BMI: Chart, Normal Range for Women and Men, and Limitations

Body mass index is arithmetic: weight in kilograms divided by height in meters squared. The formula was proposed in 1835 by the Belgian statistician Adolphe Quetelet, and since then the thresholds have been revised more than once. Below — how to calculate it, where the WHO normal boundaries lie, and why the number in the answer is not yet a diagnosis.

What body mass index is and how it is calculated

BMI allows one to assess whether a person's weight corresponds to their height and indirectly judge whether the weight is insufficient, normal, or excessive. Formula: I = m / h², where m is weight in kilograms, h is height in meters; the result is measured in kg/m². Example: with a weight of 77 kg and a height of 170 cm, BMI = 77 : (1.70 × 1.70) ≈ 26.64 kg/m². In clinical practice, it is this height-weight indicator that is used — as an approximate tool, not as a measurement of body composition.

BMI chart: categories according to the WHO classification

Interpretation of the indicators according to WHO recommendations:

• below normal — underweight;
• normal BMI by the WHO criterion — less than 25 kg/m²;
• 25 kg/m² and above — overweight;
• 30 kg/m² and above — obesity.

The same boundaries appear in WHO documents: overweight — from 25 kg/m², obesity — from 30 kg/m² in persons over 18 years of age. The threshold of 25 is not a natural constant: until 1998, in the USA a BMI up to 27.8 kg/m² was considered normal, then the upper boundary was lowered to 25, and there were approximately 29 million more people with formally overweight; the threshold changed by decision, not by discovery. In Hong Kong and Japan, obesity is considered to be values from 25 kg/m².

Normal BMI for women and men

There is no separate normal range for women and for men in the WHO classification: the interpretation of the indicators takes into account neither sex nor age. Differences are visible in statistics — BMI in men is on average higher than in women: the mean global indicator in men over 18 years of age was 25.7 kg/m² in 2010 and 26.1 kg/m² in 2014, in women — 26.7 and 26.5 kg/m². The practical conclusion is dull: the WHO criterion is the same for both sexes — normal BMI less than 25 kg/m², and sex differences are a matter of population statistics, not diagnostics.

BMI with age: what to consider

Age is not included in the formula, although BMI is usually higher in people of mature age than in the young and the elderly. In children, weight and height are tracked over time with adjustment for age. In the elderly, the key task is to preserve muscle and strength, and sudden weight loss requires a check-up with a doctor. The association of BMI with life expectancy is described by a J-shaped curve with a maximum around 25–27 kg/m² and a minimum at 33 kg/m². The data are observational, and the "obesity paradox" is explained by additional factors — the type of adipose tissue, the presence of metabolic syndrome — rather than by the benefit of excess weight.

Limitations of BMI: what it does not show

BMI does not distinguish fat, muscle, and bone mass: in athletes it may overestimate the fat proportion, in the elderly and those who have lost muscle — underestimate it. Among residents of Finland with a BMI below 25 kg/m², at least 20% of men and at least 30% of women had an elevated fat content. Weight may not change while muscle mass grows and fat is lost, so body shape reflects health better than the number on the scale.

The risk of type 2 diabetes and cardiovascular diseases increases when fat is deposited in the waist area rather than the hips; according to the American Heart Association, body measurements predict the risk of heart disease more accurately than BMI. "Normal" is not equal to average: a woman with a BMI median for her country in most countries of the world will fall into the overweight category. For clarification, waist circumference, skinfold thickness (according to the Korovin method — at the level of the 3rd rib, normally 1–1.5 cm, and parasagittally at the level of the umbilicus, 1.5–2 cm), and alternative indices are used.

When to discuss weight with a doctor

The reason for a conversation is not the number itself, but its changes. If weight grows or falls without attempts to change it, this may be a sign of a medical problem: causes of sudden weight loss — thyroid diseases, oncological, infectious, and digestive diseases, taking certain medications; of sudden gain — the same medications, thyroid pathology, cardiac, renal, and hepatic failure. The doctor assesses BMI together with waist circumference, health history, and risk factors such as elevated cholesterol, and if necessary refers to a dietitian.

A BMI value by itself is not a reason to call an ambulance; 103/112 — if, against the background of a sharp change in weight, signs of a stroke or heart attack appear.

The material is for reference and does not replace a doctor's consultation: a BMI number is not a recommendation, but a reason for a conversation with someone who sees you as a whole.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer