Adult BMI: calculation, categories and limitations
Body mass index relates body weight to height and provides a screening category for adults. This calculator accepts kilograms and centimetres or pounds and feet plus inches. It does not directly measure body fat, fitness, diet quality or overall health. A BMI result is one piece of information to discuss alongside medical history, blood pressure, laboratory results and other measurements. The adult categories shown here are not appropriate for assessing children and teenagers, whose BMI interpretation depends on age and sex.
Calculate BMI in either unit system
Metric BMI = weight in kilograms / height in metres squared. Convert centimetres to metres before squaring. Imperial BMI = weight in pounds / total height in inches squared × 703. Total inches = feet × 12 + additional inches. Never divide kilograms by centimetres squared or mix pounds with metres in the same formula.
For adults age 20 and over, CDC categories are below 18.5 underweight, 18.5 to below 25 healthy weight, 25 to below 30 overweight, and 30 or above obesity. The page displays BMI to one decimal place but classifies the unrounded calculation. Near a boundary, the rounded number can appear to be in the next band; interpretation should use the underlying value.
A screening category is not a diagnosis
BMI cannot distinguish fat from muscle or show where fat is distributed. Someone with substantial muscle can have a high BMI without the same body composition as another person with that score. Equally, a result in the healthy-weight range does not rule out a health condition. Avoid treating the number as a personal judgement or a complete assessment.
Use consistent measurements: a reliable scale, ordinary light clothing and an accurate height rather than an old estimate. Pregnancy and individual medical conditions require different context. For children and adolescents, use an age- and sex-specific percentile assessment through an appropriate healthcare source. If a result concerns you, discuss it with a qualified clinician rather than following an automatic calorie target.
Worked examples
70 kg and 175 cm
Height is 1.75 m. BMI = 70 / 1.75² = 22.857, displayed as 22.9. This falls within the standard adult healthy-weight screening range; it is not proof of complete health.
180 lb and 5 ft 10 in
Total height is 70 inches. BMI = 180 / 70² × 703 = 25.824, displayed as 25.8. The adult category is overweight; muscle mass, medical history and other measurements still matter.
Frequently asked questions
Can I use these bands for a teenager?
No. Children and teens require BMI-for-age percentiles, not the fixed adult thresholds shown on this page.
Does a healthy-range BMI mean I am healthy?
Not by itself. BMI is a screening measure and cannot assess all health risks or body composition.
Why can an athlete have a high BMI?
BMI counts total weight relative to height and cannot separate muscle from fat. A clinician can interpret the result with additional information.
Which units should I choose?
Use kilograms and centimetres in metric mode, or pounds and feet plus inches in imperial mode. Do not mix the two systems.