What is Body Mass Index?
Body Mass Index (BMI) is a simple measure calculated from height and weight, used worldwide as a quick initial tool to estimate whether a person's weight is appropriate for their height. The formula: weight in kilograms divided by height in meters squared.
BMI Categories According to the World Health Organization
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25 – 29.9 | Overweight |
| 30 and above | Obese |
Limitations of BMI
BMI is a screening tool, not a precise diagnosis, because it doesn't distinguish between muscle weight and fat weight. For example, an athlete with high muscle mass might show a BMI in the "overweight" range even though their actual body fat percentage is low. BMI also doesn't account for fat distribution in the body, age, or individual differences.
A worked example
Someone 170 cm tall weighing 80 kg: 80 ÷ (1.7 × 1.7) = 80 ÷ 2.89 = 27.7, which is “overweight”. The healthy weight range for that height is 53.5–72 kg, so they are about 8 kg above the upper limit.
Healthy weight by height
This table shows the weight range that gives adults a BMI of 18.5–24.9:
| Height | Healthy weight |
|---|---|
| 150 cm | 41.6 – 56 kg |
| 160 cm | 47.4 – 63.7 kg |
| 170 cm | 53.5 – 72 kg |
| 180 cm | 59.9 – 80.7 kg |
| 190 cm | 66.8 – 89.9 kg |
Waist size completes the picture
Because BMI doesn’t show where fat is carried, the World Health Organization also uses waist circumference: health risk rises above 94 cm for men and 80 cm for women, and rises substantially above 102 cm for men and 88 cm for women. Measure midway between your lowest rib and the top of your hip bone.
Frequently Asked Questions
Is BMI accurate for everyone?
No, it's a general indicator suitable for most adults, but it's less accurate for athletes with high muscle mass, pregnant women, older adults, and children (who have different scales). In these cases, it's best to consult a specialist for a more accurate assessment.
Does the formula differ by sex?
The calculation formula itself doesn't differ between men and women, but interpreting the result and body fat distribution can differ, which is why BMI remains a general guidance tool rather than a substitute for a comprehensive medical evaluation.
Does it work for children?
No. Children and teenagers are assessed with BMI-for-age by sex (growth charts), not the adult categories. For children up to 5, use the baby growth calculator.
How much do I need to lose to reach a healthy weight?
Multiply 24.9 by your height in metres squared to get the top of the healthy range, then subtract it from your weight. For 170 cm: 24.9 × 2.89 = 72 kg.
How many calories a day do I need to reach a healthy weight?
Use our daily calorie calculator to see your maintenance calories and safe targets for losing or gaining weight.