BMI Calculator

Body mass index (BMI) is a calculated measure of weight relative to height. For adults, it is commonly used as a screening measure to place a person's height and weight into broad categories.

BMI is calculated from weight and height only. It does not directly measure body fat, distinguish fat from muscle or bone, or show where fat is distributed. For an individual, BMI is one piece of information rather than a complete assessment of health.

This calculator is designed for adults aged 20 and older. Children and teenagers are interpreted using age- and sex-specific BMI-for-age measures rather than the fixed adult categories.

Choose Metric if you want to enter height in centimetres and weight in kilograms. Choose Imperial if you want height in feet and inches and weight in pounds.

Select Standard adult thresholds for the usual adult BMI categories, or Lower NICE thresholds when the lower practical thresholds applicable to specified ethnic-background groups are the interpretation you need.

Enter your height and weight. The calculator converts imperial measurements internally, calculates BMI, rounds it to one decimal place, and shows the corresponding screening category.

The result also shows a mathematical reference-weight interval based on the selected BMI boundaries. That interval is not an ideal-weight prescription or a personalised treatment target.

For metric measurements, BMI equals weight in kilograms divided by height in metres squared. If height is entered in centimetres, convert it to metres before applying the formula.

For US customary measurements, the equivalent shortcut is 703 multiplied by weight in pounds divided by height in inches squared. Both approaches produce the same BMI when the measurements are converted correctly.

The calculator uses the same underlying formula for both unit systems. Changing between Metric and Imperial changes the displayed inputs and converts populated measurements; it does not use a different BMI definition.

For adults aged 20 and older, the standard categories are underweight below 18.5, healthy weight from 18.5 to below 25, overweight from 25 to below 30, and obesity at 30 or higher.

Adult obesity is commonly divided into class 1 from 30 to below 35, class 2 from 35 to below 40, and class 3 at 40 or higher.

These numerical categories are screening thresholds. A category by itself does not diagnose disease or describe an individual's complete health status.

NICE uses lower practical BMI thresholds for adults from South Asian, Chinese, other Asian, Middle Eastern, Black African, and African-Caribbean backgrounds because cardiometabolic risk can occur at lower BMI values in these groups.

With this interpretation, the healthy-weight range is 18.5 to below 23, overweight is 23 to below 27.5, obesity class 1 is 27.5 to below 32.5, obesity class 2 is 32.5 to below 37.5, and obesity class 3 is 37.5 or higher.

The BMI formula itself does not change. The selected interpretation changes the category thresholds and therefore the mathematical reference-weight interval shown by the calculator.

These thresholds should not be treated as a universal substitute for individual clinical assessment. Ethnicity is one factor considered in the NICE approach, alongside other measures and clinical context.

For a given height, a BMI boundary can be converted back into a corresponding weight. The calculator uses the selected lower and upper healthy-BMI boundaries to produce this mathematical interval.

For the standard adult thresholds, the calculation uses BMI 18.5 as the lower boundary and BMI 25 as the upper boundary. Under the lower NICE interpretation, the upper boundary is BMI 23.

Calling this a reference-weight interval is deliberate: the numbers describe weights that correspond to selected BMI boundaries. They are not a statement that every person should reach or maintain a particular weight.

Suppose an adult is 175 cm tall and weighs 72 kg. Height is 1.75 m, so BMI is 72 ÷ (1.75 × 1.75), which is approximately 23.5.

Under standard adult thresholds, 23.5 falls within the healthy-weight category. The mathematical reference interval at 1.75 m is approximately 56.7 kg to less than 76.6 kg.

The example demonstrates the calculation; it should not be interpreted as a personal target or medical recommendation.

BMI uses only height and weight. It cannot distinguish muscle from fat, account directly for bone mass or fluid, or show where body fat is carried.

A person with substantial muscle mass can therefore have a higher BMI without having the same body-fat profile as another person with the same BMI. Conversely, two people with identical BMI values can have different body compositions and health risks.

BMI should be considered alongside other relevant information such as medical history, physical findings, blood pressure, laboratory results, activity, and other measures of health when an individual assessment is needed.

The standard adult BMI category thresholds are fixed numerical thresholds for adults aged 20 and older; the BMI calculation itself does not change based on sex.

That does not mean BMI has identical meaning in every individual. Body composition, fat distribution, age, health history, and other factors can affect how a BMI value should be understood.

For this reason, the calculator presents BMI as a screening measure rather than a diagnosis or a complete measure of health.

Adult BMI thresholds should not be used to interpret BMI in children or teenagers. People aged 2 through 19 are assessed using BMI-for-age percentiles that account for age and sex because children and adolescents are still growing.

This calculator is intentionally limited to adults aged 20 and older. If you need to assess a child or teenager, use an age- and sex-specific pediatric BMI method rather than applying the adult categories shown here.

The adult BMI categories in this calculator are not designed to assess normal pregnancy-related weight change.

Pregnancy involves changes in body weight and composition that require pregnancy-specific clinical context. Do not use an adult BMI result from this calculator as a pregnancy weight-gain target or assessment.

This calculator displays BMI to one decimal place and uses that rounded result for its category classification. A value close to a threshold can therefore move into a neighbouring category after rounding.

Measurement precision matters too. A small difference in recorded height or weight can change the calculated BMI, particularly when a result is close to a category boundary.

A category boundary is a statistical screening threshold, not a sharp biological line where health suddenly changes from one state to another.

BMI can summarize the relationship between height and weight in a simple, reproducible number. It is useful for screening and for tracking weight status over time.

BMI cannot tell you your exact body-fat percentage, where your fat is located, how much of your weight is muscle, or whether you have a particular medical condition.

If a BMI result concerns you or does not seem consistent with your health or body composition, a clinician can interpret it alongside other measurements and your medical history.

The calculation runs locally in the browser. Height, weight, selected units, thresholds, and the resulting BMI are not uploaded, added to the URL, or permanently stored by the tool.

That means the calculator can be used for a personal BMI calculation without sending the entered measurements to a server for processing.