BMI Calculator

A number invented for populations, applied to individuals

Someone 175 cm tall weighing 70 kg has a BMI of 22.9 — comfortably inside the range the WHO calls normal. Add ten kilograms and it becomes 26.1, which the same scale calls overweight. The arithmetic is trivial. What the number *means* for that particular person is not, and understanding the gap between the two is the only way to use BMI sensibly.

Body Mass Index is weight in kilograms divided by height in metres squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was explicitly studying the distribution of traits across *populations* — not diagnosing individuals. It entered clinical use only in the 1970s, chosen largely because it is cheap: it needs a set of scales and a tape measure, and it correlates reasonably well with body fat across large groups.

That origin explains both its usefulness and its limits. Across ten thousand people, BMI tracks health outcomes well enough to be worth collecting. For any one of those ten thousand, it can be badly wrong — because it measures mass, and takes no view on whether that mass is muscle, fat, bone or fluid, or where on the body it sits.

The formula, and the one adjustment that matters in India

BMI = weight (kg) ÷ height (m)²

Height must be in metres, which is where most manual attempts go wrong — 175 cm is 1.75 m, and squaring it gives 3.0625.

The WHO categories this calculator reports are:

• Below 18.5 — underweight
• 18.5 to 24.9 — normal
• 25.0 to 29.9 — overweight
• 30.0 and above — obese

There is an important qualification for anyone of South Asian descent. Research consistently finds that Indians carry more body fat and more visceral fat at any given BMI than white European populations, and develop insulin resistance, type 2 diabetes and cardiovascular disease at lower BMIs. In response, the WHO Expert Consultation of 2004 and subsequent ICMR guidance proposed lower Asian cutoffs: overweight from 23.0 and obese from 25.0.

The practical consequence is significant. A BMI of 23.5 reads as comfortably normal on the WHO scale and as overweight on the Asian-Indian scale. This calculator reports the WHO categories, which are the international standard; if you are of South Asian origin, read your result against the lower thresholds as well.

Working through the numbers

Height 175 cm, weight 70 kg.

Convert height: 175 ÷ 100 = 1.75 m. Square it: 1.75 × 1.75 = 3.0625.

Divide: 70 ÷ 3.0625 = 22.86, rounded to 22.9. WHO category: normal. Asian-Indian category: also normal, since it is below 23.

Now the same height at 80 kg. 80 ÷ 3.0625 = 26.12 — overweight on both scales.

And at 60 kg. 60 ÷ 3.0625 = 19.59 — normal on both, though towards the lower end.

Notice how much weight separates the categories at this height: the normal band spans roughly 56.6 kg to 76.3 kg under WHO cutoffs, a range of nearly twenty kilograms. Two people at opposite ends of it look entirely different and would be classified identically.

The height sensitivity is worth seeing too. Because height is squared, small measurement errors matter. Recording 1.75 m as 1.73 m moves the BMI for a 70 kg person from 22.9 to 23.4 — enough to cross the Asian-Indian overweight threshold on a two-centimetre error. Measure without shoes, standing straight, ideally in the morning.

What BMI is genuinely good for

Tracking yourself over time. This is its strongest use. Your height is constant, so changes in your BMI are changes in your weight, viewed on a scale that is easy to compare against guidance.

A first-pass screen. A BMI of 34 is informative regardless of body composition. Extremes are where BMI is most reliable.

Population and clinical thresholds. Eligibility for bariatric surgery, some drug protocols and public health targets are defined in BMI terms, so the number has administrative meaning whatever its individual limitations.

It is genuinely unreliable for muscular individuals — a trained athlete frequently registers as overweight or obese on a scale that cannot distinguish muscle from fat. It is also unreliable for older adults, who lose muscle mass and may carry substantial fat at a normal BMI; for pregnant women, for whom it is not applicable; and for children and adolescents, who need age- and sex-specific percentile charts rather than adult categories.

Reading your result properly

Treat the category as a prompt to look further, not as a diagnosis. Two measurements add far more than a second decimal place on your BMI:

Waist circumference, measured at the navel. This captures visceral fat — the fat around your organs, which drives metabolic risk far more than fat under the skin. Indian guidance generally flags risk above 90 cm for men and 80 cm for women, notably lower than European thresholds.

Waist-to-height ratio. Keeping your waist below half your height is a simple, well-supported rule that works across ages and ethnicities, and it needs no chart at all.

If your BMI sits in the overweight or obese band, the useful next step is a conversation with a doctor and basic bloodwork — fasting glucose, HbA1c, lipid profile, blood pressure. Those measure what BMI is only guessing at. This calculator is an estimate for general information and is not medical advice; it cannot see your body composition, family history or metabolic markers, and it should not be the basis for starting or stopping anything.

Where BMI is misused

Treating a category boundary as a cliff. There is no meaningful health difference between 24.9 and 25.1. The categories are convenient divisions on a continuum.

Applying WHO cutoffs to South Asian bodies without adjustment. As above — a normal WHO reading can sit in the overweight Asian-Indian range, and the difference matters precisely in the group most at risk of early diabetes.

Using it on athletes or heavily muscled people. A rugby player at BMI 29 and a sedentary person at BMI 29 have almost nothing in common metabolically.

Using adult categories for children. Children need growth-chart percentiles; adult BMI bands do not apply and can cause real harm when misapplied to a growing body.

Ignoring where the weight sits. Two people at BMI 27, one carrying weight around the abdomen and one around the hips, have measurably different cardiovascular risk. BMI cannot tell them apart; a tape measure can.

Weighing yourself at inconsistent times. Body weight swings a kilogram or two across a day with food, fluid and salt. For tracking, weigh at the same time under the same conditions, and read the trend across weeks rather than the daily figure.

Treating it as a measure of worth rather than a screening statistic. BMI describes one ratio between two easily measured quantities. It says nothing about fitness, diet quality, strength or health behaviours, all of which independently affect outcomes. ## BMI for children, and why the adult scale does not apply

Applying adult BMI categories to anyone under 18 is not merely imprecise — it is the wrong instrument. Children's body composition changes continuously with growth, and a BMI that is entirely healthy at seven would be concerning at seventeen.

Paediatric assessment uses BMI-for-age percentile charts instead, which compare a child's BMI against reference data for children of the same age and sex. The categories are defined by percentile rather than by absolute value: broadly, below the 5th percentile is underweight, the 5th to 85th is healthy, the 85th to 95th is overweight, and above the 95th is obese. India has its own IAP growth reference charts, which differ from the international WHO and CDC references in ways that matter for Indian children.

The practical instruction is simple: if the person you are measuring is under 18, calculate the BMI here if you wish, but interpret it only against a paediatric chart, ideally with a paediatrician. A single reading matters far less than the trajectory across visits — a child tracking steadily along the 60th percentile is developing normally, while one crossing several percentile lines upward or downward in a year warrants attention regardless of which band they currently sit in.