How BMI Is Actually Calculated (And Its Limits)
A quick look at the formula behind Body Mass Index, and why it does not tell the whole story.
Body Mass Index is the most quoted number in health screening and one of the most misunderstood. It compresses two measurements — height and weight — into a single figure, which makes it cheap, fast and reproducible, and also blind to almost everything that actually determines metabolic health.
The formula, in both unit systems
Metric BMI is weight in kilograms divided by height in metres squared. The imperial version multiplies by 703 to reconcile pounds and inches.
BMI (metric) = kg / m²
BMI (imperial) = (lb / in²) × 703A person weighing 78 kg at 1.78 m has a BMI of 78 / (1.78 × 1.78) = 24.6. The same person described as 172 lb and 70 in gives (172 / 4900) × 703 = 24.7 — the small gap is rounding in the unit conversion, not a different method.
Worked examples
| Height | Weight | BMI | WHO band |
|---|---|---|---|
| 1.60 m | 48 kg | 18.8 | Normal (just above underweight) |
| 1.75 m | 68 kg | 22.2 | Normal |
| 1.75 m | 86 kg | 28.1 | Overweight |
| 1.90 m | 112 kg | 31.0 | Obese class I |
Where the bands come from
The cut-offs of 18.5, 25 and 30 were set by the World Health Organization from mortality data in large, mostly European populations. They are statistical inflection points across millions of people, not thresholds where anything changes in one body.
| BMI | WHO classification | What it signals |
|---|---|---|
| Under 18.5 | Underweight | Screen for nutrition, illness, bone density |
| 18.5–24.9 | Normal range | Lowest average mortality in the source data |
| 25.0–29.9 | Overweight | Elevated average cardiometabolic risk |
| 30.0–34.9 | Obesity class I | Risk rises steadily with the number |
| 35.0 and above | Obesity class II–III | Strongly associated with comorbidity |
Why BMI misclassifies some people
Because the formula only knows height and weight, it cannot distinguish muscle from fat, nor where fat sits.
- Muscular athletes are routinely classed overweight at 12% body fat, because muscle is denser than fat.
- Older adults can lose substantial muscle and gain visceral fat while their BMI never moves — sarcopenic obesity that screening misses entirely.
- Risk thresholds differ by ancestry. Health authorities in several Asian countries use 23 as the overweight cut-off, because cardiometabolic risk appears at a lower BMI.
- The squared term flatters tall people slightly and penalises short people, because human bodies do not scale as a perfect square of height.
What to measure alongside it
- Waist circumference — over 94 cm for men or 80 cm for women marks raised risk, and it detects the visceral fat BMI ignores.
- Waist-to-height ratio — keeping your waist under half your height is a simple check that works across body types and ages.
- Waist-to-hip ratio — distinguishes central from peripheral fat distribution.
- Resting blood pressure, fasting glucose and a lipid panel — these measure the outcomes BMI is only guessing at.
You can run the first three in seconds with the BMI calculator, waist-to-hip ratio calculator and ideal weight calculator.
Tracking change beats a single reading
The useful signal is direction, not position. A BMI drifting from 27 to 29 over two years is more informative than a one-off 28, and it is worth logging alongside waist measurements, since losing fat while gaining muscle can hold BMI perfectly still while health improves.
Frequently asked questions
Is BMI accurate for children?
No — children use age-and-sex-specific BMI percentile charts, because body composition changes rapidly during growth. An adult BMI band applied to a child is meaningless.
Does BMI work during pregnancy?
No. Weight gain during pregnancy is expected and is tracked against pre-pregnancy BMI instead, so a mid-pregnancy BMI reading has no clinical interpretation.
Why do some countries use 23 as the overweight threshold?
Populations differ in the BMI at which type 2 diabetes and cardiovascular risk begin to climb. Several Asian health bodies lowered the cut-off to 23 after studies showed risk appearing well below the WHO figure of 25.
What is a healthy body fat percentage instead?
Broadly 8–19% for men and 21–32% for women, varying with age and measurement method. Body fat percentage requires a scan, calliper or bioimpedance device — BMI is the free approximation people reach for when none is available.