Body mass index (BMI) is your weight in kilograms divided by the square of your height in metres. For adults, the World Health Organization classes a BMI under 18.5 as underweight, 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity. It is a quick, free screening figure that is useful for spotting weight-related health risk across a population, but it does not measure body fat directly, so on its own it cannot tell you whether you are healthy.
- BMI = weight (kg) ÷ height (m)², or 703 × weight (lb) ÷ height (in)².
- Adult categories are the same for men and women: healthy 18.5 to 24.9, overweight 25 to 29.9, obesity 30 or more (classes I, II and III).
- In the UK, NICE and the NHS use lower cut-offs (overweight from 23, obesity from 27.5) for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
- Children and teenagers are assessed with BMI-for-age percentiles, not the adult categories.
- BMI can mislead for muscular people, older adults and during pregnancy; waist size adds useful information.
How is BMI calculated?
BMI uses only two numbers, your weight and your height:
- Metric: BMI = weight in kilograms ÷ (height in metres × height in metres).
- Imperial: BMI = 703 × weight in pounds ÷ (height in inches × height in inches). The factor 703 converts pounds and inches into the metric result.
Example: someone 1.80 m tall who weighs 75 kg has a BMI of 75 ÷ 3.24 = 23.1. Someone 5 ft 4 in (64 in) tall who weighs 10 st 7 lb (147 lb) has a BMI of 703 × 147 ÷ 4,096 = 25.2. The result is normally rounded to one decimal place. You can skip the arithmetic with our BMI calculator, which accepts centimetres and kilograms or feet, stones and pounds.
Squaring the height is a compromise. Taller people are naturally heavier, and dividing by height squared keeps BMI roughly independent of height for most adults. It is not perfect: very tall people tend to get slightly higher BMIs and very short people slightly lower ones than their body fat would suggest. In 2013 the Oxford mathematician Nick Trefethen suggested an alternative that divides by height to the power 2.5, but health services still use the standard formula.
Where does BMI come from?
The formula was described in the 1830s by the Belgian mathematician and statistician Adolphe Quetelet, who was studying the "average man" and noticed that weight in adults rises roughly with the square of height. For more than a century it was known as the Quetelet index. In 1972 the American physiologist Ancel Keys compared several weight-for-height formulas against body fat measurements, found that this one worked best, and named it the body mass index. It was designed to describe groups of people, which explains both its strengths and its limits when it is applied to one person.
What do the BMI categories mean?
The WHO classification for adults aged 18 and over is used in most countries:
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy (normal) weight |
| 25.0 to 29.9 | Overweight (pre-obesity) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 or more | Obesity class III |
The boundaries are not sudden cliffs. They were chosen because, across large populations, the risk of conditions such as type 2 diabetes, high blood pressure and heart disease rises as BMI goes up from the mid-20s, and rises more steeply at higher values. A low BMI also carries risks, including undernutrition, weaker bones and lower resistance to illness.
A related figure, BMI Prime, divides your BMI by 25, the upper limit of the healthy range. A BMI Prime of 0.90 means you are 10% below that limit; 1.20 means 20% above it.
Why do some people use lower cut-offs?
Research has shown that at the same BMI, people from some ethnic backgrounds tend to have more body fat around the organs and a higher risk of type 2 diabetes and heart disease. A WHO expert consultation in 2004 noted this for Asian populations and suggested 23 and 27.5 as additional points for public health action. In England, NICE guidance tells health professionals to use lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background:
| Category | Standard (WHO) | Lower thresholds (NICE/NHS) |
|---|---|---|
| Healthy weight | 18.5 to 24.9 | 18.5 to 22.9 |
| Overweight | 25.0 to 29.9 | 23.0 to 27.4 |
| Obesity | 30.0 or more | 27.5 or more |
Other countries have their own versions; several Asian health authorities, for example, use 23 or 24 for overweight and 25 or 27.5 for obesity. If you are unsure which applies to you, ask your doctor. Our BMI calculator has a tick box for the NHS lower cut-offs.
How is BMI used for children and teenagers?
Children's BMI is calculated with the same formula, but it changes naturally as they grow and differs between boys and girls, so a fixed number cannot be used. Instead, the result is compared with a reference group of children of the same age and sex on a growth chart, giving a percentile (or centile):
- In the US, the CDC charts for ages 2 to 19 class below the 5th percentile as underweight, the 5th to below the 85th as a healthy weight, the 85th to below the 95th as overweight, and the 95th or above as obesity.
- In the UK, clinicians commonly use the UK90 charts, with the 91st centile and above suggesting overweight and the 98th and above obesity.
- The WHO growth reference for ages 5 to 19 uses standard deviations from the median instead of percentiles.
So an adult BMI calculator will give a number for a 12-year-old, but its adult category would be meaningless. Use a children's BMI tool, or ask a doctor, school nurse or health visitor, who can also look at growth over time.
What BMI cannot tell you
Because BMI knows only height and weight, it cannot see what that weight is made of or where it sits on the body. The main blind spots:
- Muscle versus fat: muscle is denser than fat. Rugby players, bodybuilders and other very muscular people can have a BMI in the overweight or obese range with little body fat.
- Hidden fat: the opposite also happens. Someone with a "healthy" BMI can carry a lot of fat around the abdomen, which is linked with higher metabolic risk.
- Age: older adults lose muscle and some height, so BMI can stay the same while body fat rises. Doctors may accept a slightly different range for them.
- Sex: at the same BMI, women on average have more body fat than men. The adult categories are nevertheless the same for both.
- Pregnancy: BMI is not meaningful during pregnancy. Midwives use the weight from before or early in pregnancy.
- Other situations: limb amputation, fluid retention and some medical conditions distort the result.
Experts increasingly stress that BMI should start a conversation, not end it. In 2025, for example, a Lancet commission of obesity experts recommended that BMI on its own should not be used to diagnose obesity in an individual, and that excess body fat should be confirmed with another measure, such as waist size, alongside an assessment of the person's health.
Better measures to use alongside BMI
- Waist-to-height ratio: measure your waist (halfway between the bottom of your ribs and the top of your hips) and divide by your height in the same units. NICE suggests that a ratio of 0.4 to 0.49 indicates no increased risk, 0.5 to 0.59 increased risk and 0.6 or more high risk. The easy rule: keep your waist to less than half your height.
- Waist circumference: commonly cited thresholds for increased risk are above 94 cm (37 in) for men and above 80 cm (31.5 in) for women, with higher risk above 102 cm (40 in) and 88 cm (34.6 in). Thresholds differ for some ethnic groups.
- Body fat percentage: measured with skinfold callipers, bioelectrical impedance scales or, most accurately, DEXA scans. Home scales give a rough guide and vary with hydration.
- Health markers: blood pressure, blood sugar (HbA1c), cholesterol, fitness and how you feel day to day matter more than any single number.
What to do with your BMI result
If your BMI is in the healthy range and your waist is less than half your height, keep up whatever habits got you there. If it is above the healthy range, small, sustained changes to eating and activity usually work better than drastic diets; the NHS suggests aiming to lose 0.5 to 1 kg (1 to 2 lb) a week. Our calorie calculator (TDEE) estimates how many calories you burn and what a gradual deficit looks like. If your BMI is below 18.5, or if you have lost weight without trying, talk to your doctor.
Need to convert your measurements first? Use the kg to lbs converter and the cm to inches converter. For other screenings, see our health tools.
Frequently asked questions
What is a normal BMI?
For most adults, 18.5 to 24.9. The NHS uses 18.5 to 22.9 for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background.
Is BMI accurate?
It is a reasonable screening tool for groups of people and a starting point for individuals, but it does not measure body fat, so it can misclassify muscular people, older adults and people who carry fat around the waist.
Is BMI the same for men and women?
Yes, adults use the same formula and categories regardless of sex. Women typically have more body fat than men at the same BMI.
What BMI is considered obese?
A BMI of 30 or more for most adults, or 27.5 or more using the lower NHS cut-offs. Obesity is divided into class I (30 to 34.9), class II (35 to 39.9) and class III (40 or more).
Can I use an adult BMI calculator for my child?
Only to get the number. Children's BMI must be compared with others of the same age and sex using percentile charts, so the adult categories do not apply.
Who invented BMI?
Adolphe Quetelet described the formula in the 1830s, and Ancel Keys named it the body mass index in 1972.