BMI Calculator: Formula, WHO Chart and Full Interpretation Guide

The BMI calculator (Body Mass Index) is the most widely used tool in the world for classifying a person's weight relative to their height. It shows up in doctor's appointments, workplace checkups and every health guide because it costs nothing, needs no equipment and takes five seconds to calculate. But BMI is also one of the most misunderstood indicators: it was never designed as an individual diagnosis, and when misapplied it can produce results that make no sense (an 'obese' athlete, a 'normal' sarcopenic older adult).

In this guide we explain how BMI is calculated, what each range in the official WHO table means, how to interpret it by sex and age, what its real limitations are and which other indicators are worth checking to get a more complete picture of your health. At the end you will find the link to our free online BMI calculator so you can do the calculation in one click.

What BMI is and where it comes from

BMI (Body Mass Index) is a numerical relationship between your weight and your height. It was developed by the Belgian mathematician Adolphe Quetelet in the 19th century for statistical studies of populations, as a simple way to measure human 'corpulence'. It was not originally conceived as an individual health indicator, but as a tool for assessing the health of large demographic groups.

The World Health Organization (WHO) adopted BMI in 1985 as a nutritional screening tool for adults, and it has been standardized globally ever since. Its great virtue is simplicity: two variables (weight in kg, height in meters) and a formula anyone can apply. That accessibility made it a standard, despite its well-known limitations.

How to calculate BMI: formula and practical examples

To calculate your BMI, make sure your weight is in kilograms and your height in meters. If you measure your height in centimeters, divide it by 100 to convert it to meters before using it in the formula.

Step-by-step example. A woman weighing 65 kg and 1.65 m tall: first we square the height, 1.65 × 1.65 = 2.7225. Then we divide the weight by that number, 65 / 2.7225 = 23.87. Rounded, we get a BMI of 23.9, which falls in the normal weight range according to the WHO. For a man weighing 80 kg and 1.80 m tall, the calculation would be: 1.80 × 1.80 = 3.24. Then, 80 / 3.24 = 24.69. His BMI would be 24.7, also in the normal weight category.

If you work in the imperial system (pounds and inches), the equivalent formula is BMI = (weight in pounds × 703) / height in inches². However, in Spain and Latin America it is natural to work in kilograms and centimeters or meters.

BMI classification table according to the WHO

The WHO has established standardized ranges for classifying BMI in adults (aged 18 and over). These ranges, although universal, should be interpreted with caution and always in the individual context of each person.

These ranges are the same for adult men and women. However, in children and adolescents (5-18 years) percentiles adjusted for age and sex are used, since growth and development are key factors to consider.

BMI in women and men: are there differences in interpretation?

Although the WHO ranges are identical for both sexes, body composition and fat distribution vary significantly. Women tend to have a higher body fat percentage than men for hormonal and reproductive reasons. This means that, at the same BMI, a woman may have more body fat than a man.

For example, a BMI of 24 in an active woman could correspond to 28-30% body fat, while in an active man it could be 18-22%. This difference means that BMI, without additional context, can underestimate body fat in women and overestimate it in men with well-developed musculature. Fat distribution also differs, with gynoid patterns (fat on the hips and thighs) more common in women and android patterns (abdominal fat) in men, which has different implications for metabolic health.

BMI and body composition: beyond the number

BMI is an indicator of weight relative to height, but it does not distinguish between muscle mass, bone mass or fat mass. This is its main limitation. Two people with the same BMI can have radically different body compositions and, therefore, opposite health profiles.

A person with high muscle mass, such as a strength athlete, can have a high BMI (in the overweight or obese range) but a very low body fat percentage. On the other hand, a sedentary person could have a 'normal' BMI but a high body fat percentage and little muscle mass, which is known as 'sarcopenic obesity' or 'TOFI' (Thin Outside, Fat Inside), with significant metabolic risks. That is why complementing BMI with body composition measurements is crucial.

Limitations of BMI: when NOT to use it as your only measure

• Athletes with high muscle mass: A high BMI in athletes or bodybuilders does not always indicate excess fat. Their muscle density can wrongly classify them as overweight or obese, when in reality they have a healthy body composition and a low body fat percentage.
• Older adults with sarcopenia: With age, the loss of muscle mass (sarcopenia) and the relative increase in fat can keep BMI in the 'normal' range, hiding a real cardiometabolic risk caused by low lean mass and fat infiltration.
• Pregnant women: The BMI formula does not apply during pregnancy. Weight gain is normal and necessary for the development of the fetus, and should be monitored with pregnancy-specific guidelines.
• Children and adolescents: BMI in people under 18 is interpreted with percentiles adjusted for age and sex, since they are constantly growing and developing. Using the adult table would be a mistake.
• Asian ethnicities: Some Asian populations show higher cardiometabolic risk at lower BMIs than the standard WHO ranges. For these populations, the WHO proposes a revised table in which overweight starts at ≥ 23 and obesity at ≥ 27.5.

Complementary methods to BMI for a complete assessment

Since BMI is a screening tool and not a diagnostic one, it is essential to complement it with other measurements to get a more accurate picture of health and body composition. These methods provide additional information about fat distribution, the amount of muscle and metabolic risk.

If your BMI falls outside the normal weight range or if you are an athlete, we strongly recommend that you also check your waist circumference. A value above 102 cm in men and 88 cm in women indicates elevated cardiometabolic risk, regardless of your BMI.

For a more complete estimate of your body composition at home, bioimpedance scales are an accessible option. Advanced models with 8 electrodes offer greater accuracy by measuring electrical resistance in different segments of the body.

BMI in children and adolescents: a different perspective

Unlike in adults, BMI in children and adolescents is not interpreted with fixed values but through percentiles. This is because their body composition and height change rapidly as they grow. Percentiles compare a child's BMI with that of other children of the same age and sex, showing whether it is above or below the average.

A BMI between the 5th and 85th percentile is considered healthy. Below the 5th it is classified as underweight, between the 85th and 95th as overweight, and above the 95th as obese. This methodology allows pediatricians to monitor children's growth appropriately, identifying trends that may require nutritional or medical intervention.

Frequently asked questions

What is a healthy BMI?

According to the WHO, a BMI between 18.5 and 24.9 in adults is considered healthy or 'normal weight'. Within this range, population studies suggest that the lowest all-cause mortality is seen in the 22 to 24.9 band. However, 'normal BMI' does not always equal 'optimal body composition'.

Can I lower my BMI without losing muscle?

Yes, it is possible and advisable. A moderate calorie deficit (aiming for a weight loss of 0.5-1% of body weight per week) combined with sufficient protein intake (1.6-2.2 g/kg of body weight) and strength training ensures that most of the weight lost is fat. Losing weight too quickly increases the likelihood of losing muscle mass, which could improve your BMI numerically but worsen your body composition and metabolic health.

Is BMI useful if I am an athlete?

As an initial screening tool, BMI has significant limitations for athletes with high muscle mass. It is likely to wrongly classify you as overweight or obese. For an accurate assessment, complement BMI with body fat percentage (using bioimpedance or the Navy method with a tape measure) and waist circumference. This will give you a real picture of your body composition and health status.

How reliable is a home bioimpedance scale?

The reliability of home bioimpedance scales varies. 4-electrode models (feet) are less accurate than 8-electrode ones (hands and feet), which measure different segments of the body. Factors such as hydration level, time of day and recent food intake can affect the results. They are useful for tracking trends over time, but not for a clinical diagnosis. For greater accuracy, look for clinically validated models such as the OMRON BF511.

Does BMI change with age?

BMI itself does not change with age as far as its formula goes, but its interpretation can vary. In adults, the tendency is to gain weight and body fat with age, while muscle mass may decrease (sarcopenia). This means that a 'normal' BMI in an older person could hide a higher body fat percentage and lower muscle mass, increasing the risk of frailty and metabolic problems. That is why, in older age, it is even more important to consider body composition and waist circumference.

Calculate your BMI now

Use our free online BMI calculator (no registration): enter your weight and height and get your BMI with the WHO classification, the healthy weight range for your height and links to complementary tools (body composition, daily calories). Your data never leaves your browser.