Calculating BMI (body mass index), women and men
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BMI is calculated like this: weight in kilograms divided by height in meters squared. In pounds and inches, the same calculation is weight in pounds divided by height in inches squared, times 703. For adults, the French health authority HAS, which cites the World Health Organization, classes normal weight from 18.5 to 24.9, overweight from 25.0 to 29.9 and obesity from 30. The same table applies to women and to men. It does not apply to children.
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Example: a woman aged 40, 65 kg (143 lb) and 165 cm (5 ft 5 in). Height in meters: 1.65. BMI = 65 / (1.65 × 1.65) = 23.9. The adult table places 23.9 in normal weight (18.5 to 24.9).
Calculating your BMI
The formula published by the WHO is weight (kg) divided by height (m) squared. Age and sex do not change the figure. They serve two purposes here: refusing the calculation for a child, and recalling that the adult table is not a personal target after 65.
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Table of adult categories
The French HAS adopts the WHO classification (technical report 894): normal weight from 18.5 to 24.9, overweight from 25.0 to 29.9, class I obesity from 30.0 to 34.9, class II from 35.0 to 39.9, class III from 40. The WHO fact sheet on obesity keeps the same thresholds for overweight (from 25) and obesity (from 30). Below 18.5, the value sits under the “normal weight” class of this table.
| Class | BMI |
|---|---|
| Below normal weight | below 18.5 |
| Normal weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obesity, class I | 30.0 to 34.9 |
| Obesity, class II | 35.0 to 39.9 |
| Obesity, class III | 40 and above |
Normal and ideal BMI for women
There is no ideal female BMI distinct from the adult table. A woman and a man of the same weight and height have the same BMI, and the same class. “Normal BMI for a woman” therefore means the normal weight class, from 18.5 to 24.9.
The menopause does not change the formula. It can change fat distribution, which BMI does not see. Body composition and waist circumference provide that information.
Normal and ideal BMI for men
Same table: normal weight from 18.5 to 24.9. A very muscular man can exceed 25 with little fat. The figure classes weight relative to height; it does not say what that weight is made of.
BMI and age: what changes after 65
The table above is the adult one. It is not a personal numerical target, especially after 65. This page gives no “ideal” range for that age: build is discussed with a doctor, using measurements other than BMI alone.
The limits of BMI
BMI does not separate muscle from fat, and it ignores where the fat is. An athlete can be classed as overweight while their fat mass is low. Someone in the “normal weight” class can carry excess abdominal fat. The French HAS asks, between 25 and 35, for the examination to be completed with waist circumference.
Going further: fat mass, muscle mass, visceral fat
These three measurements describe the body where BMI stops. They can be followed from one weigh-in to the next, for example with a bioimpedance scale.
This information does not replace the advice of a healthcare professional.
Frequently asked questions
What is the ideal BMI for a woman?
The WHO adult table, adopted by the French HAS, sets no separate female ideal. The normal weight class runs from 18.5 to 24.9, for women as for men. It is not a prescribed personal target.
What is a normal BMI for a man?
The same class: 18.5 to 24.9. Above 25, the table speaks of overweight, and from 30 of obesity. A BMI above 25 in a muscular man does not by itself mean there is too much fat.
Does BMI change with age?
The formula does not contain age. Neither does the adult table. After 65, this table is not a personal target: the situation is looked at with a doctor.
Why is BMI not enough for an athlete?
Because a kilo of muscle and a kilo of fat count the same in weight. The athlete can exceed 25 with a low fat mass. Body composition makes the difference.
How do you calculate a child's BMI?
Not with this calculator. For children, the WHO uses growth curves by age and sex, not the adult thresholds. The reference is the child health record and the doctor.
Is the calculation free?
Yes. It runs in the browser, with no account. No value is sent or kept.
Is a menopausal woman's BMI calculated differently?
No. The formula and the adult table stay the same. The menopause can change fat distribution, which BMI does not measure.
Sources
Read also
BMI does not distinguish muscle from fat. Vita measures body composition at every weigh-in: fat mass, muscle mass and visceral fat. See the Vita scale