What BMI is
BMI (Body Mass Index) relates weight to height with the formula weight in kg ÷ (height in metres)². A person 1.75 m tall weighing 72 kg has a BMI of 72 ÷ (1.75 × 1.75) = 23.5. It is the indicator the World Health Organization uses to classify adult weight across large populations.
WHO categories
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class I |
| 35 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
Limits of BMI
BMI is a simple number and therefore imperfect: it does not tell fat from muscle (a muscular athlete may read as "overweight"), it ignores age, sex and fat distribution, and it does not apply to children and teenagers, who use age-specific charts, or during pregnancy. Waist circumference and body composition complete the picture. For personal assessments see a doctor or a dietitian.
The "normal" weight for your height
The range shown by the calculator corresponds to a BMI between 18.5 and 24.9: for 1.75 m (5 ft 9 in) it goes from about 57 to 76 kg (125 to 168 lb). It is a statistical reference, not a target to hit at any cost.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.70 m → 70 ÷ 2.89 = 24.2. In imperial units: weight in pounds × 703 ÷ height in inches squared.
What is the ideal BMI?
For adults the "normal" range is 18.5 to 24.9. Many studies point to 20–25 as the range with the lowest risk, but the optimal value depends on the person.
Does BMI apply to children?
No: for children and teenagers up to 18, age- and sex-specific growth charts are used, not adult thresholds.