How BMI is calculated
A 70 kg person who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9 — inside the healthy range.
WHO adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
What BMI can and can't tell you
BMI is a population-level screening tool: it correlates with body fat for most adults and predicts health risk trends across large groups. It does not measure body composition — muscular athletes often read "overweight" with low body fat, and older adults can read "normal" while carrying little muscle. Waist circumference, waist-to-height ratio, and a body fat estimate add useful context. For children and teens, BMI is interpreted against age-and-sex percentile charts instead of these fixed cutoffs.
This tool provides general estimates for educational purposes and is not medical advice. Consult a healthcare professional for guidance specific to you.
Frequently asked questions
What is a healthy BMI?
For adults, the World Health Organization defines a normal BMI as 18.5 to 24.9. Below 18.5 is underweight; 25 to 29.9 is overweight; 30 and above is classed as obesity. Some Asian populations use a lower overweight threshold of 23.
Is BMI accurate for athletes?
Not very. Muscle is denser than fat, so muscular people often have a high BMI with low body fat — many rugby players and sprinters are technically 'overweight'. For trained individuals, body fat percentage and waist measurements are far more informative.
How can I lower my BMI safely?
A sustainable target is losing 0.25–0.5 kg (0.5–1 lb) per week through a moderate calorie deficit — see our calorie calculator — combined with resistance training to preserve muscle. Crash diets lower BMI fast but mostly through water and muscle loss.
Does BMI apply to children?
The formula is the same but interpretation differs: children's BMI is compared against growth-chart percentiles for their age and sex. A pediatrician can plot this correctly; adult cutoffs like 25 or 30 don't apply.
What are BMI's main limitations?
It ignores muscle mass, fat distribution, age, and ethnicity. Two people with identical BMIs can carry very different health risks — abdominal (visceral) fat is more harmful than fat stored elsewhere, which BMI cannot see.