Calculate your Body Mass Index (BMI) instantly. Understand healthy weight ranges, obesity classification, and personalized health insights.
| Category | BMI range |
|---|---|
| Underweight | < 18.5 |
| Normal / Healthy Weight← your result | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese Class I | 30.0 – 34.9 |
| Obese Class II | 35.0 – 39.9 |
| Obese Class III | ≥ 40.0 |
Body Mass Index (BMI) is a simple screening measure that relates your weight to your height. In metric units it is your weight in kilograms divided by the square of your height in metres (kg/m²). In imperial units the same ratio is expressed as 703 × weight in pounds divided by the square of height in inches. Because it uses only two inputs, BMI is quick to compute and widely used by the World Health Organization (WHO) and the UK National Health Service (NHS) to flag whether a person's weight sits in a range associated with higher health risk.
The score itself is a continuous number, but it is usually grouped into categories. A BMI below 18.5 is underweight, 18.5 to 24.9 is a normal or healthy weight, 25.0 to 29.9 is overweight, and 30.0 and above is obese. Obesity is further divided into three classes — I (30.0–34.9), II (35.0–39.9), and III (40.0 and above) — because the health risks climb steeply as the score rises. These thresholds are the same for adult men and women and are the standard used across clinical guidance in the UK and internationally.
The table above lists every adult classification tier. The row matching your current result is highlighted so you can see at a glance where your score falls. Remember that these bands describe population risk, not an individual diagnosis — two people with the same BMI can have very different body compositions and health profiles.
BMI measures body size, not body fat directly. Because muscle is denser than fat, a high-performance athlete with substantial muscle mass can have a BMI in the overweight or obese range while carrying very little body fat — BMI can therefore overestimate adiposity in muscular individuals. Conversely, older adults often lose muscle mass with age, so a normal BMI can mask a low muscle-to-fat ratio and underestimate health risk. BMI also does not account for where fat is stored or for ethnic differences in risk: people of South Asian, Chinese, and other backgrounds may face elevated metabolic risk at lower BMI values than the general thresholds suggest, which is why some guidance recommends lower action thresholds for these groups.
Because BMI alone can mislead, clinicians pair it with other simple measures. Waist circumference is one of the most useful: keeping your waist below roughly half your height is a practical target that reflects abdominal fat, which is more strongly linked to cardiovascular and metabolic risk than fat stored elsewhere. Other useful companions include waist-to-hip ratio, body fat percentage, and blood pressure. Use BMI as a starting point, then consider these additional metrics and your overall health context before drawing conclusions.