prime, Ponderal Index, and healthy weight range instantly.