BMI Calculator

How to use this calculator

1. Choose metric or imperial units.
2. Enter your weight and height.
3. The result shows your BMI and the WHO adult classification band it falls into.

The formula

kg/m²

Metric form

703

Conversion factor that makes the imperial form agree with the metric one

How it works

BMI scales weight against the square of height, which roughly corrects for the fact that taller people naturally weigh more.

The WHO bands for adults are: underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, and obesity 30 and above.

How the categories were defined

The familiar thresholds — under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obese — come from World Health Organization guidance based on population-level associations between BMI and mortality or disease risk. They are statistical boundaries, not diagnostic cut-offs, and risk changes gradually rather than jumping at a threshold.

Several health bodies use lower thresholds for people of South Asian, Chinese, and some other ancestries, because cardiometabolic risk appears at lower BMI values in these populations. If that applies to you, discuss the appropriate range with a clinician rather than assuming the standard bands.

Where BMI misleads

BMI cannot distinguish muscle from fat. Athletes and heavily trained individuals routinely score in the overweight or obese range while carrying very little body fat. Conversely, someone with low muscle mass can sit inside the healthy range while carrying an unhealthy proportion of fat, sometimes described as normal-weight obesity.

It also ignores fat distribution, which matters clinically. Visceral fat around the abdomen carries more metabolic risk than subcutaneous fat elsewhere, which is why waist circumference and waist-to-height ratio are often measured alongside BMI. A waist-to-height ratio below 0.5 is a widely used complementary check.

BMI is not valid for children and teenagers, who need age- and sex-specific percentile charts, nor is it reliable during pregnancy, for people over roughly seventy, or for those with significant muscle-wasting conditions.

Limitations of a two-variable formula

BMI was derived from height and weight data collected on European men in the 1830s, long before body-composition scanning existed. It was never intended as an individual diagnostic tool; the mathematician who created it, Adolphe Quetelet, explicitly used it to describe population averages, not to judge a single body. Modern medicine adopted it later simply because it correlated reasonably well with disease risk across large datasets and required no special equipment.

Because the formula only divides weight by height squared, it cannot account for frame size, bone density, hydration status, or the proportion of the body made up of muscle versus fat. Two people with the same BMI can have very different amounts of visceral fat, cardiovascular fitness, and metabolic health. This is why researchers increasingly pair BMI with waist circumference, body-fat percentage, or imaging-based measures when assessing individual risk rather than population trends.

How results differ across groups

Age changes what a given BMI means. Older adults naturally lose muscle and gain relative fat even at a stable weight, so a BMI in the healthy range may still coincide with reduced strength and higher fat percentage than the same number would represent in someone younger. Some geriatric guidelines actually accept slightly higher BMI ranges in older adults, since being underweight is linked to higher frailty and mortality risk in this group.

Sex also matters: women typically carry a higher percentage of body fat than men at the same BMI, reflecting normal biological differences rather than a health problem. Ethnic background shifts the picture further, since populations with higher visceral fat at a given BMI, such as many South and East Asian groups, face elevated diabetes and cardiovascular risk at levels that would be considered low-risk under the standard Western-derived thresholds.

Common mistakes when interpreting BMI

A frequent error is treating a single BMI reading as a verdict on health rather than as one screening input among many. Another is comparing your BMI directly against a friend's or family member's without accounting for differences in muscle mass, frame, or age — the number is not a like-for-like comparison across bodies with different compositions.

People also often misapply adult categories to teenagers, or assume that a healthy-range BMI rules out metabolic problems such as insulin resistance, which can exist in people of normal weight, particularly if they carry fat viscerally rather than subcutaneously. Rounding height or weight inputs carelessly, especially near a category boundary, can also nudge someone into a different label despite negligible real-world difference.

Using the number practically

The most productive way to use BMI is as a periodic checkpoint rather than a daily metric. Recalculating every few months and watching the trend tells you far more than any single value, because it filters out day-to-day water-weight fluctuation and reflects genuine change in your body over time.

If your BMI sits outside the healthy range, pair it with a waist measurement and, if possible, a conversation with a clinician who can consider blood pressure, lipids, glucose, family history, and activity level together. If it sits inside the healthy range, that is reassuring but not a guarantee of good metabolic health, particularly if your lifestyle is sedentary or your waist circumference is elevated.

Related metrics and glossary

Waist-to-height ratio divides waist circumference by height and is a simple, widely validated proxy for abdominal fat that many researchers argue outperforms BMI for predicting cardiovascular risk; a ratio under 0.5 is generally considered favourable. Body-fat percentage, measured by skinfold callipers, bioelectrical impedance, or DEXA scanning, gives a direct estimate of how much of your weight is fat versus lean tissue.

Lean body mass refers to everything that is not fat — muscle, bone, organs, and water — and is the metric bodybuilders and clinicians often track instead of total weight. If any of these numbers point toward risk, or if you have a condition affecting fluid balance, muscle mass, or growth, a doctor or registered dietitian can interpret the combined picture far better than any calculator alone.

What BMI cannot tell you

BMI uses only height and weight, so it cannot distinguish muscle from fat, nor tell you where fat is stored — the factor most strongly linked to health risk.

Athletes with high muscle mass often register as overweight while carrying very little fat, and older adults can sit in the healthy range while having lost substantial muscle.

Waist measurement and waist-to-height ratio add the distribution information BMI lacks, and together the two give a much better picture than either alone.

Treat BMI as a population screening tool rather than a personal diagnosis, and discuss any concern with a clinician who can assess the full picture.

Tracking change rather than a single reading

A single BMI value is far less informative than its direction over months, since trend removes day-to-day fluctuation in hydration and food volume.

Weighing under the same conditions — same time of day, similar clothing — makes a series comparable.

Changes of one or two kilograms within a week are almost always fluid rather than fat, so weekly averages read more honestly than individual readings.

Recording waist alongside weight catches the case where body composition improves while the scale barely moves.

Population differences in thresholds

The familiar cut-offs were derived largely from European populations, and health bodies recommend lower thresholds for people of South Asian, Chinese, and other Asian descent.

In those guidelines, increased risk can begin around a BMI of twenty-three rather than twenty-five, because metabolic risk appears at a lower body mass.

Children and teenagers are assessed against age and sex percentile charts rather than fixed adult bands, since body composition changes rapidly with growth.

Pregnancy makes BMI meaningless as a current measure, and clinicians instead track weight gain against pre-pregnancy weight.

Acting on a reading

A reading outside the healthy band is a prompt to look closer, not a diagnosis, and a clinician can assess blood pressure, blood sugar, and lipids alongside it.

Small sustained changes in habit move the number far more reliably than short intensive efforts that are abandoned within weeks.

Strength training changes body composition even when weight is stable, which improves health markers that BMI cannot see.

If the figure is causing distress rather than informing a decision, focusing on behaviour and measurements other than weight is usually more productive.

When this calculator is useful

• A quick population-standard weight screening
• Tracking change over time
• A starting point for a conversation with a health professional