BMI: What It Is, How It's Calculated, and What Your Number Means
Body Mass Index explained — the formula, what healthy BMI ranges mean, the limitations of BMI as a health measure, and when to use other measurements instead.
BMI — Body Mass Index — is one of the most widely used health screening tools in the world, yet it is also one of the most misunderstood. It can be a useful indicator when interpreted correctly, but it has real limitations that are worth knowing.
Check your own number instantly with our BMI Calculator.
What Is BMI?
BMI is a numerical value calculated from your height and weight. It was developed in the 19th century by Belgian mathematician Adolphe Quetelet and adopted by the medical community as a quick way to classify weight relative to height.
It is not a direct measure of body fat — it cannot tell the difference between muscle and fat. But at a population level, it correlates reasonably well with more precise measures like dual-energy X-ray absorptiometry (DEXA) scans.
The BMI Formula
Metric:
BMI = weight (kg) ÷ height (m)²Imperial:
BMI = (weight (lbs) ÷ height (inches)²) × 703Example: A person who is 175 cm (1.75 m) tall and weighs 75 kg:
BMI = 75 ÷ (1.75)² = 75 ÷ 3.0625 = 24.5
BMI Categories (Adults)
| BMI | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely Obese (Class III) |
These thresholds are set by the World Health Organisation (WHO) and are used by the NHS.
Adjusted Thresholds for Asian Adults
Research has shown that people of South Asian, Chinese, Japanese, and other Asian backgrounds tend to develop health complications at lower BMI values than the WHO thresholds suggest. Many health authorities now use lower action thresholds for these groups:
| Classification | General population | Asian populations |
|---|---|---|
| Overweight | 25.0+ | 23.0+ |
| Obese | 30.0+ | 27.5+ |
What BMI Can and Cannot Tell You
What it can tell you:
- A rough indicator of whether your weight is in a healthy range for your height
- A starting point for conversations with a GP or healthcare professional
- A useful population-level screening tool
What it cannot tell you:
- How much of your weight is fat versus muscle
- Where fat is distributed on your body (visceral fat around the abdomen carries higher health risk than subcutaneous fat)
- Whether you are metabolically healthy
- Your fitness level, bone density, or hydration status
A highly muscular person — an athlete or weightlifter — may have a BMI in the overweight range while having very low body fat. Conversely, someone with a healthy BMI may carry excess fat around their organs (sometimes called “skinny fat” or normal-weight obesity).
Better Alongside BMI: Waist Circumference
Waist circumference is a better predictor of cardiovascular and metabolic risk than BMI alone. Health risk increases significantly when waist circumference exceeds:
- 88 cm (35 inches) in women
- 102 cm (40 inches) in men
Waist-to-height ratio (waist ÷ height, both in the same unit) is also useful. A ratio below 0.5 is generally considered healthy.
BMI in Children
For children and teenagers, BMI is assessed differently. Rather than fixed thresholds, children’s BMI is compared to reference data for the same age and sex, producing a centile score. The NHS uses BMI centile charts for children aged 2 to 18.
When to Talk to Your GP
BMI is a screening tool, not a diagnosis. If your BMI is outside the healthy range — particularly if it is below 18.5 or above 30 — it is worth discussing with your GP, who can consider your full picture alongside other health markers.