What is BMI?
Body Mass Index (BMI) is a simple ratio of weight to height that gives a rough idea of whether a person is at a healthy weight for their stature. It's used by doctors, public-health agencies and insurers as a quick screening tool — it doesn't measure body fat directly, but it correlates well enough with health risk to be useful in large populations.
How to interpret your number
For adults aged 20 and above, the World Health Organization categories are:
- Under 18.5 — underweight
- 18.5 – 24.9 — healthy range
- 25.0 – 29.9 — overweight
- 30.0 and above — obese (Class I, II or III at 35 and 40)
Children and teens are evaluated on age- and sex-specific percentile charts instead — use our child's BMI calculator for that.
Where BMI falls short
BMI was designed for populations, not individuals. It can't tell muscle from fat, so athletes with high lean mass often register as overweight while still being very healthy. Older adults can register as healthy while carrying high body fat (sometimes called "skinny-fat"). Treat BMI as one signal among several — waist circumference, blood pressure, lipids and activity level all matter more.
Improving your BMI safely
If your BMI is outside the healthy range, sudden crash diets and extreme training plans tend to backfire. Most evidence-based guidance recommends a sustainable 0.25–1 kg (0.5–2 lb) change per week through a modest calorie deficit or surplus, regular strength training and at least 150 minutes of moderate cardio a week. Pair this BMI tool with our macro calculator to plan the food side.