Calculate Adult BMI and Understand Its Limits

Use BMI as a screening measure

Body mass index is weight in kilograms divided by height in metres squared. It describes a height-and-weight relationship; it does not directly measure body fat, fitness or an individual’s health. A category label should not be treated as a diagnosis or a personalised target.

Reproduce the metric calculation

For an adult weighing 65 kg at a height of 170 cm, convert height to 1.70 m. The calculation is 65 / (1.70 × 1.70), approximately 22.49. The interface displays 22.5 after rounding.

1. Select metric and enter height in centimetres and weight in kilograms.
2. Check that height was not entered as 1.70 in the centimetre field.
3. For imperial inputs, enter feet, additional inches and pounds in their separate fields.
4. Read the category as a broad screening category, not an instruction to change weight.

Understand the category boundaries

The tool uses common adult cutoffs: below 18.5, 18.5 to below 25, 25 to below 30, and 30 or above. Its category calculation uses the unrounded BMI, so a number displayed close to a boundary can appear inconsistent if you classify only the rounded headline.

The CDC adult BMI guidance applies adult categories to people aged 20 and older and describes BMI as one part of a broader assessment. Children and teenagers require age- and sex-specific interpretation, which this utility does not provide.

Know when the number is insufficient

BMI does not account directly for muscle mass or where fat is distributed. Pregnancy and individual clinical circumstances also require an appropriate assessment. The interface’s calculated weight range is a mathematical range from fixed BMI cutoffs, not a personalised recommendation or treatment plan.

Troubleshoot units and measurements

Measure height and weight consistently and check each unit selector. Five feet seven inches is entered as 5 feet plus 7 inches, not 5.7 feet. Zero or negative height/weight cannot produce a useful ratio and is rejected. Use Unit Converter to check a unit conversion; discuss health questions and the relevance of a category with an appropriate healthcare professional.