Body Mass Index
Calculate your body mass index (BMI).
Description
Calculate body mass index from height and weight, understand the result, and learn where BMI is useful or potentially misleading.
Body mass index (BMI) is a ratio of weight to height that is widely used as a quick screening measurement for adults. This calculator accepts metric or US customary measurements and returns the same unit-independent BMI value. The number is useful as a starting point, but it cannot describe body composition, diagnose a health condition, or determine an individual's overall health by itself.
The standard adult categories shown below are intended for adults age 20 and older. Children and teenagers require age- and sex-specific growth-chart interpretation rather than the fixed adult ranges.
When BMI is useful
BMI is popular because height and weight are inexpensive, quick, and comparatively easy to measure in a consistent way. It gives researchers and public-health organizations a common measurement for describing weight patterns across large populations. In an individual assessment, it can serve as an initial signal that invites a closer look at health history, physical findings, laboratory results, behaviors, and other body measurements.1
Common uses
- Obtaining a quick adult weight-category screening result
- Tracking the same height-and-weight ratio over time
- Comparing broad patterns across groups or populations
- Combining BMI with waist measurements and clinical information
A BMI result is most informative when its purpose is clear. A population statistic, an eligibility threshold, and a personal health discussion use the same calculation but do not necessarily support the same conclusion.
How BMI is calculated
The metric calculation divides weight in kilograms by height in meters squared. Squaring height adjusts weight for differences in stature, although the resulting index is not a direct percentage or physical measurement of body fat.
BMI = weight (kg) / height² (m²) For a person weighing 70 kg and measuring 1.8 m, the live calculation returns a BMI of Loading tool….
For US customary measurements, multiply weight in pounds by 703 and divide by height in inches squared. Enter a height such as 5 feet 10 inches as 5'10" when the height control is set to feet and inches. Metric and US customary inputs produce the same BMI apart from ordinary rounding.
BMI = 703 × weight (lb) / height² (in²) Use total inches for height. For example, 5 feet 10 inches is 70 inches.
Measurement quality matters
Small input errors can affect the result because height is squared. Use a recent weight, measure height without shoes when practical, and enter the units shown beside each input. A rounded BMI is appropriate for ordinary interpretation; extra decimal places do not make the underlying measurement more precise.
Interpreting an adult BMI result
For adults age 20 and older, US public-health guidance commonly uses the following categories. The boundaries describe screening categories, not sharp biological changes: a person immediately above and a person immediately below a cutoff are unlikely to be meaningfully different solely because their labels differ.2
- Below 18.5
- Underweight
- 18.5 to less than 25
- Healthy weight
- 25 to less than 30
- Overweight
- 30 to less than 35
- Class 1 obesity
- 35 to less than 40
- Class 2 obesity
- 40 or greater
- Class 3 obesity
A result outside the healthy-weight category does not establish why the value is high or low, whether body fat is excessive, or whether treatment is appropriate. Likewise, a result inside that category does not rule out health risks. The useful next question is what additional information is needed for the reason the measurement was taken.
What to do with the result
First, check that height, weight, and units were entered correctly. If you are tracking change, use measurements taken under reasonably similar conditions and pay more attention to the pattern over time than to a small day-to-day difference. A result near a category boundary should not be treated as a sudden change in health.
Use the result as one question to explore, not as a diagnosis or a target chosen in isolation. Waist size, body composition, medical history, medications, pregnancy, physical examination, laboratory findings, fitness, nutrition, sleep, and personal goals may all change how useful BMI is. If the result concerns you or will affect a health decision, discuss it with a qualified health care professional.2, 1
Important limitations
BMI measures weight relative to height. It does not directly measure fat mass, lean mass, bone mass, fitness, metabolic health, or where fat is stored. Two people with the same BMI can therefore have substantially different body composition and health profiles.3, 1
- A muscular person may have a high BMI without having a high amount of body fat.
- An older adult may lose muscle while retaining or gaining fat without a proportionate rise in BMI.
- BMI does not show whether fat is concentrated around the abdomen.
- The relationship between BMI, body composition, and health risk can vary among individuals and populations.
- Adult category thresholds should not be applied to children and teenagers.
BMI can be useful without being complete. Problems arise when a screening number is treated as a diagnosis, a direct body-fat measurement, or a judgment about a person. Health decisions should be based on the broader clinical context rather than this calculator alone.
Alternative and complementary measurements
There is no universal replacement that answers every question BMI is used for. A better choice depends on whether the goal is to estimate abdominal fat distribution, study body composition, monitor population trends, or assess an individual's health risks.
Waist-based measurements
Waist circumference adds information about abdominal size that BMI cannot provide. Waist-to-height ratio and waist-to-hip ratio express related measurements as proportions. These methods remain indirect and are sensitive to measurement technique, but they can complement BMI when body-fat distribution matters.3
Body-composition methods
Skinfold measurements, bioelectrical impedance, air-displacement methods, and dual-energy X-ray absorptiometry attempt to describe body composition more directly. Their accuracy, cost, availability, and measurement assumptions differ, so a more elaborate method is not automatically the best choice for every user or purpose.
References
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BMI Frequently Asked Questions — Centers for Disease Control and Prevention
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Adult BMI Categories — Centers for Disease Control and Prevention
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Am I at a Healthy Weight? — National Institute of Diabetes and Digestive and Kidney Diseases