Two BMI results can differ even when both calculators work correctly. Check the inputs and display precision before interpreting a change as a change in your health. A different category label near a cutoff may be a rounding issue.
First check what you entered
Adult BMI uses weight in kilograms divided by height in metres squared. At 72 kg and 170 cm, the calculation is 72 ÷ (1.70 × 1.70) = 24.91349…, usually displayed as 24.9. Dividing by 170 squared instead of 1.70 squared produces a result 10,000 times too small.
Feet and inches are another common source of disagreement. Five feet ten inches is 70 inches, or 177.8 cm. It is not 5.10 decimal feet: that equals 61.2 inches. Use the separate feet and inches fields in our calculator, or convert the total inches to centimetres by multiplying by 2.54.
The US formula often uses the rounded conversion factor 703. Converting pounds to kilograms and inches to metres first corresponds to about 703.0696 instead. At 180 lb and 70 inches, those approaches produce approximately 25.8245 and 25.8270. Both round to 25.8; that tiny difference is an arithmetic convention, not a meaningful change in body size.
Why a displayed 25.0 can have two labels
The CDC adult categories put values from 18.5 up to, but not including, 25 in the healthy-weight category, and values from 25 up to, but not including, 30 in the overweight category. These apply to adults aged 20 and older and are screening categories, not diagnoses.
Consider an illustrative BMI of 24.96. It displays as 25.0 when rounded to one decimal place, but its unrounded value is below 25. A calculator that classifies before rounding can therefore show “25.0” with a healthy-weight label. One that classifies the displayed number can label the same calculation overweight. The calculation method should explain this distinction.
Do not round height or intermediate calculations early to force a preferred category. Keep available input precision, calculate once, then round the display. The dividing line is useful for consistent screening; crossing it by a hundredth does not establish an abrupt biological change.
Height differences can matter more than decimal places
Height is squared in the denominator. A slightly taller recorded height lowers the result even when weight is identical. At 72 kg, entering 169 cm produces about 25.21, while entering 170 cm produces about 24.91. That is enough to cross the adult cutoff in this example.
Try a measurement comparison
See how entering height one centimetre differently changes BMI at the same weight. This is an arithmetic example, not a confidence interval or a health assessment.
At 72 kg: 169 cm gives BMI 25.209; 170 cm gives 24.913; 171 cm gives 24.623. Extra decimal places illustrate the calculation, not measurement accuracy.
The explorer varies only height. It does not estimate your measurement error, body fat, or disease risk. A scale used on a different surface, shoes included in one measurement, or a remembered height can also make comparisons unreliable. More decimal places cannot repair an inaccurate input.
Compare like with like before drawing a conclusion
- Write down both sites' original height and weight inputs, including units.
- Check whether the result is ordinary BMI, a child percentile, or a different metric. They are not interchangeable.
- Use the same measurements on each site. Compare their formulas and rounding rules.
- If the BMI agrees but the label differs, check the population guidance used. Some clinical guidelines use different action thresholds.
- If a difference remains unexplained, use the published formula to check the arithmetic by hand.
For ages 2–19, use age- and sex-specific assessment through the child and teen guidance. For adults, discuss an unexpected result in the context of medical history, muscle mass and other measurements; our BMI limitations guide explains why the number alone is incomplete.