Complete BMI Guide: Formula, Chart, Categories and Limitations

What is BMI?

Body mass index, or BMI, is a single number derived from a person's weight and height. It was proposed in the 1830s by Belgian statistician Adolphe Quetelet as a way to describe the typical body proportions of a population. In the 1970s, researcher Ancel Keys popularised the modern name "body mass index", and the World Health Organization later adopted it as a low-cost screening tool for underweight, overweight and obesity in adults.

BMI was never designed to be a diagnosis. Its purpose is to flag, at the population level, whether weight relative to height is in a range associated with higher health risk. For an individual it is a starting point — useful when combined with other measurements, and misleading when read on its own.

The metric BMI formula

In metric units the formula is:

BMI = weight (kg) ÷ height (m)²

Worked example. An adult who weighs 72 kg and is 1.70 m tall: 72 ÷ (1.70 × 1.70) = 72 ÷ 2.89 ≈ 24.9. That sits at the very top of the healthy adult range.

The US BMI formula

In US customary units (pounds and inches) the formula uses the conversion factor 703 so the score lands on the same scale as the metric version:

BMI = (weight in lb × 703) ÷ height (in)²

Worked example. A person who weighs 165 lb and is 69 in tall: (165 × 703) ÷ (69 × 69) = 115 995 ÷ 4 761 ≈ 24.4. The small difference from the metric example above is just rounding when converting between unit systems.

Adult BMI categories (WHO)

These categories apply to adults aged 18 and over. They are not appropriate for children or pregnant people, and the cut-offs may be lower for some populations.

How to interpret your BMI

A BMI in the healthy range is a positive sign but not a stamp of perfect health. Two adults with the same BMI can have very different body composition, fitness, blood pressure and metabolic markers. A sedentary smoker with a BMI of 22 can be at higher cardiovascular risk than an active non-smoker with a BMI of 27.

Treat BMI as a flag, not a verdict. If yours is outside the healthy range, especially with other risk factors such as a high waist circumference or family history of type 2 diabetes, that is a reason to look more closely — usually with a healthcare provider — not a reason to panic.

Limitations of BMI

BMI uses only weight and height. It cannot tell what proportion of body weight is muscle, bone, water or fat, and it does not say where fat is stored. Because visceral fat (around the organs) is more strongly linked with cardiometabolic risk than fat stored under the skin, BMI alone misses an important part of the picture.

• It can overstate body fat in muscular athletes.
• It can understate body fat in older adults who have lost muscle mass.
• It does not capture fat distribution, which affects health risk.
• The cut-offs were derived mostly from European-ancestry populations.

BMI and muscle mass

Muscle is denser than fat, so people who train regularly with weights — particularly rugby players, sprinters, weightlifters and bodybuilders — often have a BMI in the overweight or obese range while carrying a low body fat percentage. In this case the number reflects the formula, not health risk. Body fat percentage measured with calipers, DEXA or bioimpedance, plus waist circumference, gives a clearer picture.

BMI and age

Older adults tend to lose muscle and gain fat, even when total body weight stays stable. As a result, a BMI of 24 in a 30-year-old and a BMI of 24 in an 80-year-old do not describe the same body. In adults over 65, slightly higher BMI ranges (often cited as 23–28) are sometimes associated with the lowest mortality, although this is still debated. Always interpret BMI with age in mind.

BMI and biological sex

On average, women carry a higher proportion of body fat than men at the same BMI, because the formula does not distinguish lean from fat tissue. WHO categories nevertheless apply equally to adult men and women — the difference shows up in complementary measurements such as body fat percentage and waist circumference, which use sex-specific thresholds.

BMI and ethnicity

Health risk linked to body fat does not start at the same BMI in every population. Several health authorities, including the UK NICE guidelines and the WHO Asia-Pacific consultation, recommend lower thresholds for adults of South Asian, Chinese or other Asian backgrounds. In these groups, increased risk often appears from a BMI of around 23, and obesity-level risk from around 27.5, rather than 25 and 30.

Waist circumference and waist-to-height ratio

Where fat is stored matters. Fat around the abdomen is more strongly linked with cardiometabolic risk than fat on the hips or thighs. Two simple complements to BMI help capture this:

• Waist circumference.Measured at the midpoint between the lowest rib and the iliac crest, after a normal exhale. Higher risk thresholds are commonly cited as >94 cm (≈37 in) for men and >80 cm (≈31.5 in) for women, with substantially raised risk above 102 cm (40 in) and 88 cm (34.5 in) respectively.
• Waist-to-height ratio.Waist measurement divided by height in the same unit. A ratio above 0.5 ("keep your waist under half your height") is the widely used cut-off for raised risk in adults.

When to speak to a healthcare professional

Consider booking an appointment if your BMI is below 18.5 or 25 or above and you have other risk factors, if your weight has changed quickly without explanation, or if BMI is causing you anxiety. A clinician can interpret your number alongside the measurements and history that BMI cannot capture, and help build a plan that fits your life rather than a generic chart.

Frequently asked questions

Is BMI a medical diagnosis?

No. BMI is a screening tool that compares weight with height. It cannot diagnose obesity, undernutrition, or any other condition on its own. A clinician interprets BMI alongside waist circumference, blood pressure, blood lipids, blood glucose, fitness level, lifestyle, family history and other findings before reaching a conclusion.

What is the healthy BMI range for adults?

The World Health Organization defines a healthy BMI for most adults as 18.5 to 24.9. This range is associated, at the population level, with the lowest risk of weight-related conditions. It does not guarantee individual health and the cut-points are slightly different for some populations, including South Asian adults.

Does BMI work for athletes?

Often poorly. Muscle is denser than fat, so a strength-trained athlete with low body fat may register a BMI in the overweight or obese range. For people with very high muscle mass, body fat percentage, waist circumference and waist-to-height ratio give a more accurate picture than BMI alone.

Can I use this BMI guide for children?

No. The adult BMI categories on this page are for people aged 18 and over. Children and teenagers are still growing, so clinicians use BMI-for-age percentiles from the WHO, the CDC or the UK NHS rather than fixed categories.

Should I worry if my BMI is just outside the healthy range?

A BMI of 25 or 26 is not the same as a BMI of 35. Risk increases gradually, not at a single threshold. If your BMI is slightly outside the healthy range, look at the bigger picture — waist size, activity level, blood pressure, sleep, diet quality — and talk to a healthcare professional if you are unsure.

References

• World Health Organization. Obesity and overweight. who.int
• Centers for Disease Control and Prevention. About Adult BMI. cdc.gov
• UK National Health Service. What is the body mass index (BMI)? nhs.uk
• WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 2004.
• National Institute for Health and Care Excellence (NICE). BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups. nice.org.uk