BMI Formula: What the Number Means and Where It Fails
The BMI formula in metric and imperial units, the WHO categories, and the three situations where BMI gives a misleading answer.
2026-09-11 · Health
The formula
Metric: weight (kg) ÷ height (m)². Imperial: 703 × weight (lb) ÷ height (in)². Both produce the same number.
The categories
Under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese. The healthy range for a 172 cm adult works out to 55–74 kg.
Where BMI fails
Athletes read high because muscle is denser than fat. Older adults lose muscle and read artificially low. South Asian populations show metabolic risk at lower BMIs, which is why some guidance caps the healthy range at 23.
What to use alongside
Waist circumference, waist-to-height ratio (keep it under 0.5) and a trend over months. A single reading is a snapshot; a direction is a signal.
What is a healthy BMI?
18.5–24.9 on the WHO scale, with a lower ceiling for South Asian populations in some guidelines.
Is BMI accurate for children?
No — children use age and sex specific percentile charts, not the adult bands.
Can I use BMI to track progress?
As one of several measures. Waist and photos track fat loss better.
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