How it works
What BMI actually measures
Body Mass Index is a simple ratio invented in the 1830s by a Belgian statistician and adopted by the World Health Organization in the 1990s as a population-level screening tool. It's popular because it needs only two measurements — weight and height — and gives a single number that correlates reasonably well with body fat across large groups. For an individual, it's a starting point, not a verdict.
NHS BMI categories for adults
| Category | BMI range (kg/m²) | Clinical note |
|---|---|---|
| Underweight | Below 18.5 | May indicate undernutrition; check with GP |
| Healthy weight | 18.5 – 24.9 | Lowest statistical health risk |
| Overweight | 25 – 29.9 | Increased cardiovascular and metabolic risk |
| Obese (Class I) | 30 – 34.9 | Substantially increased risk |
| Obese (Class II) | 35 – 39.9 | Severely increased risk |
| Obese (Class III) | 40+ | Very severely increased risk |
How to calculate BMI by hand
Metric is the natural unit. If you know your height in centimetres and weight in kilos:
Metric example
Height 1.68 m, weight 65 kg.
BMI = 65 / (1.68 × 1.68) = 65 / 2.8224 ≈ 23.0 — healthy weight.
Imperial example (stones, pounds and feet/inches)
5'8" and 11 st 4 lb → 1.7272 m and 71.67 kg.
BMI = 71.67 / 1.7272² ≈ 24.0 — upper end of healthy weight.
A quick imperial shortcut: BMI ≈ (weight in lbs × 703) / (height in inches)². For 158 lb and 68 in: (158 × 703) / 68² = 111,074 / 4,624 ≈ 24.0.
Lower thresholds for some ethnic groups
NHS and NICE guidance (2022) recommend lower BMI thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, because cardiovascular and type 2 diabetes risk rise at lower BMI in these groups.
| Category | General population | Higher-risk ethnic groups |
|---|---|---|
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obese | 30+ | 27.5+ |
When BMI is misleading
BMI is a ratio of weight to height squared, which means it can't distinguish muscle from fat or tell you where the fat sits. It misreads in several everyday situations:
- Very muscular people — rugby players and regular weightlifters often register "overweight" on BMI despite low body fat.
- Older adults — natural muscle loss can make BMI understate body-fat percentage. Waist circumference is often more informative.
- Pregnancy — BMI categories don't apply. Clinicians use pre-pregnancy BMI to guide weight-gain targets.
- Children and teenagers — different BMI-for-age percentiles apply; use NHS child BMI tool.
Better paired metrics
Pair BMI with waist circumference for a much better read on health risk. NHS guidance (2022) considers these waist-to-height ratios:
- Healthy: less than 0.5 (waist < half your height).
- Increased risk: 0.5 – 0.6.
- High risk: above 0.6.
What to do with your result
A BMI in the healthy range is reassuring at a population level, but day-to-day fitness, diet quality, sleep, smoking, alcohol and mental wellbeing matter at least as much. If your BMI is above or below the healthy range, it's a prompt to talk to a GP, not a reason to panic — especially if other markers (blood pressure, cholesterol, waist) are fine.
If you'd like to plan changes, our calorie calculadora estimates daily energy needs, and our ideal weight calculadora shows target weight windows using multiple validated formulas.