Adult BMI categories
| BMI | Category | What it signals |
|---|---|---|
| Below 18.5 | Underweight | Possible undernutrition; worth investigating rather than ignoring |
| 18.5 – 24.9 | Healthy weight | Lowest average risk across large populations |
| 25.0 – 29.9 | Overweight | Modestly raised average risk; highly dependent on other factors |
| 30.0 – 34.9 | Obesity (class 1) | Raised risk of type 2 diabetes and cardiovascular disease |
| 35.0 – 39.9 | Obesity (class 2) | Substantially raised risk |
| 40.0 and above | Obesity (class 3) | Highest risk band; sometimes called severe obesity |
How to read the chart
BMI is your weight in kilograms divided by the square of your height in metres. The bands above are thresholds applied to that single number, and they are the same for men and women — unusual among health metrics, and one of the reasons BMI is convenient rather than precise.
Two worked examples. Someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9, comfortably inside the healthy band. Someone 1.60 m weighing 78 kg has 78 ÷ (1.60 × 1.60) ≈ 30.5, which crosses into class 1 obesity. Notice how little weight separates the bands at shorter heights: at 1.60 m, roughly 13 kg covers the entire healthy range, so a few kilograms moves you a whole category. At 1.85 m the same range spans about 22 kg. This is a real weakness of a squared-height formula, and it means the bands are blunter for short people than for tall ones.
Where the cut-offs came from
The 18.5 / 25 / 30 thresholds were adopted by a WHO expert consultation in 1997 and have been the international standard since. They were chosen because that is roughly where population-level mortality and disease risk start to curve upward in the large cohort studies available at the time — not because anything physiological changes at exactly 25.0. A BMI of 24.9 and a BMI of 25.1 are indistinguishable in every way that matters to your body. Treat the boundaries as a change of shading on a gradient, not a line you cross.
The measure itself is much older. It was devised in the 1830s by the Belgian mathematician Adolphe Quetelet as a way of describing the distribution of body sizes across a population. Quetelet was explicit that it was a statistical tool for groups, and not intended to assess any individual — a caveat that got lost somewhere between the 1830s and the modern GP surgery.
Who this chart describes badly
These bands apply to most adults aged 18 to 65. They should not be applied to the following groups without a lot of caution:
- Muscular people. Muscle is denser than fat. A well-trained rugby player or powerlifter can read 28 or 30 while carrying very little fat. BMI cannot tell muscle from fat, and this is its single most-cited failure.
- Older adults. People typically lose muscle and bone with age while gaining fat. Someone at 70 can sit squarely in the healthy band while having a body composition that would concern a clinician — the number stayed still, the body underneath it did not.
- Children and teenagers. They use age- and sex-specific percentile charts instead, because a growing body's healthy ratio changes year by year. Do not use adult bands for anyone under 18.
- Some ethnic groups. People of South Asian, Chinese, and some other Asian ancestry tend to develop cardiometabolic risk at lower BMIs, and many health bodies apply reduced thresholds — often around 23 for overweight and 27.5 for obesity.
- Pregnant people. BMI is not meaningful during pregnancy.
- People much shorter or taller than average. The squared-height term systematically over-reads for tall people and under-reads for short people.
What to pair it with
BMI is a screening number, and it works best as one input among several. Two measurements add most of what it misses, and neither needs equipment beyond a tape measure. Waist circumference captures where fat is stored, which matters more for metabolic risk than how much there is in total — visceral fat around the organs behaves very differently from fat on the hips and thighs. Body-fat percentage separates the muscle-versus-fat question that BMI cannot answer at all.
If your BMI reads high and your waist measurement and body-fat estimate are both unremarkable, the BMI number is probably the one that is wrong. If all three point the same way, the signal is real.
What to do with your result
Very little, on its own. A single BMI reading is a snapshot of a ratio, not a diagnosis, and it says nothing about your blood pressure, blood glucose, cholesterol, fitness, sleep, or diet quality — all of which are better predictors of how you will actually fare. If your number sits outside the healthy band and that concerns you, the useful next step is a conversation with a doctor who can look at the rest of the picture, not a crash diet.
If you do want to shift it, the durable route is unglamorous: a modest calorie deficit rather than a severe one, enough protein and resistance training to keep the muscle you have, decent sleep, and a timeframe measured in months. Track the trend across weeks rather than reacting to daily fluctuations — normal shifts in water, food volume, and glycogen can move the scale by a couple of kilograms in either direction without any change in body fat at all.