What is BMI?
Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you're in a healthy weight range. The formula is:
BMI = weight (kg) ÷ height (m)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
It was originally developed in the 19th century by Belgian statistician Adolphe Quetelet — not as a clinical tool, but as a population-level statistical measure. It was later adopted by health organizations because it's cheap, quick, and requires no equipment.
BMI ranges: what the numbers mean
The World Health Organization defines the following categories for adults:
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
So a "healthy BMI" is generally considered to be between 18.5 and 24.9. This range is associated with lower risk of weight-related health conditions like type 2 diabetes, cardiovascular disease, and hypertension.
📏 Note on ethnicity
Some health organizations use adjusted BMI thresholds for people of Asian descent — a BMI of 23 or above may indicate increased health risk, rather than the standard 25 cutoff. If you're of Asian heritage, speak with your doctor about the appropriate range for you.
What BMI doesn't tell you
Here's the honest limitation of BMI: it measures weight relative to height, but it cannot distinguish between fat and muscle. This creates real blind spots:
- Muscular athletes often have a BMI in the "overweight" or even "obese" range despite having very low body fat.
- Older adults may have a "normal" BMI while carrying a high proportion of body fat due to age-related muscle loss (sarcopenia).
- Fat distribution matters. Visceral fat (fat around the organs) is more metabolically harmful than subcutaneous fat, but BMI doesn't capture where fat is stored. Waist circumference is often more predictive of metabolic risk.
- BMI doesn't account for sex. Women naturally carry more body fat than men at the same BMI, yet the same thresholds apply to both.
None of this means BMI is useless — it just means it's a screening tool, not a diagnosis. A high BMI is a signal worth investigating further, not a verdict on your health.
What to use alongside BMI
If you want a more complete picture of your health, these metrics complement BMI well:
- Waist circumference: A waist over 88 cm (35 in) for women or 102 cm (40 in) for men is associated with increased metabolic risk, regardless of BMI.
- Waist-to-height ratio: A ratio under 0.5 is generally considered healthy. Simply divide your waist circumference by your height — both in the same unit.
- Body fat percentage: More direct than BMI. Healthy ranges are roughly 21–33% for women and 8–20% for men (varies by age).
- Blood markers: Fasting glucose, HbA1c, lipid panels, and blood pressure give a far more clinically meaningful picture than any single body measurement.
BMI for children and teens
Standard BMI categories don't apply to people under 18. For children and adolescents, BMI is plotted on age- and sex-specific growth charts, and expressed as a percentile rather than an absolute number. This accounts for the fact that healthy weight ranges change significantly during development.
- Below 5th percentile: Underweight
- 5th to 85th percentile: Healthy weight
- 85th to 95th percentile: Overweight
- 95th percentile and above: Obese
Want to calculate your BMI right now? Use our free BMI calculator — it also shows your healthy weight range for your height.
Calculate my BMI →The bottom line
A healthy BMI is between 18.5 and 24.9 for most adults. If your BMI falls outside this range, it's worth paying attention to — but it's not the only thing that matters. Body composition, where you carry fat, your fitness level, and your blood markers all tell a fuller story than a single number can.
Use BMI as a starting point, not a final answer.