Body Mass Index is just weight divided by height squared. It was developed in the 1830s by a statistician studying population averages, not individual health, and it became a common clinical screening tool in the 20th century because it is cheap to calculate and correlates reasonably well with body fat across large groups of people.
What it's reasonably good at
Across large populations, BMI correlates with health risks tied to excess body fat, which is why it is still used as a quick screening tool by clinics and researchers. For most adults with an average body composition, a BMI in the high range does flag a real increase in certain health risks worth discussing with a doctor.
Where it breaks down
BMI cannot distinguish muscle from fat, so muscular athletes are routinely classified as 'overweight' or 'obese' despite low body fat. It also does not account for where fat is distributed, which matters more for some health risks than total body fat alone. Bone density, age, and ethnicity all shift the relationship between BMI and actual body fat percentage, which is why the same BMI number means something different at different ages.
It is also a single snapshot. Someone in excellent cardiovascular condition and someone who is sedentary can land on the exact same BMI.
How to use it sensibly
Treat BMI as one quick screening signal, not a verdict. It is more useful for tracking your own trend over time than for comparing yourself to someone else, and it is most reliable for average body types and least reliable at the extremes (very muscular, very tall or short, older adults). The BMI calculator and metric BMI calculator both compute the same standard formula -- imperial or metric input, your choice -- so you can see your number, but pair it with how you actually feel and perform, not as a number to optimize on its own.