Quick answer: BMI is your weight in kilograms divided by your height in metres squared: 70 kg at 1.70 m gives 70 ÷ 1.70² = 24.2, which is in the normal range (18.5 to 24.9).
How it's calculated
BMI = weight (kg) ÷ height (m)². In pounds and inches the same value is 703 × weight (lb) ÷ height (in)².
The healthy range for your height is the weight that gives a BMI from 18.5 to 24.9. BMI Prime is your BMI divided by 25, the upper limit of normal: under 1 means below the overweight line. Some calculators take 25 as the top of the range, which gives a slightly higher top weight. Ponderal Index is your weight divided by your height cubed; it changes less with height than BMI does.
BMI is a screening number, not a diagnosis. It does not separate fat from muscle, and it is not used for children and teenagers, who are assessed on age- and sex-specific percentiles.
Worked examples
- 70 kg, 170 cm: 70 ÷ 1.70² = 24.2 (normal weight)
- 95 kg, 175 cm: 95 ÷ 1.75² = 31.0 (obesity class I)
- 154 lb, 5 ft 7 in (67 in): 703 × 154 ÷ 67² = 24.1
WHO BMI categories for adults
| BMI | Category |
|---|---|
| Below 16 | Severe thinness |
| 16 – 16.9 | Moderate thinness |
| 17 – 18.4 | Mild thinness |
| 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 |
Frequently asked questions
What is a healthy BMI?
For adults, 18.5 to 24.9 is the normal range in the WHO classification.
Is BMI accurate for athletes?
Not very. Muscle is heavier than fat, so muscular people can have a high BMI with little body fat. A body fat measurement gives a better picture.
Can I use BMI for children?
No. Children and teenagers are assessed with BMI-for-age percentiles, which this calculator does not do.
Is BMI different for men and women?
The formula and the adult categories are the same for both. Women usually carry more body fat than men at the same BMI, which BMI alone can't show.
What is BMI Prime?
Your BMI divided by 25, the top of the normal range. A value of 0.92 means you are 8% below that line, and 1.10 means 10% above it.
What is the Ponderal Index?
Your weight in kilograms divided by your height in metres cubed. Because it uses height cubed, it changes less than BMI for very tall or very short people.