BMI Calculator — kg/cm, Feet and Inches, or Pounds

Enter your weight and height to calculate your BMI, see your WHO weight category, healthy weight range for your height, and where you stand compared to the global adult population — all in seconds.

Created by Ashar PervaizMedically reviewed by Dr. Syeda Khadija AkbarLast reviewed 14 October 2026

Working It Out in Whatever Units You Have

Most people know their height in one system and their weight in another — centimetres and kilograms, or feet and inches with a weight in kilos, or pounds and inches. The formula only accepts one combination at a time, so the arithmetic starts with a conversion more often than not.

Someone who is 5 feet 7 inches and weighs 70 kg converts as 67 inches × 2.54 = 170.2 cm = 1.702 m. Squaring gives 2.897, and 70 ÷ 2.897 = 24.2.

The 703 in the imperial version is not a mysterious constant. It is simply the conversion factor that turns pounds per square inch into kilograms per square metre, folded into one number so the formula can be applied without leaving imperial units. Using the imperial formula with a metric height, or the metric formula with pounds, produces answers so far out that they are usually obvious — a BMI of 3 or of 300 means the units were mixed, not that anything is wrong with you.

Squaring the height is what makes small height errors matter. A two-centimetre difference in a recorded height shifts BMI by roughly half a point, which is enough to move somebody across a category boundary if they were already sitting on one. Measure without shoes rather than using a figure from memory.

Does Age or Sex Change the Calculation?

For adults, no. There is one adult BMI formula and it takes height and weight only. A woman and a man of identical height and weight have identical BMI, and so do a thirty-year-old and a seventy-year-old. Calculators advertised as being for women, for men, or for a particular age are running the same arithmetic as every other one.

What genuinely changes is what the number is worth once you have it.

Body Composition Differs by Sex

Women carry more essential fat than men as a matter of physiology, so at the same BMI a woman typically has a higher body fat percentage than a man. The category label does not adjust for this. A BMI of 23 describes a different body composition in the two cases even though the number is identical, which is why a body fat estimate carries more information than BMI for anyone comparing themselves against a partner or a sibling.

Age Changes the Body Behind the Number

Muscle mass tends to decline from middle age onward while fat mass rises, often with no movement on the scale at all. A man of 55 at the same BMI he held at 25 is very likely carrying more fat and less muscle. The figure has not moved; the body underneath it has. This is why a stable BMI across decades should not be read as a stable body composition, and why waist measurement becomes a more useful companion metric with age.

Children Are a Different Calculation Entirely

For anyone under 18, a raw BMI number is not interpretable against adult categories. Children are assessed on BMI-for-age percentiles, which compare a child against reference data for their exact age and sex, because normal body composition changes substantially through growth. A BMI of 17 can be entirely healthy at one age and a concern at another. Paediatric assessment belongs with a clinician using growth charts, not with an adult calculator.

Where the Categories Come From, and What They Assume

The familiar cut-offs — 18.5, 25, 30 — are population thresholds drawn from studies of health outcomes across large groups. They were never intended as individual diagnoses, and they carry an assumption that is easy to miss: that the relationship between BMI and body fat is consistent across populations.

It is not. Health authorities in several Asian countries apply lower thresholds, because metabolic risk in those populations tends to appear at a lower BMI than the standard cut-offs suggest. Someone classified as normal weight by the general thresholds may already be in an elevated-risk band under the adjusted ones. If your own health service publishes population-specific cut-offs, those are the ones that apply to you.

Category boundaries are also hard lines drawn across a continuous scale. A BMI of 24.9 and one of 25.1 sit in different named categories and describe practically identical bodies. Treating the boundary as a cliff rather than a marker on a gradient reads more into the number than it can support.

The Four Situations Where BMI Misleads

• Substantial muscle mass. Muscle is denser than fat, so a well-trained person can register as overweight or obese while carrying very little fat. This is the best-known failure and the reason BMI is a poor screening tool in athletic populations.
• Low muscle with normal weight. The mirror image attracts far less attention and is more common: someone whose BMI sits comfortably in the normal band while carrying a high proportion of fat and very little muscle. The category reads as reassuring and the composition is not.
• Older adults. Height loss with age inflates BMI without any change in body mass, since the denominator shrinks. Muscle loss compounds the effect in the opposite direction on composition.
• Pregnancy. BMI is not applicable during pregnancy. Weight gain is expected and monitored against pregnancy-specific guidance rather than against the standard categories.

What to Measure Alongside It

BMI earns its place by being fast, free, and requiring nothing but a scale and a tape. It stops being useful the moment it is asked to stand alone.

Waist circumference is the most valuable companion and takes about thirty seconds. Fat stored around the abdomen carries more metabolic risk than fat elsewhere, and BMI cannot see the difference. Waist divided by height, kept under about half, is a widely used and easily remembered check that works across heights without needing a table.

For an estimate of composition rather than mass, the body fat calculator uses neck, waist and hip measurements to distinguish fat from lean mass — the exact distinction BMI cannot make. If the goal is changing the number rather than interpreting it, the calorie calculator estimates daily energy needs as a starting point.

Using It Sensibly

• Treat it as a screening figure, not a diagnosis. It flags that a conversation might be worth having, and nothing more.
• Measure height properly rather than recalling it, since the denominator is squared.
• Read your result as a position on a gradient, not as membership of a category.
• Pair it with a waist measurement before drawing any conclusion.
• If you train seriously, or are over about sixty, or are pregnant, expect BMI to describe you poorly and use something else.

Educational reference only. Confirm any clinical calculation independently before acting on it. How this page is reviewed