Body Mass Index is one of the most searched health metrics online, and for straightforward reasons: it takes two numbers anyone knows, height and weight, and produces a single figure that roughly indicates whether a person's weight falls within a healthy range for their height. It is not a perfect measure and never claimed to be, but it is fast, free to calculate, requires no equipment, and has enough real-world utility that it remains a standard screening tool in clinical settings worldwide despite decades of debate about its limitations.
How BMI is calculated
The formula is straightforward. In metric units, BMI equals weight in kilograms divided by height in meters squared. Someone who weighs 70 kg and stands 1.75 meters tall has a BMI of 70 divided by 1.75 squared, which is 70 divided by 3.0625, giving a BMI of approximately 22.9.
In imperial units, the same calculation uses a conversion factor. BMI equals weight in pounds multiplied by 703, then divided by height in inches squared. Someone who weighs 154 pounds and stands 5 feet 9 inches tall, which is 69 inches, has a BMI of 154 multiplied by 703, divided by 69 squared, giving approximately 22.7, consistent with the metric result for the same person.
This calculator handles both sets of units automatically. Enter your weight and height in whichever system you are more comfortable with and the result is calculated instantly, along with your BMI category and the healthy weight range for your height.
What the four BMI categories mean
The World Health Organization defines four standard adult BMI categories that are used consistently in clinical and public health contexts worldwide.
Underweight is a BMI below 18.5. Being underweight can reflect insufficient caloric intake, malnutrition, or certain medical conditions, and it carries its own set of health risks including weakened immune function, bone density loss, and in severe cases, organ complications. Someone in this category who is not intentionally at a low weight, such as an athlete or someone with a naturally lean build, should speak with a healthcare provider.
Normal weight covers BMI from 18.5 to 24.9. This range is associated with the lowest statistical risk of weight-related health conditions in large population studies. Being in this range does not mean a person is necessarily healthy in all respects, since factors like cardiovascular fitness, diet quality, and body fat distribution matter independently of BMI, but it does indicate that weight alone is not a primary risk factor.
Overweight covers BMI from 25 to 29.9. At this range, risk of certain conditions including type 2 diabetes, high blood pressure, and cardiovascular disease begins to increase compared to the normal weight range, though the increase is modest and strongly influenced by other lifestyle factors. Many people in this BMI range are metabolically healthy, particularly if they have high muscle mass or are physically active.
Obese is BMI of 30 and above. At this range, population-level associations with chronic disease risk become substantially stronger. The obese category is sometimes further subdivided into Class I, II, and III obesity at BMI thresholds of 30, 35, and 40 respectively, with higher classes associated with progressively greater health risk.
BMI for men versus women
A common question when calculating BMI is whether the result differs between men and women. The standard BMI formula and categories are the same regardless of sex, but the interpretation of BMI does differ meaningfully between men and women in practice.
Women naturally carry a higher percentage of body fat than men at the same BMI. A woman with a BMI of 22 and a man with the same BMI will typically have different body fat percentages, with the woman carrying more fat and the man carrying more lean mass. This difference is biological rather than problematic, since women require higher baseline body fat for reproductive and hormonal function, and the BMI categories account for this at the population level by applying to both sexes.
The more practically relevant difference is that muscle mass is a larger confounding factor for men than for women when using BMI as a proxy for body fat. Men with significant muscle mass through strength training frequently have BMIs in the overweight or even obese range while having low body fat percentages and excellent cardiovascular health. For athletic men especially, BMI is a particularly poor indicator of actual health status.
BMI and age
The standard adult BMI categories apply to adults aged 20 and older. For children and teenagers, BMI is calculated the same way but interpreted differently, using age and sex-specific percentile charts rather than fixed category thresholds. This is because a healthy BMI for a 10-year-old is very different from a healthy BMI for a 35-year-old, even of the same sex.
For older adults, some evidence suggests that the normal weight range is slightly less predictive of health outcomes than it is for middle-aged adults. A modest amount of extra weight may have a protective effect in older adults due to increased reserve in the face of illness, a finding sometimes called the obesity paradox in gerontological research. This does not change the standard categories, but it is a reason why clinical interpretation of BMI becomes more nuanced with age.
Limitations of BMI as a health measure
BMI's limitations are real and worth understanding clearly. The measure was originally developed by a statistician in the nineteenth century as a population-level tool to describe the average body weight of a population, not as a diagnostic tool for individual health assessment. Its widespread clinical use developed later and has always been accompanied by the caveat that it is a screening tool, not a diagnosis.
The most significant limitation is that BMI cannot distinguish between fat mass and lean mass. Muscle is denser than fat, so a highly muscular person can have a BMI in the overweight or obese range while actually having a low body fat percentage and excellent metabolic health. This is not a theoretical edge case: competitive athletes and anyone who does significant strength training frequently falls into this category. A bodybuilder with 8 percent body fat and a sedentary person with 35 percent body fat can theoretically have the same BMI.
BMI also does not account for fat distribution. Visceral fat, which accumulates around the abdominal organs, is substantially more metabolically harmful than subcutaneous fat, which sits under the skin. Two people with the same BMI can have very different distributions of fat and consequently very different health risk profiles. Waist circumference and waist-to-hip ratio are better proxies for visceral fat accumulation but are not captured by BMI.
There are also documented differences in how BMI correlates with health risk across different ethnic populations. Research has consistently found that South Asian, East Asian, and some other populations tend to have higher proportions of body fat and higher metabolic risk at lower BMI values than the standard categories suggest. Several health organizations have proposed adjusted BMI thresholds for specific ethnic groups, though standardized international guidelines continue to use the original thresholds.
What to do with your BMI result
A BMI result is most useful as a starting point for a broader conversation about health rather than as an endpoint in itself. If your BMI falls within the normal range, that is a useful data point but not a reason to stop monitoring other health indicators. If your BMI falls outside the normal range, that is a reason to pay attention, but it is not by itself a diagnosis of any condition.
The most actionable thing to do with a BMI result that falls outside the healthy range is to discuss it with a healthcare provider alongside other information: blood pressure, blood glucose, cholesterol levels, physical activity habits, diet, and family history. A provider who has this fuller picture is in a much better position to assess actual health risk and recommend appropriate next steps than a single BMI number can convey on its own.
For tracking progress over time, BMI can be a useful consistent metric precisely because it requires nothing beyond weight and height, both of which are easy to measure. Even if the absolute BMI value is an imperfect measure of health, changes in BMI over time in the same person track changes in body weight relative to height, which is a useful and reliable signal when the goal is weight management. The percentage calculator is useful alongside this if you want to track what percentage of weight you have lost or gained relative to a starting point.
Healthy weight range
This calculator shows the healthy weight range for your specific height alongside your BMI result. This range is the weight span that corresponds to a BMI between 18.5 and 24.9 for your height, and it gives a concrete target range rather than just a category label. For someone 175 cm tall, the healthy weight range is approximately 56.7 to 76.3 kg, or 125 to 168 pounds for someone 5 feet 9 inches tall. These figures are derived from the standard BMI formula applied to the healthy range boundaries at that height.