First, compare the exact inputs
Write down the unrounded height and weight used in both tools. A remembered height, a converted weight, or a default value left in one field can make the tools appear inconsistent before the formula is involved.
The standard adult equations are equivalent: kilograms divided by metres squared, or pounds divided by inches squared and multiplied by 703. When equivalent measurements are used with sufficient precision, the results should be very close. [1]
Conversion and rounding create the common small gap
Height is squared, so rounding it before the calculation can have more effect than expected. One tool might calculate from 175 cm while another uses a rounded feet-and-inches conversion. The final display may also round to one decimal, two decimals, or a whole number.
Keep the original measurement, carry full precision through the formula, and round only the final answer. If two outputs are 22.8 and 22.9, conversion precision is a more likely explanation than a different definition of adult BMI.
Mixed units are easy to enter incorrectly
Five feet nine inches is 69 total inches, not 5.9 feet. Eleven stone six pounds is 160 total pounds, not 11.6 stone. A calculator with separate fields reduces that ambiguity; a single free-form field may not.
For stones, multiply the stone value by 14 and add the remaining pounds. For height, multiply feet by 12 and add the remaining inches before applying the US formula. [1]
Check whether both tools are for adults
CDC adult categories apply from age 20. Children and teenagers use age- and sex-specific BMI-for-age percentiles, so a paediatric tool asks for more information and interprets the result differently. [1] [2]
Pregnancy is another important suitability check. The NHS advises against using its standard adult BMI calculator during pregnancy. A tool designed for a different population should not be expected to give the same label. [3]
The formula can match while risk thresholds differ
The BMI number itself does not change with ethnicity. However, NHS guidance says people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean backgrounds may develop health problems at lower BMI values. It uses 23 to 27.4 for overweight and 27.5 or above for obesity in these groups. [4]
CDC’s standard adult categories begin overweight at 25 and obesity at 30. Two tools can therefore calculate the same raw number but attach different risk guidance when they follow different public-health frameworks. That is an interpretation difference, not a different weight-and-height formula. [2] [4]
The number may match while the label differs
One interface may show “healthy weight” while another says “normal” or “standard range.” More importantly, a tool may round 24.96 to 25.0 for display before deciding the category, even though BMI 25 belongs to the overweight category and the unrounded value is still below it. [2]
Calculators can also choose different rounding policies near a boundary. Massy rounds the calculated BMI to one decimal and classifies that displayed value; it keeps metric, US, and stone inputs separate and shows the adult category boundaries beside the result.
Sources
- BMI Frequently Asked QuestionsCenters for Disease Control and Prevention
- Adult BMI CategoriesCenters for Disease Control and Prevention
- Calculate your body mass index (BMI) for adultsNHS
- Overweight and obesityNHS