Body mass index is the most widely used indicator for placing a weight in relation to a height. Simple to calculate, it is still often misinterpreted. Here is how to obtain it with the official WHO formula, read the result correctly, avoid the classic mistakes and understand its limits so you never mistake it for a diagnosis.
BMI (body mass index) was popularized in the 19th century by the Belgian statistician Adolphe Quetelet, then adopted by the World Health Organization as a tool for classifying weight. Its appeal lies in its simplicity: two measurements are enough, height and weight, to obtain a figure that can be compared from one person to another. It is a useful reference point, provided you know exactly what it measures, and what it does not.
In this guide, we walk through the formula step by step, a full worked example, the table of official thresholds, the most common calculation pitfalls and the situations where BMI becomes misleading. The goal is simple: to make sure you can calculate your index in thirty seconds and interpret it with the right level of perspective.
The official WHO formula
BMI is calculated by dividing weight in kilograms by the square of height in meters. The formula is the same for adult men and women, whatever the age within the reference range.
BMI = weight (kg) ÷ height² (m²)
Let's take a concrete example. A person who is 1.75 m tall and weighs 72 kg first calculates the square of their height: 1.75 × 1.75 = 3.0625. They then divide their weight by this result: 72 ÷ 3.0625 ≈ 23.5. Their BMI is therefore 23.5, which places them in the healthy weight range. The figure is expressed in kilograms per square meter (kg/m²), a unit you don't need to remember but which explains why height is squared.
The step-by-step calculation
- 1Convert your height to meters (for example 175 cm = 1.75 m).
- 2Square that height: multiply it by itself (1.75 × 1.75 = 3.0625).
- 3Divide your weight in kilograms by that number (72 ÷ 3.0625 ≈ 23.5).
- 4Round to one decimal place, which is more than enough for interpretation.
- 5Compare the result with the threshold table below.
If mental math isn't your strong suit, know that the operation relies on a simple division, exactly like when you calculate a percentage or a share of a budget. The key is to get the units right, because that's where most errors slip in.
Watch out for units
The most common mistake is using height in centimeters instead of meters. With 175 instead of 1.75, the result makes no sense at all. Always check that your height is expressed in meters before squaring it, and that your weight is in kilograms.
Interpreting the result: the reference thresholds
The WHO defines BMI ranges associated with weight categories. These thresholds apply to adults aged 18 to 65, outside of pregnancy. The same figure therefore does not tell the same story for a young, athletic adult and for an older, sedentary person.
| BMI (kg/m²) | WHO interpretation |
|---|---|
| Below 16.5 | Severe underweight |
| 16.5 to 18.5 | Underweight |
| 18.5 to 25 | Healthy weight |
| 25 to 30 | Overweight |
| 30 to 35 | Moderate obesity (class I) |
| 35 to 40 | Severe obesity (class II) |
| 40 and above | Morbid obesity (class III) |
These categories are statistical benchmarks, not a diagnosis. A BMI in the healthy range does not guarantee good health, and a BMI in the overweight range does not necessarily mean a problem. The figure indicates a probability of risk across a population, which then has to be refined case by case with other factors.
Don't overdramatize crossing a threshold
Going from 24.9 to 25.1 changes nothing about your actual health: these are conventional boundaries, not walls. A stable BMI of 26 and a BMI of 26 rising rapidly do not carry the same meaning. The trend over time matters more than a single isolated value.
Calculate your BMI in an instant
Enter your height and weight, and our tool applies the WHO formula and automatically places your result in the right category, with no risk of a unit error.
The most common calculation and interpretation mistakes
A BMI calculation rarely fails because of the formula itself, but because of small misleading habits. Here are the pitfalls that come up most often, and how to avoid them.
- Confusing centimeters and meters: this is by far the number one mistake. Height must always be in meters before being squared.
- Forgetting to square the height: dividing weight by height without multiplying it by itself gives a figure three to four times too high.
- Using pounds or feet: the WHO formula works in kilograms and meters. Convert first if your measurements are in another unit.
- Treating BMI as a target to hit at all costs: aiming for a precise figure makes no sense; a healthy, stable range is far more relevant.
- Comparing your BMI with a friend's: body shape, age and muscle mass differ from one person to another, so a direct comparison tells you nothing.
BMI is a starting point for conversation, not a verdict. It points toward further tests when they are needed.
The limits of BMI to keep in mind
BMI does not distinguish between fat mass and muscle mass. That is its main weakness: it measures overall body size, not body composition. Several situations make it misleading.
- Athletes and very muscular people: muscle is denser than fat, so a high BMI may reflect good physical condition rather than excess weight.
- Older adults: muscle mass decreases with age, so a normal BMI can hide a significant proportion of fat.
- Children and teenagers: adult thresholds do not apply; growth charts specific to age and sex are used instead.
- Pregnant women: BMI is not relevant during pregnancy.
- Fat distribution: BMI ignores location, yet abdominal fat is more strongly linked to cardiovascular risk than fat around the hips.
Complement it with waist circumference
Waist circumference is an excellent complement to BMI. Above 94 cm in men and 80 cm in women, it signals excess abdominal fat. Measured halfway between the lowest rib and the top of the hip, it refines the interpretation where BMI stays silent.
Track the trend rather than a single figure
A BMI taken just once says very little. What matters is its trajectory over several months. Record your index at regular intervals, for example once a month under the same conditions, and watch the trend. To smooth out the daily variations tied to hydration or meals, you can work with an average across several measurements rather than the last weigh-in. A stable trend within a healthy range is a much better signal than a perfect figure obtained on a given day.
The idea of an ideal weight
Many people want to know their "ideal weight." In reality, there is no single figure but a range of healthy weights corresponding to a BMI between 18.5 and 25. You can calculate it by reversing the formula.
For a height of 1.75 m, you multiply the square of the height (3.0625) by the two thresholds: 3.0625 × 18.5 ≈ 56.6 kg and 3.0625 × 25 ≈ 76.6 kg. The healthy weight therefore falls between roughly 57 and 77 kg, which leaves a comfortable margin rather than a rigid target.
| Height | Healthy weight range (BMI 18.5 to 25) |
|---|---|
| 1.60 m | 47 to 64 kg |
| 1.70 m | 53 to 72 kg |
| 1.80 m | 60 to 81 kg |
| 1.90 m | 67 to 90 kg |
A zone, not a point
The good news is that the healthy range is wide: for many heights, it spans around twenty kilograms. So you don't have to aim for a value down to the gram. Staying lastingly within this zone, without sudden swings, is the realistic and reassuring goal to keep in mind.
An "ideal" weight also depends on age, body shape, personal history and how you feel. Aiming for the middle of the range is not mandatory: staying stable within a healthy zone matters more than reaching a precise figure.
Frequently asked questions
How do you calculate your BMI yourself?
Divide your weight in kilograms by the square of your height in meters. For example, for 72 kg and 1.75 m: 1.75 × 1.75 = 3.0625, then 72 ÷ 3.0625 ≈ 23.5. Above all, make sure the height is in meters and not centimeters.
What is a good BMI for an adult?
According to the WHO, a BMI between 18.5 and 25 corresponds to a healthy weight for an adult aged 18 to 65. Below that is underweight, above that is overweight. These are population benchmarks, not an absolute personal target.
Why is my BMI high even though I'm athletic?
BMI does not distinguish muscle from fat, and muscle is denser. A very muscular person can therefore show a BMI in the overweight range while being in excellent physical condition. In that case, waist circumference and medical advice are more telling.
Is BMI reliable for everyone?
No. It is not suited to children and teenagers, pregnant women, very muscular people, and not always to older adults. For these profiles, other benchmarks are used, such as growth charts or body composition measurement.
How do you calculate your healthy weight range?
Multiply the square of your height in meters by 18.5 and then by 25. For 1.75 m: 3.0625 × 18.5 ≈ 57 kg and 3.0625 × 25 ≈ 77 kg. Your healthy weight therefore falls between these two values, with a comfortable margin.
In summary
BMI is calculated by dividing weight in kilograms by the square of height in meters, following the formula adopted by the WHO. It quickly places body size on a scale ranging from underweight to obesity, with the healthy range between 18.5 and 25. It is a reliable screening tool at the population level, but imperfect at the individual level: it ignores muscle mass, fat distribution, age and sex. Use it as a first indicator, complement it with waist circumference and, when in doubt, talk it over with a healthcare professional.
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