Özet
Body Mass Index (BMI) is a simple screening measure that uses your weight and height to estimate whether you’re in a healthy weight range. It’s used by the NHS and WHO as a population-level indicator, though it has well-known limitations for individuals.
Nasıl çalışır
BMI divides your weight (in kilograms) by the square of your height (in metres). The resulting number is compared against standard categories to classify your weight status.
The calculation itself is straightforward — the nuance lies in which thresholds you compare against. Standard WHO thresholds may underestimate health risk for people of South Asian, Chinese, and other ethnic backgrounds, who tend to develop metabolic complications at lower BMIs.
Formül
Where
Standard WHO/NHS categories
| BMI range | Category |
|---|---|
| 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+ | Obese (Class III) |
Ethnic-adjusted thresholds (NICE PH46)
For South Asian and Chinese populations, NICE recommends lower thresholds:
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 22.9 | Healthy weight |
| 23.0 – 27.4 | Overweight (increased risk) |
| 27.5+ | Obese (high risk) |
Çözülmüş örnek
Person: 80 kg, 1.75 m tall
Square the height
= 3.0625
Divide weight by height squared
= 26.12
Classify against WHO thresholds
= Overweight
Result
BMI = 26.1 — Overweight (WHO standard)
Girdiler açıklaması
- Weight — your body weight. Can be entered in kg or stones/pounds — the calculator converts to kg internally.
- Height — your height. Can be entered in cm or feet/inches — converted to metres internally.
- Ethnicity — optional. If South Asian or Chinese is selected, the calculator uses NICE PH46 adjusted thresholds.
Çıktılar açıklaması
- BMI value — the calculated number (typically between 15 and 50)
- Category — underweight, healthy, overweight, or obese classification
- Healthy weight range — the weight range that would give you a BMI of 18.5–24.9 at your height
- Distance from healthy — how many kg above or below the healthy range you currently are
Varsayımlar ve sınırlamalar
- BMI does not distinguish fat from muscle. A muscular athlete may have a “overweight” BMI despite low body fat. Use body fat % or waist-to-hip ratio for a more complete picture.
- Not designed for children — BMI-for-age percentile charts are used for under-18s.
- Not accurate during pregnancy.
- Population-level tool, not a diagnosis. BMI is a screening indicator. Clinical assessment considers waist circumference, blood pressure, blood sugar, and family history alongside BMI.
- The formula was developed by Adolphe Quetelet in the 1830s for population statistics, not individual health assessment.
Doğrulama
| Test case | Weight (kg) | Height (m) | Expected BMI | Category |
|---|---|---|---|---|
| Underweight | 50 | 1.75 | 16.3 | Underweight |
| Healthy | 70 | 1.75 | 22.9 | Healthy |
| Overweight | 80 | 1.75 | 26.1 | Overweight |
| Obese | 100 | 1.75 | 32.7 | Obese (Class I) |
| Very tall | 90 | 2.00 | 22.5 | Healthy |
All values verified against the NHS BMI calculator.
Sources
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