What BMI tells you and what it doesn't — and why the “normal” range is so wide
Published: 15 September 2026 · Izračunaj.ba
BMI (body mass index) divides weight in kilograms by height in metres squared and places adults in the WHO categories — normal weight is a BMI of 18.5 to 24.9. It is a screening tool, not a diagnosis: at a height of 175 cm the “normal” range spans as much as 19.6 kg, the same 80 kg is normal weight at 180 cm but overweight at 170 cm, and none of these numbers says what the kilograms are made of — muscle or fat. The calculator is informational and not medical advice; for a health assessment, see a doctor.
How it is calculated and what the number means
BMI = weight (kg) ÷ height² (m)
Example: a person of 82 kg and 180 cm. Height in metres is 1.8, its square 3.24, so BMI is 82 ÷ 3.24 ≈ 25.3 — overweight in the WHO classification, and only just: at that height a BMI of 25.0 falls at exactly 81 kg.
Under the World Health Organization classification for adults, underweight is a BMI below 18.5, normal weight 18.5 to 24.9, overweight 25.0 to 29.9, and obesity starts at 30.0: class I up to 34.9, class II up to 39.9 and class III from 40.0. The cut-offs are the same for men and women and apply to adults (18+) only.
Height enters the formula squared, so the same kilogram does not move BMI equally for everyone: at 160 cm one kilogram moves BMI by 0.39, at 175 cm by 0.33, and at 190 cm by 0.28.
BMI calculatorBody mass index with WHO categories and healthy weight range.Open the calculator →The normal range is wide — and wider the taller you are
“Normal weight” is not one number but a range, and a wide one. At a height of 175 cm it covers 56.7 to 76.3 kg, a spread of 19.6 kg. Two people of the same height who differ by almost 20 kg can both be in the normal weight category.
The range grows with height, because both cut-offs are multiplied by height squared: at 155 cm it is 15.4 kg wide, at 195 cm already 24.4 kg. So the same number of kilograms above or below the range does not mean the same for a shorter and a taller person.
| Height | Normal weight (BMI 18.5–24.9) | Width of the range |
|---|---|---|
| 155 cm | 44.4 – 59.8 kg | 15.4 kg |
| 160 cm | 47.4 – 63.7 kg | 16.3 kg |
| 165 cm | 50.4 – 67.8 kg | 17.4 kg |
| 170 cm | 53.5 – 72.0 kg | 18.5 kg |
| 175 cm | 56.7 – 76.3 kg | 19.6 kg |
| 180 cm | 59.9 – 80.7 kg | 20.8 kg |
| 185 cm | 63.3 – 85.2 kg | 21.9 kg |
| 190 cm | 66.8 – 89.9 kg | 23.1 kg |
| 195 cm | 70.3 – 94.7 kg | 24.4 kg |
Same weight, different categories
The same number on the scale means different things at different heights. 80 kg at 155 cm gives a BMI of 33.3 — obesity class I. At 170 cm it is 27.7, overweight; at 180 cm 24.7, normal weight; and at 190 cm 22.2.
That is why “I weigh 80 kilos” says nothing about the category without the height — and the same BMI in two people does not say how much of those kilograms is muscle and how much is fat.
| Height | BMI at 80 kg | WHO category |
|---|---|---|
| 155 cm | 33.3 | Obesity class I |
| 165 cm | 29.4 | Overweight |
| 170 cm | 27.7 | Overweight |
| 175 cm | 26.1 | Overweight |
| 180 cm | 24.7 | Normal weight |
| 185 cm | 23.4 | Normal weight |
| 190 cm | 22.2 | Normal weight |
What BMI cannot see
BMI does not distinguish muscle mass from fat tissue. Muscle tissue is denser and heavier, so people with a lot of muscle mass can have a BMI above 25 with quite a low body fat percentage. For them, body composition measurements are more reliable.
It does not take age, sex, build or fat distribution into account either. Two people with the same BMI can therefore have entirely different body composition, and the formula cannot show it.
For children and adolescents the formula and the WHO cut-offs do not apply: their BMI is read from growth percentile charts by age and sex, and that is a question for a paediatrician.
How to use the number you get
BMI is a screening tool, not a diagnosis. An adult's height does not change, so BMI follows weight: 2 kg less at 175 cm means a BMI lower by 0.65.
Near a category boundary a few hundred grams change the name of the category, not what BMI can tell you about the body. At 180 cm the overweight boundary is exactly 81 kg.
If you want the range from the classic ideal weight formulas or an estimate of how much energy you use per day, that is what the ideal weight calculator and the calorie calculator are for — both are estimates with limitations of their own.
The calculator is informational and not medical advice. For a real assessment of your health and body composition, see a doctor.
Check the example in the calculator
The example from this article opens with 82 kg and 180 cm already filled in: BMI 25.3, overweight, and a healthy range of 59.9 – 80.7 kg for that height. Enter 81 kg and BMI is exactly 25.0.