BMI Explained: What Your Body Mass Index Really Means
Body Mass Index, or BMI, is the number doctors, insurers and health apps most often use to describe whether a weight is in a healthy range. It is quick to calculate and easy to compare, but it also has well-known limits. This guide explains what BMI is, how to calculate it, how to read the result and when to look beyond it. You can get your own number with our BMI calculator.
What BMI is
BMI relates your weight to your height. It was devised in the 19th century by the Belgian statistician Adolphe Quetelet to describe populations, and since the 1990s the World Health Organization has used it as a simple screening tool for adults. It does not measure body fat directly; it estimates whether weight is likely to be in a healthy range for a given height.
How to calculate BMI
- Metric: weight in kilograms ÷ (height in metres)². Example: 68 kg and 1.70 m → 68 ÷ 2.89 = 23.5.
- Imperial: weight in pounds ÷ (height in inches)² × 703. Example: 150 lb and 67 in → 150 ÷ 4,489 × 703 = 23.5.
How to read the result
| BMI | Category (adults) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
Some countries use lower thresholds for people of South Asian, Chinese and other Asian backgrounds, because research shows health risks such as type 2 diabetes appear at a lower BMI in these groups.
Why BMI is used
- It needs only a scale and a tape measure.
- It allows comparison between people and over time.
- At a population level, higher BMI is linked to higher risk of heart disease, type 2 diabetes and some cancers, and very low BMI to other risks.
The limits of BMI
- Muscle vs. fat: muscle is dense, so athletes can have an “overweight” BMI while being lean.
- Fat distribution: fat around the waist carries more risk than fat elsewhere, and BMI does not show where fat is.
- Age: older adults lose muscle, so a normal BMI can hide a higher body fat percentage.
- Children and teenagers: they need BMI-for-age percentile charts, not adult categories.
- Pregnancy: BMI is not meaningful during pregnancy.
Useful measures to use alongside BMI
- Waist circumference: many guidelines flag higher risk above about 94 cm (37 in) for men and 80 cm (31.5 in) for women, with higher thresholds in some populations.
- Waist-to-height ratio: keeping your waist under half your height is a simple, widely used target.
- Body fat percentage: measured with scales, calipers or scans; accuracy varies by method.
- Blood pressure, blood sugar and cholesterol: these say more about health than any single body measurement.
If your BMI is outside the healthy range
A single number is a starting point for a conversation, not a verdict. If your BMI is below 18.5 or above 25, consider talking to a doctor or a registered dietitian, especially before starting a diet or an intense exercise programme. Sustainable changes in sleep, activity and eating usually matter more than reaching a specific number.
Key takeaways
- BMI = weight ÷ height², a quick screening measure for adults.
- 18.5 to 24.9 is the healthy range for most adults.
- It does not distinguish muscle from fat, so combine it with waist measurements and medical checks.
Calculate yours with the free BMI calculator. Sources: WHO and Wikipedia: Body mass index. This guide is general information, not medical advice.