BMI Calculator

About the BMI Calculator

Body mass index is a single number that relates your weight to your height, calculated as weight in kilograms divided by height in meters squared. It was never meant to measure fat directly. Its appeal is that it needs only a scale and a tape measure, which is why doctors, insurers, researchers, and public health agencies use it as a first-pass screen across large groups of people. This calculator does the arithmetic for you in either U.S. units (feet, inches, pounds) or metric (centimeters, kilograms) and places the result in a standard adult weight category.

The categories it uses are the widely published adult cutoffs: below 18.5 is underweight, 18.5 up to 25 is a healthy weight range, 25 up to 30 is overweight, and 30 or higher is obesity. Those lines come from population studies linking higher and lower BMI values with differences in health risk. They are statistical, so they describe groups better than they describe any one person. Two people with identical BMIs can have very different amounts of muscle, bone, and body fat, and different risks.

That is why BMI is most useful as a starting point. A result outside the healthy range is a reason to look at other information, such as waist circumference, blood pressure, blood sugar, cholesterol, fitness, and family history, ideally with a clinician. A result inside the range is not a guarantee of good health either. Used that way, as one data point among several, the number is a reasonable way to track change over months and years.

When to Use It

How to Use It, Step by Step

1. Choose your units with the toggle: U.S. for feet, inches, and pounds, or Metric for centimeters and kilograms. The fields change to match, so pick the system you know your measurements in.
2. In U.S. mode, enter Height (feet) and Height (inches) as two separate numbers. A person who is 5 feet 9 inches types 5 in the first box and 9 in the second. Inches range from 0 to 11, and anything larger belongs in the feet box.
3. In Metric mode, enter your Height in centimeters in a single box. If you only know your height in meters, multiply by 100 first, so 1.75 meters becomes 175.
4. Enter your Weight in pounds or kilograms, depending on the mode. Use a recent reading from the same scale, ideally in the morning after using the bathroom and before eating, to keep the figure consistent from one check to the next.
5. Select Calculate BMI and read the number, which is shown to one decimal place, along with the category name below it.
6. Compare the result with the cutoffs of 18.5, 25, and 30. If you are close to a line, remember that a pound or two or a different scale can move you across it, so look at the pattern, not one reading.
7. Try the tool with a different weight to see which weights keep you in the healthy range for your height. This is a quick way to estimate a range rather than a single number.
8. Save the result along with the date. If you are tracking change, repeat the measurement under similar conditions every few weeks instead of every day.

Understanding Your Results

The BMI figure is shown to one decimal place, and the category follows the standard adult bands. A value of 22 to 23 sits in the middle of the healthy band, while a value of 24.9 sits at its top edge and 18.5 at its bottom. Because the numbers are continuous, someone at 25.2 is hardly different from someone at 24.8. Treat the bands as signposts for a conversation, not as sharp boundaries between healthy and unhealthy.

To sanity-check the result, multiply your height in meters by itself, then multiply that by the BMI. You should get your weight in kilograms. For example, a BMI of 24 at 1.75 meters corresponds to about 73.5 kg. Typical adult values range from the high teens to the thirties, with extremely low or high readings usually pointing to a data entry error, such as a missing digit in the height.

What you do next depends on context. If the result is in the healthy range and you feel well, there is nothing to change. If it is above or below the range, consider more informative measures, such as waist circumference and blood tests, and talk with a clinician before making changes. Rapid or severe weight changes are worth discussing with a professional, whichever direction they go.

More Worked Examples

Tips for Better Results

• Weigh yourself at a consistent time of day, on the same scale, in similar clothing. Daily swings of a few pounds from fluids and food are normal and can move the BMI by a few tenths.
• Measure your height in bare feet against a wall rather than relying on a number from years ago. People commonly lose a small amount of height with age, and an outdated figure shifts the result.
• Pair BMI with your waist circumference. A larger waist relative to height points to more abdominal fat, which is the type most associated with metabolic risk, and this adds context that BMI alone does not provide.
• Look at the direction of change over several months instead of fixating on a single reading. A slow, steady move is easier to interpret than a daily number.
• If you are very muscular, very short, or very tall, give BMI less weight. The formula tends to overstate fat for people with a lot of muscle and may mislead at the extremes of height.
• Use the tool to find the weights that bracket your healthy range, then talk with a professional about where in that range is right for you based on your build, age, and health history.

Common Mistakes to Avoid

• Entering feet and inches in the wrong boxes. Typing 69 in the feet field or 5 in the inches field produces nonsense, so put 5 in feet and 9 in inches for 5 feet 9 inches.
• Mixing units, for example using the Metric toggle while typing pounds. Check the unit label next to each field, because the tool will happily calculate with whatever number you provide.
• Treating BMI as a diagnosis. It is a screening measure, and a result on either side of a cutoff does not by itself tell you whether you are healthy or ill.
• Applying the adult bands to children and teenagers. For people under 20, BMI is interpreted using age- and sex-specific percentile charts, so use a pediatric growth chart or ask a pediatrician.
• Comparing yourself with someone else's number. Two people with the same BMI can have very different body composition, fitness levels, and health risks, so compare your own results over time instead.

Assumptions and Limits

• The calculator uses the standard adult cutoffs of 18.5, 25, and 30 for everyone, with no adjustment for age, sex, ethnicity, athletic training, or pregnancy. Some health organizations recommend lower action points for people of Asian descent.
• The category is based on the exact calculated value, while the number shown is rounded to one decimal. A reading displayed as 25 can still be listed as Healthy weight if the exact value is just under 25.0.
• BMI does not distinguish muscle from fat, does not consider where fat is stored, and tends to be less informative for athletes, older adults who have lost muscle, and people who are very tall or very short.
• It is not valid for children, and it should not be used during pregnancy. For personal health decisions, rely on a physician, who can combine BMI with exams and lab tests.

Key Terms

Body mass index (BMI)

A ratio of weight in kilograms to the square of height in meters, used as a rough screening measure of whether weight is appropriate for height.

Healthy weight range

The band of BMI values from 18.5 to just under 25, which on a population level is associated with the lowest average health risk for adults.

Waist circumference

The distance around your waist, measured at a consistent point. It reflects abdominal fat and is often used alongside BMI to assess health risk.

Body composition

The proportion of your body weight that is fat, muscle, bone, and water. It explains why two people with the same BMI can look and perform very differently.

Screening tool

A quick, inexpensive measure that flags people who may need further testing. It indicates possible concern but does not confirm a diagnosis.

Quetelet index

The original name of BMI, after the nineteenth-century Belgian scientist Adolphe Quetelet, who studied how weight varies with height across populations.