Calculate your Body Mass Index with age-adjusted interpretation — understand what your number actually means for your health.
Body Mass Index (BMI) is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²). Developed in the 19th century as a population-level tool, it remains the most widely used weight-health screening measure in clinical practice — primarily because it's free, instant, and broadly correlates with body fat at a population level.
| BMI Range | Category | Associated Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk — malnutrition, osteoporosis, immune function |
| 18.5 – 24.9 | Healthy weight | Lowest risk for most chronic conditions |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40.0+ | Obese Class III | Extremely high risk |
The standard BMI thresholds were established using data from younger adults. As we age, body composition shifts even when BMI stays the same — muscle mass declines while fat (especially visceral fat) tends to increase. A BMI of 27 means something different at 30 than at 70.
For adults over 65, research suggests a BMI of 25–27 may be associated with lower mortality than the standard "healthy" range — a well-documented "obesity paradox." Some reserve weight appears protective in older age against illness-related weight loss and frailty.
A highly muscular person can have an "overweight" BMI while carrying very little body fat. This is why athletes and people with high muscle mass should interpret BMI with caution.
Visceral fat (around organs, creating an "apple" shape) is far more metabolically dangerous than subcutaneous fat. Two people with identical BMIs can have very different health risks. Waist circumference is a better predictor of metabolic risk — high risk is defined as above 94cm for men and 80cm for women.
People of South Asian, East Asian and other non-European backgrounds have higher cardiometabolic risk at lower BMI values. NHS guidance recommends lower thresholds (overweight: 23+, obese: 27.5+) for South Asian and Chinese populations.
Waist circumference, waist-to-height ratio (aim below 0.5), and body fat percentage from DEXA scanning provide a more complete picture of body composition and metabolic risk than BMI alone.