How BMI is calculated
Body mass index is a simple ratio of weight to height. In metric units you divide your weight in kilograms by your height in meters squared: BMI = kg ÷ m². A person who weighs 70 kg and stands 1.75 m tall has a BMI of 70 ÷ 3.0625 = 22.9. In US units the same idea needs a conversion factor of 703, so BMI = 703 × pounds ÷ inches². Someone who is 5 ft 9 in (69 inches) and 165 lb has a BMI of 703 × 165 ÷ 4,761 = 24.4.
This calculator converts feet and inches to centimeters behind the scenes and always uses the metric formula, so both unit systems give identical answers. It also shows BMI Prime, which is your BMI divided by 25, the upper limit of the healthy range. A BMI Prime of 1.10 means you are 10% above that limit.
BMI categories for adults
The World Health Organization and the U.S. Centers for Disease Control and Prevention use the same adult cut-offs. A BMI under 18.5 is classed as underweight, 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or above as obesity. Obesity is often split further into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 and above). Some Asian health agencies use lower thresholds, such as 23 for overweight, because risk of type 2 diabetes rises at lower BMIs in those populations.
The healthy weight range shown under your result is simply the weights that would put someone of your height between a BMI of 18.5 and 24.9. It is a useful sanity check when setting a goal, but it is a range for populations, not a target that is right for every individual.
What BMI does not tell you
BMI cannot tell muscle from fat. A muscular athlete can be classed as overweight while carrying very little body fat, and an older adult who has lost muscle can have a normal BMI with a high fat percentage. It also says nothing about where fat is stored. Fat around the waist is more strongly linked to heart disease than fat on the hips, which is why doctors often pair BMI with a waist measurement. As a rule of thumb, a waist above 40 inches (102 cm) for men or 35 inches (88 cm) for women signals higher risk.
For a fuller picture, try our body fat calculator, which uses tape-measure readings, or compare your result with the ideal weight formulas. BMI for children and teens is interpreted with age- and sex-specific percentiles, so this adult calculator should not be used for anyone under 18.
Using your result
Treat BMI as a starting point for a conversation rather than a diagnosis. If your result is outside the healthy range, a doctor can look at blood pressure, blood sugar, cholesterol and family history to judge whether your weight is affecting your health. If you want to change your weight, the calorie calculator will estimate a daily intake that moves you toward your goal at a sustainable pace of about 0.5 to 1 kg (1 to 2 lb) per week.
Frequently asked questions
What is a healthy BMI?
For most adults a BMI between 18.5 and 24.9 is considered a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or above is obesity.
Is BMI different for men and women?
The adult BMI formula and categories are the same for men and women. Women tend to have more body fat than men at the same BMI, which is one reason BMI is only a screening tool.
Why does the imperial formula use 703?
703 converts pounds per square inch into kilograms per square meter, so the imperial and metric formulas give the same number.
Can I use this BMI calculator for children?
No. Children and teens are assessed with BMI-for-age percentile charts, because healthy body fat changes as they grow.