Body mass index: how to calculate it and where it goes wrong
Body mass index takes ten seconds to calculate and is therefore used where it shouldn't be: for an individual. It was designed to describe population groups, and all its known flaws are a consequence of exactly that. Knowing the formula is useful, but knowing its limitations is more useful.
How to calculate
Body mass in kilograms is divided by the square of height in meters.
Example: 78 kg at a height of 1.74 m. The square of height is 3.03. 78 divided by 3.03 is approximately 25.7.
Category thresholds according to the WHO classification for adults:
| BMI | Category |
|---|---|
| less than 18.5 | underweight |
| 18.5–24.9 | normal weight |
| 25.0–29.9 | overweight |
| 30.0 and above | obesity |
For children and adolescents, different criteria are used, adjusted for age and sex; the adult scale is not applicable to them.
What the indicator actually measures
The ratio of mass to height. It contains nothing about body composition: it does not distinguish between muscle and fat, does not know where that fat is located, and does not account for bone density.
At the population level, however, the indicator works: the link between high values and health risks is consistently reproduced in large studies. This was precisely the goal—a quick way to describe body mass distribution in large groups. Which risks are confirmed and which are not is analyzed in the article on excess weight and health.
Four situations where it is misleading
High muscle mass. Muscle tissue is denser than fat, and a person with developed musculature will have a BMI above 25 despite a low body fat percentage. This is the most well-known limitation.
Low muscle mass with normal weight. The opposite and more common case: BMI within the normal range with a high body fat percentage and low muscle mass. This is the most insidious case because it does not raise concerns for anyone.
Old age. Over the years, muscle mass decreases, height decreases due to the spine, and both changes shift the indicator.
Very tall or very short height. The formula divides by the square of height, although body mass does not grow strictly quadratically with height; at the edges of the range, this leads to a systematic bias.
Separately: pregnancy, edema, and significant fluid retention make the indicator uninformative.
What to supplement it with at home
One indicator is rarely sufficient, and two others work well alongside BMI, both of which are measured with a tape measure.
- Waist circumference. Assesses the amount of fat in the abdominal area—the very fat associated with the main metabolic risks. It is measured in the morning, on an empty stomach, while exhaling, at the same spot. More details in the article on visceral fat.
- Waist-to-height ratio. A simple rule: it is advisable to keep waist circumference less than half of your height. The indicator does not require tables and works well for people of different body types.
What bioimpedance from smart scales provides and why its figure should not be taken as a measurement is covered in the article on smart scales.
How to use this
- Consider BMI a starting point, not a diagnosis. It indicates whether there is a reason to look closer, not what is happening.
- Look at the trend. Changes in the indicator over months are more informative than a single value.
- Supplement with waist measurement. Two indicators together describe the situation significantly better than one.
- Do not recalculate after every weighing. Daily weight fluctuations are water and intestinal contents.
And the reason it is usually calculated: if, based on the combination of indicators, there is a reason to lose weight, the mechanism is the same and is analyzed in the article on calorie deficit, and the calculation of the daily norm is in the article on daily calorie requirements.
Frequently asked questions
- How to calculate body mass index?
- Divide body weight in kilograms by the square of height in meters. For example, 78 kg with a height of 1.74 m: 78 divided by 3.03 equals approximately 25.7.
- What BMI is considered normal?
- According to the WHO classification for adults: less than 18.5 is underweight, 18.5–24.9 is normal, 25.0–29.9 is overweight, and 30.0 or higher is obesity. For children and adolescents, different criteria are used, adjusted for age and sex.
- Why is BMI not suitable for athletes?
- Muscle tissue is denser than fat, so with well-developed muscles and a low body fat percentage, the index may exceed 25. The formula does not distinguish between muscle and fat; it only accounts for mass and height.
- Can BMI be normal with excess body fat?
- Yes, and this is more common than the opposite: a normal value with a high body fat percentage and low muscle mass. This is precisely why this single indicator is supplemented with waist circumference.
- What can be used to supplement BMI at home?
- Waist circumference, measured in the morning on an empty stomach while exhaling, and the waist-to-height ratio: it is recommended to keep the circumference at less than half of your height. Both indicators are measured with a standard measuring tape.
Read next
- Excess weight and health: what risks are confirmed — This topic is usually presented in a way meant to induce fear.
- Visceral fat: why it matters and how to measure it — Two people at the same weight, with the same BMI, can carry very different metabolic risk.
- Daily calorie intake — There is no universal norm: it depends on weight, height, age, sex, and activity level.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.