BMI Calculator

Body Mass Index (BMI) is calculated as weight in kilograms divided by height in meters squared (kg/m²). In imperial units the formula is (lbs ÷ inches²) × 703. The World Health Organization defines four adult categories:

• Underweight — BMI below 18.5
• Normal weight — BMI 18.5 to 24.9
• Overweight — BMI 25 to 29.9
• Obese — BMI 30 and above

Worked examples:

• 5 ft 9 in (69 in), 160 lbs → BMI = (160 ÷ 69²) × 703 = (160 ÷ 4761) × 703 ≈ 23.6 — Normal weight.
• 175 cm, 90 kg → BMI = 90 ÷ 1.75² = 90 ÷ 3.0625 ≈ 29.4 — Overweight.

The number is coarser than it looks

BMI is reported to one decimal place, which makes it feel precise. It isn't. At 5 ft 10 in, one whole BMI point is about 7 lb — so the difference between 24.4 and 25.1 is roughly five pounds, which is inside the range a single day's food, salt and hydration can move you. Weighing yourself on a different morning can change your category.

The healthy band is also far wider than most people picture. At 5 ft 10 in a "normal" BMI of 18.5–24.9 runs from 128.6 lb to 173.9 lb — a spread of over 45 lb. At 5 ft 4 in it is 107.5–145.3 lb, and at 6 ft 2 in it is 143.8–194.3 lb. Two people at the same height, 40 lb apart, can both be squarely "normal". If you are trying to place yourself within that range rather than inside or outside it, BMI is not the instrument.

And the boundaries are conventions rather than cliffs. 18.5, 25 and 30 are round numbers adopted for consistency; the underlying health risk curve is continuous and very shallow near them. Crossing from 24.9 to 25.0 does not change anything about your body — it changes which row of a table you are printed in.

Why tall people read high and short people read low

This is the criticism of BMI that gets least airtime and matters most for reading your own number. The formula divides weight by height squared, but bodies are three-dimensional: scale a person up while keeping their shape identical and their weight grows with height cubed. Dividing by the square therefore leaves a height bias in the result.

The size of it is easy to show. Take a 5 ft 10 in person at 160 lb — BMI 23.0 — and scale that exact build to other heights:

• 5 ft 2 in, 111.2 lb → BMI 20.3
• 5 ft 6 in, 134.1 lb → BMI 21.6
• 5 ft 10 in, 160.0 lb → BMI 23.0
• 6 ft 2 in, 189.0 lb → BMI 24.3
• 6 ft 6 in, 221.4 lb → BMI 25.6

Every one of those is the same body shape. The BMI moves by more than five points, and the tallest crosses into "overweight" on height alone. So a tall person should read a borderline BMI with some scepticism, and a short person should not take a comfortable one as reassurance.

It misclassifies in both directions

Everyone knows the first direction: muscle is denser than fat, so muscular athletes read high. A rugby forward or a serious lifter can post an "obese" BMI at a low body-fat percentage, and this is the standard objection.

The second direction is quieter and affects far more people. Someone with little muscle can carry a high body-fat percentage while sitting mid-"normal" — sometimes called normal-weight obesity. It is common in people who have never trained, in older adults losing muscle with age, and in anyone who has lost weight rapidly without resistance work. BMI reports these people as fine, which is the failure mode that actually costs something, because nobody goes looking for a problem the screening tool says they don't have.

The thresholds are not the same for everyone

The 25 and 30 cut-offs were derived largely from European-ancestry populations. The WHO recognises that people of South Asian, Chinese, Korean and Japanese descent develop diabetes and cardiovascular risk at lower BMI values, and identifies lower trigger points for public-health action — around 23 for increased risk and 27.5 for high risk. At 5 ft 10 in those correspond to about 160.0 lb and 191.4 lb, rather than 174.0 lb and 208.8 lb. This calculator shows the standard WHO adult categories; if the lower thresholds apply to you, shift your reading of the result accordingly.

Age matters too. The categories are defined for adults aged 20 and over. For children and teenagers BMI is interpreted against age-and-sex percentile charts instead, because the healthy value changes as they grow, and a raw adult category is meaningless. BMI is also not interpretable during pregnancy.

The measurement worth taking alongside it

BMI's core weakness is that it knows your mass but not where it is, and abdominal fat carries most of the metabolic risk. The cheapest fix is a tape measure. The waist-to-height ratio is the usual rule: keep your waist under half your height. At 5 ft 6 in that is 33 in, at 5 ft 10 in it is 35 in, at 6 ft 2 in it is 37 in. Measure at the navel, standing, at the end of a normal breath out, without pulling the tape tight.

Read alongside BMI, it resolves most of the cases BMI gets wrong: the lifter with a high BMI and a 32-inch waist, and the untrained person with a mid-normal BMI and a 38-inch waist. Where the two disagree, the waist figure is usually the more informative one.

What BMI is actually for

It was devised in the 1830s by Adolphe Quetelet as a way of describing populations, not individuals, and that is still what it is best at: it is cheap, needs no equipment, and tracks group-level risk well enough to be useful across millions of people. Applying a population instrument to one person is where the error comes from — not from the arithmetic, which is exact, but from what the arithmetic leaves out.

Used sensibly it is a prompt, not a verdict: a reason to look further if the number is unexpected, and one input among several — waist measurement, blood pressure, blood glucose and lipids, fitness, family history — that a clinician can weigh together. It cannot diagnose anything on its own, and nothing on this page is medical advice. If your result concerns you, or if it disagrees with how you feel, that is a conversation with a healthcare provider rather than a conclusion to draw from a calculator.