What does BMI tell us about health and fitness? The Body Mass Index, or BMI, is one of the most widely used tools to assess whether someone’s weight is in a healthy range. It’s a simple calculation: weight divided by height squared. The resulting number places a person into categories such as underweight, normal weight, overweight, or obese.
Because it is so simple, BMI is often misunderstood. Many people dismiss it by saying “BMI doesn’t matter if you’re muscular.” While it is true that BMI does not directly measure body composition, the full story is a little more nuanced.
Why BMI is still useful
What does BMI tell us that is valuable? Even though BMI cannot distinguish between fat mass and muscle mass, it gives a strong indication of health risks in the general population. Studies consistently show that as BMI increases above the “normal” range, the risk for conditions like type 2 diabetes, cardiovascular disease, and joint problems also increases.
This is because, for the majority of people, a higher BMI is caused by excess fat rather than muscle. It is extremely rare for someone to have so much lean mass that their BMI is elevated without carrying extra body fat. That means that in most cases, BMI does tell us something important about body fat and health risk.
The muscle exception
Yes, athletes and people who train with weights can have a higher BMI than non-athletes of the same height. Muscle is dense and heavy, so more muscle mass increases weight and therefore BMI. But here’s the important part: in cases where high BMI comes from both muscle and fat, the excess fat still matters.
If BMI is elevated and the person does not have visible core muscle definition, it is almost always a sign of carrying too much body fat. A six-pack is a clear signal that BMI is high because of muscle. Without that, the high BMI is not “just muscle.”
The mirror test
So, what does BMI tell us compared to what the mirror shows? The mirror is often the simplest truth-check. If muscles in the abdominal area are clearly visible, you are likely the rare person whose high BMI comes from muscle rather than fat. In this case, a body composition analysis is unnecessary.
If, on the other hand, your BMI is high and your abdominal area shows no definition, the explanation is straightforward: too much body fat. A body composition analysis would only confirm what the mirror already shows.
Why this matters for your training
At CrossFit Kreis 9, we work with busy professionals who want results. For us, BMI is not about labeling people, but about understanding where to start. Combined with tools like InBody measurements and regular goal reviews, BMI gives context to progress and health.
- If your BMI is in the normal range, we help you build muscle, strength, and fitness while keeping fat in check.
- If your BMI is above the normal range, we focus on sustainable fat loss and muscle gain.
- If you are the rare case of a muscular build with a high BMI, we shift the conversation toward performance, recovery, and long-term health.
Practical takeaways
- BMI doesn’t measure fat directly, but in most cases, it indicates higher body fat.
- The exception is rare: only very muscular people can have a high BMI without excess fat.
- If BMI is high and core definition is absent, it means too much fat, regardless of muscle mass.
- The mirror is often enough: if you see visible muscle, you don’t need a composition scan.
Conclusion
So, what does BMI tell? It tells us a lot about health risk, even if it does not give a direct breakdown of fat and muscle. For most people, a higher BMI means higher fat. In rare cases, muscle can elevate BMI, but without visible abdominal definition, excess fat is always part of the story.
At CrossFit Kreis 9, we use simple tools like BMI, combined with professional coaching, to help you understand where you stand and how to move forward. The goal is not just numbers—it is long-term results in health, strength, and fitness.
