What Is BMI and Why Does It Matter?

Body Mass Index (BMI) is a quick screening number that relates your weight to your height. It's widely used by doctors as a first-pass indicator of whether someone is underweight, at a healthy weight, overweight, or obese.

The Formula

BMI = weight (kg) / height (m)²

For example, someone who is 1.70m tall and weighs 70kg has a BMI of 70 / (1.70 × 1.70) = 24.2.

The Standard BMI Categories

• Below 18.5 — Underweight
• 18.5 – 24.9 — Normal weight
• 25 – 29.9 — Overweight
• 30 and above — Obese

Where BMI Falls Short

BMI doesn't distinguish between muscle and fat. A muscular athlete can have a "high" BMI despite having very low body fat, because muscle is denser than fat. It also doesn't account for where fat is distributed on the body, which matters for health risk. That's why BMI is best treated as a starting point for a conversation with a doctor, not a diagnosis on its own — tools like body fat percentage and waist circumference add useful context.

Want your number instantly? Enter your height and weight into our BMI calculator below.