What your BMI measures and why it’s a flawed but useful tool
By Dan Kauna, July 4, 2026Many people encounter the Body Mass Index, or BMI, during routine hospital visits or when joining a gym. It is the quickest tool health professionals use to group people into underweight, normal, overweight, or obese categories.
Yet, despite its widespread use across the world, this two-digit number remains widely misunderstood. It functions well as a basic screening tool, but it fails as a complete measure of individual fitness.
Where the formula gets it wrong
At its core, the math behind BMI is incredibly basic. It simply divides total weight by height squared.
Because of this simplicity, the formula is completely blind to what makes up that weight. It cannot distinguish between heavy bones, toned muscles, or actual body fat.
This creates a massive blind spot for different body types. A manual labourer or a dedicated gym enthusiast might carry a lot of dense muscle and very little fat. Because muscle weighs more than fat by volume, the formula often wrongly flags these healthy individuals as overweight.
On the flip side, someone who looks thin but carries dangerous, hidden fat around their internal organs might get a perfect score. The formula treats all weight the same, which means it can easily offer a false sense of security.
Better health tracking for Kenyan bodies
This approach is particularly problematic for African body compositions. Research consistently shows that where the body stores fat matters much more than total weight. Many people of African descent tend to accumulate fat around the midsection, which poses a serious risk for type 2 diabetes and heart complications.
Medical researchers continue to warn against relying solely on this single metric. A study in the International Journal of Environmental Research and Public Health points out that “BMI does not measure body fat distribution (e.g., it does not recognize more harmful abdominal fat), is a limited measure of body fat (inviting misclassification of persons who are muscular or have larger bone structures).”
To get a clearer picture of metabolic health, pair BMI with simple alternatives like waist circumference. A basic tailor’s tape measure provides an immediate reality check.
Keeping a waist measurement below 94 cm for men and 80 cm for women is a strong indicator of good health. While BMI is a cheap, accessible starting point, it should never be the final word on personal well-being.