BMI Explained: What Your Number Really Means (and Its Limits)

Body Mass Index is the most common health number in the world and one of the most misunderstood. It is a quick screening tool that flags potential weight problems across large populations — but it was never designed to judge any single person, and treating it as a verdict misses the point.

How BMI is calculated

BMI is your weight in kilograms divided by your height in metres, squared. So someone who is 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. The squaring of height is what makes it a ratio rather than a simple weight, but it is also the source of most of its quirks.

The standard categories

• Under 18.5 — underweight.
• 18.5 to 24.9 — a healthy range for most adults.
• 25 to 29.9 — overweight.
• 30 and above — obese, often split further into classes.

These cut-offs come from population data linking BMI to health risk. They are useful averages, not bright lines: someone at 24.9 is not meaningfully healthier than someone at 25.1.

Where BMI misleads

• Muscle weighs more than fat by volume, so muscular athletes often register as "overweight" despite low body fat.
• It ignores where fat sits. Visceral fat around the organs carries more risk than the number suggests.
• It does not adjust well for age, sex, pregnancy, or ethnicity — several populations have different risk at the same BMI.
• It says nothing about fitness, diet, blood pressure, or blood sugar, which matter more for actual health.
• Very tall or very short people are systematically mis-measured by the height-squared formula.

How to use it sensibly

Treat BMI as one signal among many. A number outside the healthy range is a prompt to look closer — at waist circumference, activity, and how you actually feel — not a diagnosis. Track it over time rather than fixating on a single reading, and bring it to a professional who can weigh it against the rest of your health.

Privacy

The FairDrawBox BMI calculator runs entirely in your browser. Your height and weight are never sent to a server or stored, so you can check a number privately without creating an account or leaving a trace.