How BMI Is Calculated (and What It Really Tells You)
BMI — Body Mass Index — is a single number relating weight to height, used worldwide as a first-pass weight screening tool. Here is exactly how it is calculated and how to interpret it honestly.
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The Formula
For imperial measurements the formula is adjusted: BMI = 703 × weight (lb) ÷ height (in)². The 703 converts pounds to kilograms and inches to metres — it is the same calculation in different units.
Example: a 70 kg person who is 1.75 m tall has BMI = 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9.
The WHO Categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These cut-offs are the World Health Organization adult standard, applied the same way to men and women.
BMI Prime and Healthy Weight Range
BMI Prime divides your BMI by 25 (the top of the normal range), giving a quick "how far from normal" ratio: 0.9 is below normal, 1.0 is at the boundary, 1.2 is 20% over. The healthy weight range for your height is simply the weight band corresponding to BMI 18.5–24.9.
The Limitations You Should Know
BMI does not measure body fat — it is a ratio of weight to height. Muscular people can score "overweight" despite low body fat, older adults may have higher body fat at the same BMI, and the same BMI can mean different things for different body compositions.
BMI is a screening tool, not a diagnosis. For a fuller picture, pair it with body-fat estimates, waist measurements and professional advice.
Article FAQ
How is BMI calculated in kg and cm?
Divide weight in kilograms by height in metres squared. For 170 cm, use 1.70 in the formula.
What is a healthy BMI?
18.5 to 24.9 is the WHO normal range. Below is underweight, 25–29.9 overweight, 30+ obese.
Why is my BMI "overweight" when I lift weights?
BMI cannot distinguish muscle from fat. Very muscular individuals often exceed 25 despite low body fat — a known limitation.