Understanding BMI
BMI is everywhere — doctors use it, fitness apps show it, and it's easy to calculate. But it's often misunderstood. Here's what it actually tells you and where it falls short.
What BMI Actually Is
Body Mass Index is a simple ratio: your weight in kilograms divided by your height in metres squared. It was invented in the 1830s by a statistician — not a doctor — as a way to study populations, not individuals.
It doesn't measure fat. It doesn't measure health. It measures how heavy you are relative to how tall you are. That's it.
The Standard Categories
Under 18.5
Underweight
May indicate insufficient nutrition or underlying health issues.
18.5 – 24.9
Normal
Associated with the lowest health risk at a population level.
25.0 – 29.9
Overweight
Elevated risk, but context matters — body composition plays a big role here.
30.0+
Obese
Higher health risk. Medical guidance recommended.
These ranges are based on population-level data. Individual health depends on far more than which bucket you fall into.
Where BMI Falls Short
Ignores muscle mass
A 90kg person at 12% body fat and a 90kg person at 35% body fat get the same BMI. The formula can't tell the difference.
Doesn't account for frame size
Bone structure, body proportions, and where you carry weight all matter for health — BMI ignores all of them.
Population tool, not individual
BMI was designed for statistical analysis of populations. It's a rough screening tool for individuals, not a diagnosis.
Snapshot, not trend
A single BMI reading says little. The direction it's moving over months — and why — tells you much more.
BMI Works Better Alongside
BMI is most useful as one data point in a bigger picture. Pair it with these for a more accurate view of your health and progress.
Waist circumference
Abdominal fat is more strongly linked to health risks than total weight. Waist measurement captures this directly.
Body fat percentage
Separates fat from lean mass. Two people with the same BMI can have very different body fat levels.
Progress photos
Visual changes often show progress that BMI misses, especially during body recomposition.
How you feel
Energy levels, strength, sleep quality, and daily function matter more than any single number.
Use BMI When
You want a quick, free baseline number. You're at a healthy body fat level and don't do heavy strength training. You want to track weight trends over time alongside other metrics.
Don't Rely on BMI When
You carry significant muscle mass. You're going through body recomposition (losing fat and gaining muscle). You need to assess actual health risk — talk to a doctor instead.
The Bottom Line
BMI is a starting point, not a verdict. Use it alongside body measurements, photos, and how you actually feel. The number matters far less than the trend and the context.