Why doctors still use BMI
BMI needs only a tape measure and a scale, costs nothing and gives comparable numbers across clinics and countries. Large studies link a BMI well above or below the healthy range with higher rates of type 2 diabetes, heart disease, some cancers and early death. As a first screening step it works well; the next steps — waist measurement, blood tests and a conversation about lifestyle — add the detail BMI cannot give.
Tracking your BMI over time
One reading is a snapshot. Checking your BMI every month or two shows the trend, which matters more than any single number. Use the same scale, the same time of day and similar clothing each time. If you are changing your weight on purpose, a steady change of about 0.5–1 kg (1–2 lb) a week is what most health services recommend. Press Copy result under your result to keep a note of each reading.
Adult BMI categories
From the age of 20, everyone uses the same World Health Organization categories, whatever their sex or age:
| BMI (kg/m²) | Adult category (WHO) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 or more | Obesity class III |
Unit conversions
- 1 inch = 2.54 cm; 1 foot = 30.48 cm; 5 ft 7 in = 170.2 cm.
- 1 kg = 2.2046 lb; 1 lb = 0.4536 kg.
- 1 stone = 14 lb = 6.35 kg; 10 st 7 lb = 147 lb = 66.7 kg.
The calculator converts automatically, so you can enter whichever units you know and read your healthy weight range back in the same units.