BMI Calculator

Body Mass Index (BMI) is a simple screening measure that estimates whether your weight is proportional to your height, calculated as weight in kilograms divided by height in meters squared: BMI = kg ÷ m². In imperial units, the equivalent formula is BMI = 703 × lbs ÷ in², where the 703 constant converts pounds and inches into the same kilograms-per-meters-squared scale used by the metric formula. This tool accepts height and weight in either metric or imperial units and converts internally, so the BMI value it reports is identical regardless of which units you choose to enter.

The World Health Organization's standard adult BMI categories are: below 18.5 is Underweight, 18.5 to 24.9 is Normal (healthy) weight, 25 to 29.9 is Overweight, and 30 and above is Obese (which is further subdivided into Class 1, 2, and 3 by some health organizations, though this tool reports the single broader 'Obese' category). These thresholds were established through large-scale population studies correlating BMI with health outcomes, and they remain the most widely used general-population reference ranges internationally, even though BMI has well-documented individual limitations discussed elsewhere in these FAQs.

BMI is a useful, quick screening tool at a population level, but it has real, well-known limitations at an individual level, since it only measures the ratio of weight to height and has no way to distinguish between weight from muscle, bone, fat, or water. A very muscular person (many athletes and bodybuilders) can have a BMI classified as Overweight or Obese despite having low body fat, since muscle is denser than fat and adds weight without adding proportional fat mass. Conversely, someone with a Normal BMI can still have a relatively high body fat percentage and low muscle mass — a pattern sometimes called 'skinny fat.' BMI also doesn't account for where fat is distributed on the body, even though fat concentrated around the abdomen carries different health risk than fat distributed elsewhere, at the same total body fat percentage. For these reasons, BMI is best used as a general screening indicator or a rough population-level statistic, not a definitive individual diagnosis — a doctor or health professional considering additional measures (waist circumference, body fat percentage, blood markers) gives a much fuller picture than BMI alone.

No — this tool's categories reflect the standard adult BMI reference ranges, which don't apply to children and teenagers, who require age- and sex-specific BMI percentile charts instead, since body composition changes substantially throughout growth and development. BMI interpretation also shifts somewhat for older adults, since some research suggests a slightly higher BMI may be less risky (or even protective) in advanced age compared to younger adults, an effect not captured by the standard adult thresholds. Pregnancy causes substantial, expected weight gain that has nothing to do with body fat accumulation in the way adult BMI categories assume, so BMI is not a meaningful or appropriate measure during pregnancy at all. If any of these apply to you, consult a healthcare provider for appropriate guidance rather than relying on the standard adult BMI categories shown by this tool.

Some research has found that certain populations — for example, some South Asian and East Asian populations — tend to experience elevated health risks (such as type 2 diabetes) at somewhat lower BMI values than the standard thresholds reflect, which is why some national health guidelines (including in parts of Asia) use adjusted, lower BMI cutoffs for 'Overweight' and 'Obese' categories for those populations. This tool uses the single most widely recognized international WHO standard thresholds for simplicity and broad applicability, but if your national or regional health guidelines specify different cutoffs for your background, it's worth being aware of that distinction alongside this tool's general-purpose result.

The healthy weight range shown is the span of weights that would produce a BMI between 18.5 and 24.9 — the standard Normal category — at your specific entered height, calculated by solving the BMI formula for weight instead of BMI: weight = BMI × height (in meters) squared. For a height of 1.70m, for example, the healthy range is 18.5 × 1.7² = 53.5 kg at the low end and 24.9 × 1.7² = 72.0 kg at the high end. This range shifts for every different height, since taller people have proportionally more healthy weight range (in absolute kilograms or pounds) than shorter people, even though both are targeting the exact same BMI range.

Because height is squared in the BMI formula, even a small change in height has an outsized effect on the resulting BMI compared to an equivalent small change in weight — this is a direct mathematical consequence of the squared term, not a quirk of this specific calculator. A measurement error of even a couple of centimeters (easy to make if you're estimating height rather than measuring it precisely) can shift your calculated BMI more than you might expect, which is worth keeping in mind if your result seems slightly off from what you anticipated — double-check your height measurement first before assuming the BMI figure itself is wrong.

The horizontal scale represents the standard BMI range from 15 to 40, divided into four colored segments matching the Underweight, Normal, Overweight, and Obese categories, with a marker showing exactly where your calculated BMI falls within that range. It's meant to give an at-a-glance sense of not just which category you're in, but how close you are to a boundary — someone with a BMI of 24.8 is barely inside the Normal range and very close to Overweight, while someone with a BMI of 21 is comfortably in the middle of it, a distinction the single word 'Normal' alone doesn't convey.

No — every calculation in this tool runs entirely in your browser using plain JavaScript arithmetic. Nothing you enter is transmitted anywhere or stored on any server, so your height and weight — data many people consider personal — never leaves your device at any point in the calculation.

BMI can track broad trends over time reasonably well if your body composition (muscle-to-fat ratio) stays roughly stable, since in that case a change in BMI does correspond fairly closely to a change in body fat. But if you're actively changing your body composition — building muscle through strength training while also losing fat, for instance — BMI can stay flat or even rise slightly even as your actual health and fitness meaningfully improve, since you may be gaining muscle weight at the same time you're losing fat weight. In that scenario, tracking body measurements, a body fat percentage estimate, or simply how your clothes fit will tell you far more about your actual progress than BMI alone.