What BMI measures
Body mass index is your weight in kilograms divided by the square of your height in metres. It was devised in the 1830s by Adolphe Quetelet as a way to describe populations, and it was adopted for individual screening much later. The single number places you on a scale that correlates, at the population level, with body fat and with the risk of several conditions.
For adults of either sex and any age, the World Health Organization uses the categories below. The healthy range is deliberately wide: it is the band in which population-level health risk is lowest, not a target weight.
| Category | BMI (kg/m²) |
|---|---|
| Severe thinness | below 16.0 |
| Moderate thinness | 16.0 – 16.9 |
| Mild thinness | 17.0 – 18.4 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | 40.0 and above |
How BMI is calculated
The formula is the same in both unit systems; the US version folds the kilogram and metre conversions into a single constant. Multiplying pounds per square inch by 703 gives kilograms per square metre, because 0.45359237 kilograms per pound divided by 0.0254 squared metres per square inch comes to 703.07.
| Units | Formula | Worked example |
|---|---|---|
| Metric | weight (kg) ÷ height (m)² | 70 kg at 1.75 m → 70 ÷ 3.0625 = 22.9 |
| US | 703 × weight (lb) ÷ height (in)² | 180 lb at 5′11″ (71 in) → 703 × 180 ÷ 5,041 = 25.1 |
BMI for children and teenagers
Between the ages of 2 and 19, BMI is calculated with the same formula but interpreted completely differently. Children's body fat changes as they grow and differs between boys and girls, especially through puberty, so a BMI of 21 means one thing at age 8 and another at 17. Instead of fixed ranges, the Centers for Disease Control and Prevention plot the value on a growth chart for the child's age and sex and read off a percentile: the share of children of that age and sex with a lower BMI.
The percentile then maps to a weight status. A child at the 90th percentile has a higher BMI than 90% of children of the same age and sex, and is classed as overweight.
- This calculator reports the BMI value for a child but does not compute the percentile — that needs the CDC growth reference for the child's exact age in months and sex. Place the value on the CDC BMI-for-age chart, or ask a paediatrician, to get the category.
- Under age 2, BMI is not used at all. Infants and toddlers are assessed by weight-for-length on the WHO growth standards.
- A single measurement matters less than the trend. Paediatricians watch whether a child is tracking along a percentile line or crossing several of them.
| Category | BMI-for-age percentile |
|---|---|
| Underweight | below the 5th percentile |
| Healthy weight | 5th to below the 85th percentile |
| Overweight | 85th to below the 95th percentile |
| Obesity | 95th percentile and above |
| Severe obesity | 120% of the 95th-percentile BMI, or a BMI of 35 or more |
BMI Prime and the Ponderal Index
BMI Prime is your BMI divided by 25, the top of the healthy range. It turns the scale into a plain ratio: 1.00 sits exactly at the upper healthy limit, 0.90 is 10% below it, and 1.20 is 20% above it. That makes distance from the threshold obvious at a glance, and it lets populations with different cut-offs be compared on one scale.
The Ponderal Index divides weight by height cubed rather than squared. Because weight scales roughly with volume, the cube removes a bias BMI carries: very tall people read as heavier than they are on BMI, and very short people as lighter. The Ponderal Index has no formal clinical categories, but values of roughly 11 to 14 kg/m³ correspond to the healthy BMI range for someone of average height.
| BMI Prime | Equivalent BMI | Category |
|---|---|---|
| below 0.74 | below 18.5 | Underweight |
| 0.74 – 1.00 | 18.5 – 25 | Healthy weight |
| 1.00 – 1.20 | 25 – 30 | Overweight |
| above 1.20 | above 30 | Obesity |
Health risks associated with being overweight
The CDC and other health agencies link excess weight, and abdominal fat in particular, to a higher risk of the conditions below. These are population-level associations: risk rises gradually with BMI rather than switching on at 25 or 30, and waist circumference and fitness change it substantially at any given BMI.
- High blood pressure.
- High LDL cholesterol, low HDL cholesterol, or high triglycerides.
- Type 2 diabetes and insulin resistance.
- Coronary heart disease and stroke.
- Gallbladder disease and gallstones.
- Osteoarthritis, from the extra load on weight-bearing joints.
- Sleep apnoea and other breathing problems.
- Several cancers, including endometrial, breast, colon, kidney, gallbladder, and liver.
- Lower quality of life, and higher rates of depression and anxiety.
- Chronic body pain and difficulty with everyday physical functioning.
- Higher all-cause mortality.
Health risks associated with being underweight
Being underweight carries its own risks, and it is often a sign of something else — an overactive thyroid, a digestive condition, an eating disorder, or another illness — that deserves attention in its own right.
- Malnutrition and vitamin deficiencies, including anaemia.
- Osteoporosis from reduced bone density, and a higher fracture risk.
- Weakened immune function and slower recovery from illness.
- Growth and development problems in children and teenagers.
- Reproductive problems, including irregular or absent periods and a higher risk of complications in pregnancy.
- Higher complication rates after surgery.
- Higher all-cause mortality.
Where BMI misleads
BMI uses only height and weight, so it cannot distinguish muscle from fat or tell where fat is stored. That produces predictable failure cases worth knowing before you act on the number.
- Muscular people are frequently misclassified. Many athletes register as overweight or obese despite low body fat, because muscle is denser than fat.
- Older adults may have a healthy BMI while carrying elevated fat and reduced muscle, a pattern sometimes called sarcopenic obesity.
- Fat distribution is invisible to BMI. Visceral fat around the organs carries more metabolic risk than subcutaneous fat, and two people with identical BMIs can differ substantially here.
- Risk thresholds differ across populations. Health authorities recommend lower cut-offs for people of South Asian, Chinese, and some other Asian ancestries, where metabolic risk rises at a lower BMI.
- BMI is not valid for children and teenagers as an absolute number; paediatric assessment uses the age- and sex-specific percentile chart described above.
Better measurements to use alongside it
BMI is most useful as one input among several. Waist circumference is a strong and simple indicator of abdominal fat, with elevated risk generally above 40 inches (102 cm) for men and 35 inches (88 cm) for women. Waist-to-height ratio, kept under roughly 0.5, is another straightforward check that works across body sizes.
For body composition specifically, a body fat percentage estimate adds information BMI cannot capture. Beyond anthropometrics, blood pressure, fasting glucose, and a lipid panel measure the outcomes that matter more directly than any size measurement does.
Using your result sensibly
Treat BMI as a screening prompt rather than a verdict. A result outside the healthy range is a reason to look at the fuller picture — waist measurement, activity level, diet, family history, and standard blood work — ideally with a clinician who can interpret them together.
It is also worth remembering that changes in health behaviour improve outcomes even when weight does not move much. Regular physical activity, better sleep, and improved diet quality independently reduce cardiovascular and metabolic risk regardless of what the scale reports.