BMI Explained: Categories, Asian Cut-offs and Limitations
BMI explained: how to calculate it, the WHO adult categories, lower cut-offs for Asian populations, what BMI misses and how to find a healthy weight range.
By Awais, Founder and editor-in-chiefPublished
Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. For adults, the World Health Organization classes a BMI of 18.5 to 24.9 as normal weight, 25 to 29.9 as pre-obesity (overweight) and 30 or more as obesity.
BMI is a quick screening number, not a diagnosis. It cannot tell muscle from fat, and the same BMI can carry different health risks for different people. This guide explains how it is calculated, what the categories mean, why some countries use lower cut-offs for Asian populations, and what to look at alongside it. It is general information, not medical advice; talk to a doctor about your own health.
How to calculate BMI
The CDC gives the same formula in metric and imperial units:
BMI = 703 × weight (lb) ÷ [height (in)]2
Metric example: 78 kg and 1.75 m tall. 1.752 = 3.0625, and 78 ÷ 3.0625 = 25.5.
Imperial example: 172 lb and 5 ft 9 in (69 inches). 692 = 4,761, and 703 × 172 ÷ 4,761 = 25.4.
The BMI calculator does this in either unit and shows your category.
WHO adult BMI categories
These categories apply to adults aged 20 and over, according to the WHO Regional Office for Europe:
| BMI (kg/m²) | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Normal weight |
| 25.0–29.9 | Pre-obesity |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| 40 and above | Obesity class III |
The CDC uses the same thresholds with slightly different names: Healthy Weight for 18.5 to under 25, Overweight for 25 to under 30, and Obesity from 30, with Class 3 (severe) obesity at 40 or greater. In its obesity fact sheet, WHO defines “overweight” as a BMI of 25 or more, which includes obesity.
Lower cut-offs for Asian populations
Studies have found that many Asian populations have a higher proportion of body fat, and higher risk of type 2 diabetes and heart disease, at a lower BMI than European populations. In 2004, a WHO expert consultation published in The Lancet reviewed this evidence. It kept the standard international categories but identified additional “public health action points” at BMI 23, 27.5, 32.5 and 37.5, so that countries could choose lower thresholds for action.
| Standard WHO cut-off | Additional action point |
|---|---|
| 25 | 23 |
| 30 | 27.5 |
| 35 | 32.5 |
| 40 | 37.5 |
In practice, BMI 23 is often used as the point where risk starts to rise for people of South Asian, Chinese and other Asian descent, and 27.5 as the point of high risk. Some national guidelines set their own values, so ask a doctor which cut-offs apply where you live.
Worked example: a healthy weight range
To find the weight range that gives a BMI of 18.5 to 24.9 at your height, rearrange the formula:
For someone 1.70 m tall, height squared is 2.89:
- Lower end: 18.5 × 2.89 = 53.5 kg
- Upper end: 24.9 × 2.89 = 72.0 kg
- Using the Asian action point of 23: 23 × 2.89 = 66.5 kg
| Height | BMI 18.5 | BMI 24.9 |
|---|---|---|
| 1.60 m | 47.4 kg | 63.7 kg |
| 1.70 m | 53.5 kg | 72.0 kg |
| 1.80 m | 59.9 kg | 80.7 kg |
| 5 ft 6 in (66 in) | 114.6 lb | 154.3 lb |
A range this wide is a reminder that “healthy weight” is not one number. Where you sit in it depends on your build, muscle and health.
What BMI does not tell you
BMI is popular because it is free, fast and needs only a scale and a tape measure. But it has real limits:
- It does not measure body fat. The CDC states that BMI does not distinguish between fat, muscle, and bone mass. A muscular athlete can have a BMI over 25 with little body fat.
- It ignores where fat sits. Fat around the waist carries more health risk than fat on the hips and thighs.
- Risk differs between groups. As the WHO consultation found, the same BMI can mean different risk in different populations. Body composition also changes with age.
- It is not for children. For ages 2 to 19, the CDC uses BMI-for-age percentiles that compare a child with others of the same sex and age, not the adult categories.
- It is not designed for pregnancy, when weight gain is expected and monitored differently.
Waist circumference adds context
The NIH’s National Heart, Lung, and Blood Institute notes that a waist circumference of more than 35 inches (89 cm) for women or more than 40 inches (102 cm) for men increases your risk. To measure, place a tape measure around your bare waist just above the hip bones and breathe out normally.
What to do with your number
- BMI below 18.5: being underweight can also carry health risks. A doctor can check for causes.
- BMI 18.5 to 24.9: keep up healthy habits; check your waist measurement too.
- BMI 25 or above (or 23 and above if Asian cut-offs apply to you): it is worth discussing with a doctor, who can look at blood pressure, blood sugar, cholesterol and family history.
If weight loss is the goal, the size of the calorie deficit matters. The TDEE calculator estimates how many calories you burn a day, and our guide on how many calories to eat to lose weight explains safe deficits and why weight loss slows over time. The calorie deficit calculator turns that into a daily target.
FAQ
What is a healthy BMI?
For most adults, WHO classes 18.5 to 24.9 as normal weight. For people of Asian descent, many doctors use lower thresholds, with risk starting to rise from about 23.
Is BMI accurate for muscular people?
Not very. BMI cannot tell muscle from fat, so athletes and people who do a lot of strength training may get a high BMI without excess fat. Waist size or a body-fat measurement gives more context.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. For 150 lb at 65 inches: 703 × 150 ÷ 4,225 = 25.0.
Does BMI apply to children?
No. For ages 2 to 19, BMI is compared with growth charts for the same age and sex and reported as a percentile. Adult categories do not apply.
Why does WHO have lower BMI cut-offs for Asians?
Research reviewed by a WHO expert consultation in 2004 found that many Asian populations face higher health risks at lower BMI. It suggested extra action points at 23, 27.5, 32.5 and 37.5 for countries to use.