BMI calculator (body mass index)

The BMI formula and WHO weight categories

Body mass index (BMI) was devised by the Belgian statistician Adolphe Quetelet in the 1800s. The World Health Organization and the CDC use it to sort adult weight into categories.

BMI = weight (kg) ÷ height (m)² BMI = 703 × weight (lb) ÷ height (in)²

What BMI does not tell you

BMI is a screening number, not a diagnosis. It ignores where fat sits on your body, how much muscle you carry, your bone structure and your age. A muscular athlete can land in the overweight range with little body fat, and an older adult can have a normal BMI with very little muscle.

• For children and teens 2 to 19, the CDC uses BMI-for-age percentiles instead of the adult cutoffs above.
• Waist size adds useful context: a waist over 40 inches in men or 35 inches in women is linked to higher cardiovascular risk.
• Use the body fat calculator for an estimate of your fat percentage.

Healthy weight range by height

BMI calculator: a worked example

Here is a full BMI calculator example. Take a person who weighs 154 lb and stands 5'9" (69 inches). First, square the height: 69 × 69 gives 4,761. Then divide the weight by that number: 154 ÷ 4,761 gives 0.03235. Finally, multiply by 703, which gives about 22.7. So the BMI falls in the healthy range of 18.5 to 24.9.

To find your healthy range, work backward. At 5'9", a BMI between 18.5 and 24.9 matches roughly 125 to 169 lb. Therefore, two people of the same height can weigh quite differently and both land in the healthy range.

BMI calculator: WHO categories and limits

This table lists the adult categories. However, these cutoffs are statistical guides, not a diagnosis.

In fact, a muscular athlete can have a high BMI with little fat. Conversely, an older adult can have a normal BMI with little muscle. Talk to your doctor if you have doubts.

BMI calculator: go further with other tools

BMI gives a first reference, but other measures add detail. For example, the body fat calculator estimates your fat share. Then the ideal weight calculator compares several formulas. Finally, the calorie calculator helps you adjust your eating.

Also, waist size adds useful information about belly fat. So you combine several indicators instead of trusting one number.

BMI calculator: kids, older adults and athletes

A BMI calculator does not read the same for everyone. For a child, doctors use growth charts by age and sex. So the same number means something different at 8 and at 40. Then, in an older adult, a slightly higher BMI can even protect during illness. Finally, a muscular athlete may top 25 without excess fat.

Therefore, read the result with care, and ask a health professional if you have questions. The BMI calculator is a starting point, not a verdict. Also, weigh yourself in the morning to compare from month to month.

BMI calculator: frequently asked practical questions

Here are practical answers to common questions. First, how often should you check? Once a month is enough, because weight moves slowly. Second, which units should you use? Either works. The tool converts pounds and inches or kilograms and centimeters on its own. Third, what if your result sits just outside the healthy range? A value of 25.2 is not a medical event, so look at your waist, your energy and your habits too.

Also, remember that a doctor may look at blood pressure, blood sugar and family history alongside your numbers. Therefore, treat the result as one clue among several. In short, small steady changes in food and movement matter more than a perfect number. For example, a daily 30 minute walk helps most adults.

Finally, keep your measuring routine simple. Stand straight against a wall for height, step on the same scale at the same hour, and write the numbers in a notebook or phone note. After three months, you will see a clear line, and that line tells you far more than a single day.