Body Mass Index was developed in the 1830s by a Belgian mathematician studying population averages, not individual health, and it has one well-documented blind spot that matters enormously for runners: it can't distinguish muscle mass from fat mass. It's a ratio of weight to height, nothing more.
Where the number breaks down
A distance runner with low body fat and above-average muscle mass for their build will often read as "overweight" on the BMI scale — a well-documented pattern, and the same effect shows up in a milder form for recreational runners with solid muscular development from years of training. The formula has no way to know the difference between 15 extra pounds of muscle and 15 extra pounds of fat; it treats them identically.
Running the numbers
The BMI Calculator is here because it's a commonly requested reference number, not because it's the right tool for assessing a runner's health or race weight. It takes height and weight and returns the standard BMI figure and category.
Worked example: A runner at 5'10" and 165 lb returns a BMI of about 23.7 — solidly in the "normal" range. A more muscular runner at the same height and 185 lb returns a BMI around 26.5, landing in "overweight" territory by the chart, despite potentially having lower body fat and better running economy than the first runner. The number alone can't tell you which runner is actually carrying more useful mass versus more body fat.
Better numbers for runners
If race weight or body composition is the real question, BMI isn't the tool — body fat percentage (via skinfold calipers, DEXA scan, or bioelectrical impedance, in roughly ascending order of accuracy) gives a far more useful picture. For most runners, though, the more actionable number is simpler than any of these: performance trend over time. If your training paces are improving and you're recovering well, the scale — in any of its forms — is a secondary signal at best.