Weight by height is a reference range, not an ideal number
“What should I weigh?” has no single answer from height alone. A reference range can explain the arithmetic, but health assessment extends beyond a scale.
Where the range comes from
CDC’s healthy adult BMI category, for ages 20 and older, is 18.5 to less than 25. Since BMI = kilograms ÷ meters squared, the inverse is weight = BMI × height squared. Current weight is not needed to calculate this reference.
References: CDC
Why the midpoint is not everyone’s target
BMI does not separate fat, muscle and bone. The middle is not a prescribed target and the boundaries are not diagnoses. Children and teens require age- and sex-related interpretation. Do not use this tool for pregnancy or to set treatment or a diet.
References: CDC — BMI frequently asked questions
An example that exposes unit errors
For 170 cm, use 1.70 m; its square is 2.89. The mathematical endpoints are 18.5 × 2.89 = 53.465 kg and 25 × 2.89 = 72.25 kg. The displayed range is approximately 53.47 to less than 72.25 kg.
The upper boundary is excluded, not “up to and including.” Do not use rounded output to diagnose a tiny difference: measurement and context matter more than a fraction of a kilogram.
Entering 170 as meters instead of centimeters magnifies the error because height is squared. Check the unit first, then use the result for an appropriate professional discussion if needed, not to compare bodies or impose one target.