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BMI for women, what the number tells you and what it leaves out
Body mass index is the most widely used weight measure in the world and one of the most misunderstood. Women in particular are often told their BMI without being told what it means, why it is the same calculation as for men, or where it falls short. Here is the short version.
The formula
BMI is your weight divided by your height squared.
BMI = weight in kg ÷ (height in m × height in m)
A woman who is 1.65 m tall and weighs 62 kg has a BMI of 62 ÷ (1.65 × 1.65) = 22.8.
In imperial units there is an extra constant:
BMI = 703 × weight in lb ÷ (height in inches × height in inches)
So 5'5" (65 inches) and 137 lb gives 703 × 137 ÷ 4225 = 22.8. The same person, the same answer.
The adult categories
The World Health Organization bands are the same for adult women and men:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
The obese band is split further into classes I, II and III at 35 and 40. Some health services use lower action points for people of Asian descent, because health risks tend to rise at lower BMIs in those populations. The full set of WHO classes, the Asian cut-offs and the children's percentile groups are in the BMI categories table.
Why it is the same for women
It is a fair question, because women and men differ in body composition. At the same BMI, women on average carry more body fat than men. So why not have separate bands?
The answer is that BMI was never designed to measure body fat. It was designed as a quick population screen that correlates, roughly, with health risks such as type 2 diabetes and heart disease. Those risks rise with BMI in both sexes, and the cut-offs were chosen from large studies of where risk starts to climb. Separate bands would make the screen more complicated without making it much more accurate for an individual.
Children are different: their BMI is compared with percentiles for their age and sex, because both change so much while they grow.
What BMI cannot see
BMI only knows two numbers. Everything else about your body is invisible to it.
Muscle. Muscle is denser than fat, so a strong, athletic woman can sit in the overweight band while carrying little fat. This is the most common objection to BMI, and it is a real one for people who train seriously.
Where fat sits. Fat around the waist is more strongly linked to health risk than fat on the hips and thighs. Two women with the same BMI can have quite different risk depending on shape. Waist circumference, or waist-to-height ratio, adds information BMI misses.
Age. Women tend to lose muscle and bone density with age, especially after menopause, and gain fat. An older woman's BMI can stay the same while her body composition changes significantly.
Pregnancy. BMI does not apply during pregnancy. Pre-pregnancy BMI is used to guide recommended weight gain, but a reading taken while pregnant is not meaningful.
Height extremes. Squaring height is a simplification. It slightly flatters tall people and penalises short people, so very short or very tall women can get a skewed number.
A healthy weight range for your height
Turn the formula around and you get a weight range for any height. For 1.65 m:
- 18.5 × 1.65 × 1.65 = 50.4 kg
- 24.9 × 1.65 × 1.65 = 67.8 kg
So a healthy-band weight at that height runs from about 50 to 68 kg. That is a wide range, around 17 kg, and it is a reminder that BMI is a coarse tool. It tells you which broad band you are in, not a target to the kilogram.
How to use the number
Treat BMI as a first look rather than a verdict.
- If it is in the healthy band and you feel well, it is one more reassuring sign.
- If it is outside the band, it is a reason to look at other measures, such as waist size, blood pressure, blood tests and fitness, rather than a diagnosis on its own.
- If you are muscular, pregnant, older or very short or tall, expect it to be less accurate for you, and lean on other measures more.
Any concerns about weight are best discussed with a GP or other health professional, who can look at the whole picture.
Work it out
The BMI calculator for women takes height and weight in metric or imperial, including feet and inches in one field, and shows the WHO category, the healthy weight range for your height, and notes on what the number does and does not say about body fat. The general BMI calculator does the same without the women-specific notes, and the rest of the health tools are in the health calculators section.