Body mass index is your weight divided by the square of your height. That is the whole formula. It was devised in the 1830s by a Belgian statistician, Adolphe Quetelet, who was studying populations rather than patients, and it was never designed to assess an individual person standing in front of you.
It survives because it is cheap, fast and needs no equipment beyond a scale and a tape. For screening large groups it works well enough. For deciding whether you are healthy, it is a blunt instrument, and knowing exactly how it is blunt is more useful than the number itself.
The standard adult ranges
These are the World Health Organization cutoffs used across most of Europe, North America and Australia for adults aged 20 and over.
| Category | What it flags | BMI |
|---|---|---|
| Underweight | Possible undernutrition or underlying illness | Below 18.5 |
| Healthy range | Lowest average risk across populations | 18.5 – 24.9 |
| Overweight | Raised average cardiometabolic risk | 25.0 – 29.9 |
| Obesity class I | Substantially raised risk | 30.0 – 34.9 |
| Obesity class II | Severely raised risk | 35.0 – 39.9 |
| Obesity class III | Very severely raised risk | 40.0 and above |
Note the wording in the middle column. Every one of these categories describes an average across a large group. A category is a prompt to look closer, not a diagnosis.
Where BMI gets it wrong
It cannot tell muscle from fat
Muscle is denser than fat, so an athlete and a sedentary person of identical height and weight get an identical BMI despite completely different body composition. Rugby forwards, sprinters and most competitive lifters land in the overweight or obese bands with single-digit body fat. If you train hard with weights, expect BMI to overstate your risk.
It ignores where fat sits
Fat around the organs — visceral fat — carries far more metabolic risk than the same mass sitting on hips and thighs. Two people with a BMI of 27 can have very different risk profiles depending on distribution, and BMI cannot see the difference. Waist measurement can.
It shifts with age
Adults lose muscle and bone density gradually from roughly the fourth decade onward. An older adult can hold a stable BMI while quietly replacing muscle with fat. Some research also suggests slightly higher BMI is protective in later life, which is why several bodies use a raised healthy band for people over 65.
The cutoffs were drawn from mostly white European data
Risk of type 2 diabetes and cardiovascular disease rises at lower BMI values in people of South Asian, Chinese, and several other Asian backgrounds. Many health authorities therefore use lower thresholds — around 23 for overweight and 27.5 for obesity — for those populations. If that applies to you, read your result against the lower scale.
It doesn't apply at all to: anyone under 18 (children are assessed on age-and-sex percentile charts instead), pregnant or breastfeeding women, and people with conditions that alter fluid balance or limb mass, such as heart failure or limb loss.
Two measurements worth more than BMI
Waist-to-height ratio
Measure your waist at the midpoint between the bottom rib and the top of the hip bone, breathing out normally, and divide by your height in the same units. A ratio under 0.5 is the usual guide — the shorthand is that your waist should be less than half your height. It picks up central fat, which is the part that matters most, and it needs nothing but a tape measure. Several large studies have found it predicts cardiometabolic risk better than BMI does.
What your bloods say
Fasting glucose or HbA1c, a lipid panel, and blood pressure between them describe metabolic health far more directly than any measurement of your outline. Plenty of people in the healthy BMI band have poor markers, and plenty in the overweight band have excellent ones. If you only ever act on one thing from this page, make it getting those numbers checked.
If your result surprised you
A BMI outside the healthy band is information, not a verdict, and the single most useful next step is the same in either direction: talk to a GP or a registered dietitian who can look at the whole picture — your measurements, your bloods, your history, what you eat and how you move. Numbers from a web page cannot do that, and a plan built around one number tends not to hold.
Be wary of anything promising fast change. Sustainable shifts in body composition happen over months, and the approaches that work are dull: enough protein, enough sleep, resistance training, and a way of eating you can still be doing next year.
Common questions
What is a healthy BMI?
18.5 to 24.9 for most adults, or roughly 18.5 to 22.9 if you use the lower Asian-population cutoffs. It is the band with the lowest average risk across populations, not a personal target.
Is BMI accurate for muscular people?
No. It uses only height and weight, so it reads dense muscle exactly the same as fat. Body fat percentage or a waist measurement will tell you far more.
Can I use this for my child?
No. Children and teenagers are assessed against age- and sex-specific percentile charts, because healthy body composition changes throughout growth. Ask your paediatrician or GP.
How often should I check it?
Rarely. BMI moves slowly and daily weight swings of a kilo or two are mostly water. Monthly is plenty; for many people, at annual check-ups is plenty.