BMI Calculator and Interpretation
Body mass index is a simple statistical yardstick used by the WHO to classify body size. It is informative as long as its limits are known: it cannot tell muscle mass from fat mass.
The BMI formula
A 70 kg adult measuring 1.75 m gets 70 ÷ 3.0625, a BMI of 22.9 — squarely in the normal range. The index is unitless and read against WHO thresholds.
| BMI | Interpretation |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal range |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class I |
| 35 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
The limits of the index
BMI says nothing about body composition. A heavily muscled athlete can post a BMI of 28 without excess fat, while a sedentary person can show a normal BMI with an unfavourable fat share. For children and teenagers, interpretation uses age-and-sex percentile curves, not adult thresholds.
- Older adults: a BMI slightly above the normal range is often better tolerated, as undernutrition is the main risk.
- Pregnancy: the index is meaningless during pregnancy; weight gain follows dedicated curves.
- Waist circumference: it complements BMI by measuring abdominal fat. Above 94 cm for men and 80 cm for women, risk rises.
This tool is informational. It makes no diagnosis and never replaces a medical consultation.
FAQ
Which BMI is considered normal?
Between 18.5 and 24.9 for adults under 65, according to the WHO classification adopted by health authorities.
Is BMI reliable for athletes?
No. Muscle is denser than fat, so an athlete can post a high BMI without excess fat. Waist circumference or body-composition measurement is more relevant then.
How should BMI be read after 65?
A slightly elevated BMI is often better tolerated in older adults, since undernutrition is the major risk. Only a doctor can interpret the index in that context.
Is waist circumference more useful than BMI?
They are complementary. BMI locates overall body size; waist circumference locates abdominal fat, which drives part of the cardiometabolic risk.