BMI Calculator

How it works

How do I calculate BMI in kg and feet?

Choose kg + feet & inches, enter your weight in kilograms, and enter feet and inches in separate fields. For example, 70 kg at 5 ft 9 in gives a BMI of 22.8. Enter 5 feet 9 inches as 5 and 9, rather than 5.9 feet.

height(m) = (feet × 12 + inches) × 0.0254

BMI = weight(kg) / height(m)²

These are worked examples, not recommended weights. Adult BMI is a screening measure, not a diagnosis.

How do I calculate BMI in kg and cm?

Choose Metric, enter kilograms and centimetres, and read the live result. Divide height in centimetres by 100 before squaring it: 70 kg and 175 cm gives 70 ÷ 1.75² = 22.86. You can also calculate directly with centimetres:

BMI = weight(kg) × 10,000 / height(cm)²

What does BMI measure, and what does it not measure?

A BMI calculator using feet, inches and pounds gives the same answer as a metric one — Body Mass Index is a single ratio, and the units are just how you feed it in. The formula is your weight in kilograms divided by the square of your height in metres:

BMI = weight(kg) / height(m)²

For US units the tool converts first: pounds are multiplied by 0.45359237 to get kilograms, and feet and inches are converted at 2.54 cm per inch. These are the exact international definitions, so switching units never changes your result.

If you know your weight in kilograms but your height in feet and inches — the usual mix in much of the world outside the US — choose the kg + feet & inches option: only the height is converted, so 70 kg at 5 ft 9 in (175.26 cm) gives a BMI of 22.8.

What are the standard adult BMI categories?

BMI places adults into four broad bands, defined by the World Health Organization:

These bands are the same regardless of age or sex for adults, which is both what makes BMI convenient and what limits it.

How do I calculate BMI by hand?

Square your height in metres, then divide your weight in kilograms by that number. For the 70kg, 175cm example this calculator uses by default: 1.75 squared is 3.0625, and 70 ÷ 3.0625 is 22.86 — matching the calculator exactly, since that is the same arithmetic it runs. In pounds and inches, the common shortcut is weight(lb) ÷ height(in)² × 703; run 154 lb at 5 ft 9 in through it and you land within a rounding hair of the same result, since 703 is itself a rounded stand-in for the exact unit-conversion factor.

What is a healthy weight range for my height?

Because the categories are defined on BMI rather than weight, the useful question is usually the reverse one: what weight range does the normal band correspond to at my height? This is what the third result card computes for you.

Notice how wide each range is — often 30 to 40 pounds. BMI is a screening band, not a target weight.

Is BMI accurate for every ethnicity?

Not equally. The WHO’s standard cutoffs (18.5, 25, 30) were developed primarily from data on European populations, and multiple studies since have found some Asian populations carry meaningfully higher body-fat percentage and cardiometabolic risk at a given BMI than European populations do at the same number. The WHO’s own 2004 expert consultation recognised this and published additional, lower "public health action points" for Asian populations — 23 and 27.5, rather than 25 and 30 — for use alongside, not instead of, the standard bands. This calculator uses only the standard WHO bands; if you know your background falls into an adjusted-risk group, treat a "Normal" reading with that context rather than as a guarantee.

What are the obesity sub-classes?

Clinicians divide the "Obese" band itself into three: Class I (30 to 34.9), Class II (35 to 39.9), and Class III (40 and above) — the last of these was historically labelled "morbidly obese," a term current CDC guidance has moved away from in favour of Class III or "severe obesity," since it stigmatises without adding information the number itself does not already give. This calculator reports the single WHO "Obese" band; a clinician may reference the sub-class specifically, since it can affect treatment guidance.

Is there a better alternative to BMI?

Waist-to-height ratio is the one researchers point to most often: divide your waist circumference by your height, both in the same units, and a result below 0.5 is broadly considered healthy for most adults. It correlates more closely with visceral fat — the metabolically active fat around organs, most linked to cardiovascular risk — than BMI does, because it captures where weight sits rather than only how much of it there is. This calculator does not compute it; a tape measure around your waist gives you the one input BMI cannot see. Worth checking if your BMI and your own sense of your body composition disagree.

Why do doctors still use BMI if it has limits?

Because it is free, fast, and needs no equipment beyond a scale and a tape measure — the same properties that make it useful here. Decades of population studies correlate BMI with health outcomes across large groups, which is exactly the kind of question BMI is good at answering. It is a screening tool for populations and a rough first read for one person, not a diagnostic instrument on its own — the same distinction this calculator's own disclaimer makes.

Is the healthy BMI range different for men and women?

No — the WHO and CDC apply the exact same four bands to adult men and women alike; sex is not a factor in the standard formula or its categories, unlike some alternative measures. Waist circumference is one example: the commonly cited healthy thresholds there do differ by sex (roughly under 40 in / 102 cm for men, under 35 in / 88 cm for women), which BMI has no equivalent for. Men and women also differ, on average, in typical body-fat percentage at the same BMI — women typically carry more essential fat than men at an equivalent reading — one more reason BMI alone rarely settles the whole picture for either sex.

Where does BMI break down?

BMI is a ratio of weight to height. It has no way to know what that weight is made of, which produces predictable failures:

• Muscle reads as excess weight. Dense muscle weighs more than fat by volume, so athletes and heavily trained people often score overweight with low body fat.
• It ignores fat distribution. Two people with the same BMI can carry weight very differently, and abdominal fat carries different health associations than fat elsewhere.
• It is not validated for children or teenagers. Growing bodies are assessed on age-and-sex percentile charts instead.
• It is not appropriate during pregnancy.
• It does not adjust for age or ethnicity, though body composition and risk thresholds vary with both (see above).

How should I use BMI despite its limits?

BMI is genuinely useful as a fast, free, single-number screen — that is the job it was designed for. Two habits make it more informative:

• Watch the direction, not the reading. A BMI moving steadily over months tells you more than a single number does.
• Pair it with something else. Waist measurement, how your clothes fit, resting heart rate, or how you feel on stairs all add information BMI cannot.

If your reading sits near a category boundary, or contradicts everything else you know about your health, treat it as a prompt to ask a clinician rather than a conclusion.

BMI Prime and Ponderal Index

Two related numbers appear alongside your BMI, both computed from the same height and weight rather than any new information:

BMI Prime is simply your BMI divided by 25, the top of the normal band. A BMI Prime of 1.0 sits exactly at that boundary; 0.8 is 80% of the way to it; 1.2 is 20% over. It is the same information as your BMI category, just expressed as a single ratio that some people find easier to track over time than watching a number move between arbitrary band edges.

Ponderal Index divides weight in kilograms by height in metres *cubed*, rather than squared:

PI = weight(kg) / height(m)³

Changing the exponent changes how height affects the ratio. This tool does not provide clinical categories for Ponderal Index, and the number alone cannot establish body composition.

Neither figure is a second, independent measurement — they are the same weight and height, run through two different formulas that trade off differently at the extremes.

This calculator runs entirely in your browser. Your height and weight are never transmitted or stored.

Sources and interpretation

CDC adult BMI categories explains the adult screening bands and obesity classes. WHO expert consultation on Asian populations discusses additional public-health action points; this tool does not apply population-specific thresholds. NICE guidance on central adiposity recommends interpreting waist-to-height ratio alongside BMI for adults with BMI below 35. These sources do not endorse this calculator. This article is written by a software engineer and has not been clinically reviewed.

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