The Two Formulas
Imperial BMI = weight (lb) x 703 / height (in)2
The imperial formula includes 703 so pounds and inches produce the same ratio as kilograms and meters. When the same height and weight are converted precisely, the results should be nearly identical.
Why Results Differ By A Decimal
Small differences usually happen when height or weight is rounded before conversion. For example, 175 cm is close to 5 ft 9 in, but not exactly the same as every rounded foot-and-inch entry. The calculator rounds only the final displayed BMI to one decimal place.
Metric Example
70 kg and 175 cm becomes 70 / 1.752, producing a BMI of 22.9 after rounding. This is the cleanest path when your source measurements are already metric.
Imperial Example
154 lb and 5 ft 9 in becomes 154 x 703 / 692, producing a BMI near 22.7. If you convert 70 kg and 175 cm more precisely before rounding, the two paths move closer together.
When Formula Precision Is Not The Main Issue
BMI is still a screening ratio, not a direct body-fat measurement. A precise BMI can still be incomplete for muscular people, older adults, pregnancy, edema, children, or unusual body composition.
When BMI feels mismatched with your body context, compare it with waist-to-height ratio, body-fat estimate, or professional health markers rather than recalculating the same formula repeatedly.
Frequently Asked Questions
Is metric BMI more accurate than imperial BMI?
Not inherently. Metric BMI is simpler because it uses kilograms and meters directly, while imperial BMI uses the 703 conversion factor.
Why does my BMI change when I switch units?
The most common reason is rounded height or weight conversion. Use the original measured units when possible.
Which BMI calculator should I use?
Use the unit path that matches your original measurements: kg/cm for metric or lb/ft-in for imperial.
Sources And Further Reading
- About Body Mass IndexCenters for Disease Control and Prevention
- Adult BMI CategoriesCenters for Disease Control and Prevention