Calculation of Body Mass Index (BMI) | Calculate Your Ideal Weight

Body mass index bmi

Calculation of Body Mass Index (BMI)

The body mass index (BMI) allows you to quickly assess your corpulence simply with your weight and height, regardless of your sex. Quickly calculate your BMI and find out in which category you fall.

The body mass index (BMI) is the only index validated by the World Health Organization to assess the corpulence of an individual and therefore the possible health risks. The BMI allows you to determine whether you are thin, overweight or obese, for example.

BMI Formula

Metric units (in kg and cm)

BMI = weight x height x height

Imperial or US units (in lbs and inch)

BMI = 703 ÷ weight x (height x height)

Normal weight range calculation (minimum and maximum weight)

Normal weight BMI is 18.5 to 24.9. To calculate your normal weight range use the following:

Metric units:

minimum weight (kg) = 18.5 × height² (m)
maximum weight (kg) = 24.9 × height² (m)

US units:

minimum weight (lb) = (18.5 ÷ 703) × height² (in)
maximum weight (lb) = (24.9 ÷ 703) × height² (in)

BMI, basic categories

The WHO regards an adult BMI of less than 18.5 as underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI equal to or greater than 25 is considered overweight and 30 or more is considered obese. In addition to the principle, international WHO BMI cut-off points (16, 17, 18.5, 25, 30, 35 and 40), four additional cut-off points for at-risk Asians were identified (23, 27.5, 32.5 and 37.5)

These ranges of Body Mass Index values are valid only as statistical categories.

BMI, basic categories
CategoryBMI (kg/m2)BMI Prime
Underweight (Severe thinness)< 16.0< 0.64
Underweight (Moderate thinness)16.0 – 16.90.64 – 0.67
Underweight (Mild thinness)17.0 – 18.40.68 – 0.73
Normal range18.5 – 24.90.74 – 0.99
Overweight (Pre-obese)25.0 – 29.91.00 – 1.19
Obese (Class I)30.0 – 34.91.20 – 1.39
Obese (Class II)35.0 – 39.91.40 – 1.59
Obese (Class III)≥ 40.0≥ 1.60

Children (aged 2 to 20)


The calculation of the BMI is based on an identical formula for men for women. Similarly, the BMI calculation does not take age into account. Whether a woman is 20, 50, or 70, the formula used to calculate body mass index remains the same.

You must therefore use the BMI calculator to find out if your BMI is within the norm.

From the menopause, between 50 and 60 years old, women tend to gain weight. The reason? Menopause disrupts the metabolism: we spend fewer calories at rest.


It should be noted that the calculation of the body mass index does not take into account the distribution of liquids. For example, in case of water retention (liquid infiltrates the tissues), it happens that we notice a rapid weight gain. However, it is not a question of kilos linked to excess food but of a transitory variation in weight.

Body Mass Index also does not take into account the distribution of bone, muscle and fat mass in the body. This is why two people with the same Body Mass Index can have completely different builds.

The BMI nevertheless remains a useful indicator and recommended by specialists. If your BMI seems abnormal and you’re wondering if you’re overweight, talk to your doctor. They can help you interpret your Body Mass Index, prescribe additional analyzes if necessary (balance of fats and sugars) and advise you.


A man’s BMI is based on the same formula used to calculate a woman’s BMI, i.e. BMI = weight (kg) / height (m²).

It is nevertheless difficult to rely with eyes closed on the BMI alone because a muscular man and a “paunky” man can have the same Body Mass Index for opposite body types.

Athletes, whose muscle mass is highly developed, have a high BMI without being obese and presenting risks to their health.

Obesity also increases with age, from 10.4% among men aged 30 to 39 to 20.8% for men aged 60 to 69.


According to the WHO (World Health Organization), we speak of obesity when the body mass index (BMI) is greater than or equal to 30. But we now know that excess fat is more dangerous when it is localized. at the level of the abdomen.

It is also necessary to distinguish obesity from “severe” obesity.

There are actually 3 stages according to Body Mass Index:
  • Between 30 and 34.9: class I (moderate obesity)
  • Between 35-39.9: class II (severe obesity)
  • ≥ 40 kg/m²: class III (massive or morbid obesity)

It is essential to take waist circumference into account in addition to calculating Body Mass Index. Waist circumference is measured with a tape measure in centimeters (cm). Abdominal obesity is defined according to the recommendations of the Haute Autorité de Santé, namely a waist circumference greater than 94 cm for men and greater than 80 cm for women.

Obesity promotes the onset of many pathologies, including type 2 diabetes, arterial hypertension, atherosclerosis, but also liver disease, kidney disease, and even many cancers, particularly of the breast, uterus or liver. It is also necessary to underline the psychological and social repercussions of obesity.


The sleeve gastrectomy, or longitudinal gastrectomy or partial gastrectomy by plication, or tubular gastrectomy, is a restrictive technique of bariatric surgery intended for the treatment of severe obesity, which consists in removing the left two thirds of the stomach

The management of so-called “morbid” obesity is based on bariatric surgery. There are several solutions: the installation of a gastric band, the sleeve gastrectomy, the Bypass.

These surgeries are indicated in the following cases:
  • ≥ 40 kg/m²
  • ≥ 35 kg/m² associated with at least one pathology (in particular arterial hypertension, sleep apnea and other severe respiratory disorders, type 2 diabetes, osteo-articular diseases, liver pathologies).

For people whose weight is not high enough to qualify for this surgery, or those who do not wish to have surgery, there are two other non-surgical methods: gastric balloon and sleeve gastroplasty. This last non-surgical technique is indicated for patients with a BMI greater than 27 and less than 40.


In a person of normal build, muscle mass represents 35% of weight in men, 28% of weight in women. Through our daily physical activity and even at rest, we use our muscles, which generates energy expenditure and causes us to lose calories.

Read also: Radioactivity Effects on The Human Body

In the event of a diet, the food intake decreasing, it is necessary to ensure that the muscle mass remains stable. Indeed, if nutritional intake decreases but muscle mass also decreases, then metabolism decreases, i.e. less energy is spent at rest. To compensate, you need to do more physical activity.

Sources: PinterPandai, National Health Service UK, CDC U.S. Department of Health & Human Services

Photo credit: Piqsels (CC0 Public Domain)

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