What is body mass index (BMI)?
How BMI is calculated, what its ranges mean, and why this widely used formula has important limits worth knowing.
When you go to the doctor or use a health app, a number often comes up: your BMI. It’s one of the most widely used figures in the world for talking about weight, but many people don’t know exactly what it means or, above all, what its limits are. Let’s clear it up.
What it is and how it’s calculated
The body mass index (BMI) is a simple formula that relates your weight to your height to estimate whether your weight is appropriate. It’s calculated like this:
BMI = weight (kg) ÷ height² (m)
For example, a person weighing 70 kg who is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 ≈ 22.9.
Its great advantage is that it’s easy, cheap, and non-invasive: all you need is a scale and a tape measure.
What the ranges mean
Once calculated, the number is compared to standard ranges defined by the World Health Organization:
- Under 18.5: underweight.
- 18.5 to 24.9: normal or healthy weight.
- 25 to 29.9: overweight.
- 30 or more: obesity.
These values give a quick guide to possible health risks associated with weight.
What it’s for (and what it’s not)
BMI is a population screening tool: it’s used to quickly detect, in large groups of people, trends of underweight, overweight, or obesity. It’s useful as a first indicator and for tracking one person’s changes over time.
What BMI is not: a diagnosis. As the CDC reminds us, it doesn’t directly measure body fat or a person’s health.
Its important limits
Here’s the part many people ignore. BMI has blind spots because it doesn’t distinguish what your weight is made of:
- It doesn’t tell muscle from fat: a very muscular athlete can have a “high” BMI without excess fat.
- It doesn’t show where the fat is: abdominal fat is riskier than fat in other areas, and BMI doesn’t capture that.
- It varies with age, sex, and ethnicity: the same value doesn’t mean the same thing for everyone.
That’s why BMI should be interpreted alongside other measures (like waist circumference) and always by a professional.
A useful number, but not the last word
BMI is a simple compass, not a precision GPS. It’s useful for getting a first idea, but it doesn’t define your health or your fitness. If you’re worried about your weight, the sensible thing is to consult a doctor, who will assess your body and habits as a whole. A number doesn’t tell the whole story, but knowing how to read it well is already a good starting point.