BMI — Body Mass Index — is the world's most widely used measure for assessing healthy weight. It's calculated by dividing your weight in kilograms by the square of your height in metres. Simple, fast, and free. But is it right for you?
The Standard BMI Ranges (WHO)
The World Health Organization defines four categories for adults aged 18 and over:
| BMI Range | Category | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | Possible nutritional deficiency, increased infection risk |
| 18.5 – 24.9 | Normal weight | Lowest risk of weight-related health conditions |
| 25.0 – 29.9 | Overweight | Elevated risk of cardiovascular disease, diabetes |
| 30.0 and above | Obese | Significantly elevated risk — medical guidance recommended |
Does BMI Change With Age?
For adults, the same BMI ranges apply regardless of age. However, body composition changes as we get older — older adults tend to carry more fat and less muscle at the same BMI. Some researchers suggest that adults over 65 may benefit from a slightly higher BMI (up to 27) for protection against bone loss and illness.
For children and teenagers, BMI is assessed differently. Paediatricians use age- and sex-specific growth charts rather than fixed cut-off values, because children's body composition changes significantly during development.
The Limitations of BMI
Important: BMI is a screening tool, not a diagnostic measure. It has well-documented limitations that every user should understand.
BMI does not distinguish between muscle and fat. A professional athlete may register as "overweight" with a BMI of 27, while carrying very little body fat. Conversely, an older adult with a "normal" BMI of 23 may have high body fat due to muscle loss.
BMI also doesn't measure where fat is stored. Abdominal (visceral) fat carries higher health risks than fat stored around the hips. Two people with identical BMIs can have very different health profiles.
Research also shows that BMI cut-offs may underestimate obesity risk in certain Asian populations, while potentially overestimating risk in people of African descent.
What Should You Use Alongside BMI?
- Waist circumference: Above 88cm (women) or 102cm (men) indicates increased cardiovascular risk
- Body fat percentage: More precise than BMI for assessing actual fat levels
- Waist-to-height ratio: Divide waist by height — below 0.5 is generally healthy
- Blood markers: Blood pressure, cholesterol, blood glucose tell a fuller story
How to Interpret Your BMI Result
If your BMI falls in the normal range (18.5–24.9), that's a positive indicator — but not a guarantee of health. Continue with regular physical activity, a balanced diet, and routine health checks.
If your BMI is outside the normal range, use it as a starting point for a conversation with your doctor rather than a cause for alarm. A healthcare professional can assess your full health picture — not just one number.