What BMI Tells You
One division, four categories, and a measure that was designed for populations and is read as a verdict on individuals. Here is the arithmetic and its limits.
Last updated: 30 September 2026
The arithmetic
BMI = weight in kg ÷ (height in metres)²
Someone 178 cm tall weighing 75 kg:
1.78 × 1.78 = 3.1684, and 75 ÷ 3.1684 = 23.7
That is the whole calculation. There is nothing else in it, which is both why it is used everywhere and why it is wrong about so many people.
The categories
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and over | Obese |
These are the World Health Organization cut-offs for adults. They are not age adjusted, not sex adjusted, and not adjusted for anything else.
Work it out: the BMI calculator does the division and names the category.
What it actually measures
Mass, against height squared. Nothing in the equation knows what the mass is made of, where it sits, or what you can do with it.
That is not a flaw so much as a design decision. BMI was built to describe populations, where the muscular and the sedentary average out and the correlation with health outcomes across thousands of people is genuinely strong. Applied to one person it inherits none of that strength.
The four people it gets wrong
Anyone carrying real muscle. A 100 kg lifter at 180 cm has a BMI of 30.9 and is categorised as obese. Muscle is denser than fat, and the equation cannot tell them apart.
Older adults. Muscle is lost with age while fat is gained, so a BMI that has not moved in twenty years can sit on a body that has changed considerably.
Anyone very short or very tall. Squaring the height flatters short people and penalises tall ones, because human bodies do not scale as squares.
Anyone whose fat is mostly visceral. Two people at the same BMI, one carrying it around the middle and one not, face substantially different risk. Waist circumference catches this and BMI cannot.
What to use instead
For health risk, waist measurement is the better single number, and waist against height is better still: keeping your waist under half your height is a more useful target than a BMI category.
For training, BMI tells you nothing worth knowing. Bodyweight, a waist measurement and what your lifts are doing will answer every question you actually have.
Common questions
Is BMI useless?
No. It is a good population measure and a poor individual one, and almost all the argument about it comes from applying it to individuals.
My BMI says overweight but I lift. Should I worry?
Check your waist. If that is in a sensible range and your lifts are progressing, the BMI category is describing your muscle.
Does it work for children?
Not with these numbers. Children are assessed on percentile charts adjusted for age and sex.
Sources
NHS, body mass index - https://www.nhs.uk/conditions/obesity/diagnosis/
World Health Organization, obesity and overweight - https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
NHS healthy weight guidance - https://www.nhs.uk/live-well/healthy-weight/