BMI: What It Is, How to Calculate It and How to Read It [Guide]

BMI (Body Mass Index), known in Italian as Indice di Massa Corporea (IMC), is the most widely used tool in the world to assess whether a person's weight is within a healthy range relative to their height. Doctors, nutritionists, and international health organizations use it daily as a first-level indicator to identify underweight, overweight, and obesity. But how exactly is it calculated? What are the reference values? And what are the limitations of this index? In this comprehensive guide, we will answer all of these questions with practical examples and reference tables.

What Is BMI (Body Mass Index)

BMI is a numerical indicator that relates body weight to a person's height. It was devised by the Belgian mathematician and statistician Adolphe Quetelet in 1832, which is why it is also known as the "Quetelet Index." Despite its age, it remains the international standard for classifying body weight in the adult population.

The World Health Organization (WHO) uses it as a reference parameter in its guidelines on nutrition and prevention of chronic weight-related diseases. Its strength lies in its simplicity: just two measurements (weight and height) are needed to obtain an immediate result.

The BMI Formula

The formula for calculating BMI is very simple:

BMI = Weight (kg) / Height (m)^2

Weight is expressed in kilograms and height in meters (not centimeters).

Calculation Examples

Example 1: Woman, 65 kg, 1.68 m tall

BMI = 65 / (1.68)^2 = 65 / 2.8224 = 23.0 → Normal weight

Example 2: Man, 92 kg, 1.78 m tall

BMI = 92 / (1.78)^2 = 92 / 3.1684 = 29.0 → Overweight

Example 3: Man, 75 kg, 1.82 m tall

BMI = 75 / (1.82)^2 = 75 / 3.3124 = 22.6 → Normal weight

Example 4: Woman, 110 kg, 1.60 m tall

BMI = 110 / (1.60)^2 = 110 / 2.56 = 43.0 → Class III Obesity

You can calculate your BMI instantly using our online BMI calculator, which also provides result interpretation and personalized advice.

BMI Table: WHO Classification

The World Health Organization classifies BMI into precise categories:

The normal weight range (18.5-24.9) is quite broad. For a person 1.70 m tall, normal weight corresponds to a weight between 53.4 kg and 72.0 kg, a range of nearly 19 kg.

Ideal BMI: What Value to Aim For

Although the entire 18.5-24.9 range is considered "normal weight," several epidemiological studies suggest that the minimum risk of all-cause mortality falls within the 20-25 range for the general population, with an optimum around 22-23.

However, the "ideal" BMI varies based on several factors:

• Age: in older adults (over 65), a slightly higher BMI (23-27) is associated with better survival
• Sex: women naturally tend to have a higher body fat percentage than men at the same BMI
• Ethnicity: for Asian populations, the cut-offs are lower (overweight starting at BMI 23, obesity at BMI 25)
• Physical activity level: athletes may have high BMIs due to muscle mass

Limitations of BMI: What It Cannot Tell Us

BMI is a useful but imperfect tool. Here are its main limitations:

It does not distinguish between fat mass and lean mass

The most significant limitation of BMI is that it does not differentiate between body fat and muscle mass. A bodybuilder with a lot of muscle mass may have a BMI of 30 (classified as "obese") while having a very low body fat percentage. Conversely, a sedentary person with little muscle mass could have a BMI of 23 (normal weight) while having excess visceral fat.

It does not consider fat distribution

BMI does not indicate where body fat accumulates. Fat distribution is a crucial cardiovascular risk factor: abdominal (visceral) fat is far more dangerous than subcutaneous fat distributed on hips and thighs. Two people with the same BMI can have completely different risk profiles based on fat distribution.

It is not suitable for everyone

BMI has significant limitations for:

• Athletes and sports practitioners: high muscle mass distorts the result
• Pregnant women: weight gain is physiological and cannot be classified
• Elderly individuals: loss of height and muscle mass alters the ratio
• Children and adolescents: they require age- and sex-specific growth curves

BMI in Children and Adolescents

For individuals under 18, BMI is calculated using the same formula but interpreted differently: the resulting value is compared against age- and sex-specific growth curves (percentiles). The categories are:

• Underweight: BMI below the 5th percentile for age and sex
• Normal weight: BMI between the 5th and 85th percentile
• Overweight: BMI between the 85th and 95th percentile
• Obesity: BMI above the 95th percentile

In Italy, pediatricians use WHO growth curves for children up to 5 years old and those from the Italian Society of Pediatric Endocrinology and Diabetes (SIEDP) for older ages.

Alternatives and Complements to BMI

For a more comprehensive assessment of weight status, several indicators can supplement or replace BMI:

Waist circumference

Measuring abdominal circumference is a simple and effective indicator of visceral fat. The risk values are:

Waist-to-hip ratio (WHR)

The WHR (Waist-to-Hip Ratio) evaluates fat distribution. It is calculated by dividing the waist circumference by the hip circumference. High values indicate abdominal fat accumulation:

• Men: risk if > 0.90
• Women: risk if > 0.85

Waist-to-height ratio

An emerging and very simple indicator: waist circumference should be less than half of one's height. A man 180 cm tall should have a waist circumference less than 90 cm.

Bioelectrical Impedance Analysis (BIA)

BIA is an instrumental technique that measures body composition (fat mass, lean mass, body water) using a weak electrical current. It is more accurate than BMI but requires a dedicated device. Many "smart" scales integrate BIA sensors, although with lower precision compared to professional devices.

DEXA (Dual-Energy X-ray Absorptiometry)

This is the gold standard for measuring body composition. It provides precise data on fat mass, lean mass, and bone density for each body segment. It is used in clinical and research settings but is expensive and not easily accessible.

BMI and Disease Risk

Numerous scientific studies have demonstrated a correlation between high BMI and increased risk of various diseases:

• Cardiovascular diseases: hypertension, heart attack, stroke
• Type 2 diabetes: risk increases even with a BMI above 25
• Certain types of cancer: colorectal, breast (post-menopausal), endometrial, kidney
• Sleep apnea: closely correlated with excess weight
• Musculoskeletal conditions: knee and hip osteoarthritis
• Hepatic steatosis: non-alcoholic fatty liver disease

On the other hand, a BMI that is too low (below 18.5) is also associated with risks: osteoporosis, infertility, compromised immune system, anemia.

How to Improve Your BMI

If your BMI indicates overweight or obesity, here are some practical tips based on scientific evidence:

1. Moderate caloric deficit: a reduction of 500 kcal/day leads to a loss of approximately 0.5 kg per week
2. Regular physical activity: at least 150 minutes per week of moderate aerobic activity (brisk walking, swimming, cycling)
3. Resistance training: weight exercises 2-3 times per week to preserve muscle mass
4. Balanced diet: prioritize proteins, fiber, fruits and vegetables; reduce added sugars and ultra-processed foods
5. Adequate sleep: 7-9 hours per night; sleep deprivation is associated with weight gain
6. Stress management: elevated cortisol promotes abdominal fat accumulation

Frequently Asked Questions

What is the ideal BMI for a woman?

The normal weight range for women is the same as for men according to the WHO: 18.5-24.9. However, women naturally have a higher body fat percentage (20-25% is considered healthy, compared to 15-20% for men), so a BMI in the 20-24 range is generally considered optimal.

Is BMI reliable for athletes?

No, for athletes with developed muscle mass, BMI tends to overestimate the degree of overweight. A professional soccer player with a BMI of 26 could have a body fat percentage of 10%. In these cases, body composition measurement with BIA or skinfold calipers is preferable.

What BMI should I have at 60 years old?

For those over 60, several studies suggest that a slightly higher BMI (23-27) is associated with better survival compared to a BMI in the lower range of normal weight. This phenomenon, known as the "obesity paradox," may be related to energy reserves useful in case of illness. Always consult your doctor for a personalized assessment.

How is BMI calculated in the imperial system?

The formula in imperial units is: BMI = (weight in pounds x 703) / (height in inches)^2. However, it is easier to convert to metric units and use the standard formula or our online calculator.

Is BMI the same for all ethnicities?

No. Asian populations tend to have more visceral fat at the same BMI, which is why the WHO suggests lower thresholds: overweight starting at BMI 23 and obesity at BMI 25 (instead of 25 and 30). Conversely, for some Polynesian populations, the standard cut-offs may be too restrictive.

How much weight do I need to lose to drop one BMI point?

It depends on your height. For a person 1.70 m tall, one BMI point corresponds to approximately 2.9 kg. For a person 1.80 m tall, it corresponds to approximately 3.2 kg. The formula is: weight change = BMI change x height^2.