How to Calculate BMI (and What It Misses)

Body Mass Index is a quick screening tool used by doctors and insurers worldwide. It's simple to compute, but it's often misunderstood. Here's how it works — and what it can't tell you.

The Formula

BMI divides your weight by the square of your height. In metric units:

In imperial units, you multiply by a conversion factor:

For a 70 kg person who is 1.75 m tall: 70 ÷ (1.75 × 1.75) ≈ 22.9.

The Categories

• Below 18.5 — underweight
• 18.5 – 24.9 — healthy weight
• 25 – 29.9 — overweight
• 30 and above — obesity

What BMI Gets Right

As a population-level screening tool, BMI correlates with health outcomes and is cheap and fast to compute. It flags people who may benefit from a closer look, and trends over time are useful for tracking weight changes.

What BMI Misses

BMI only knows your weight and height — it can't see your body composition:

• Muscle mass. An athlete with high muscle density can have a high BMI while being very lean.
• Age. BMI categories are fixed, but healthy body-fat ranges shift with age.
• Fat distribution. Waist circumference is often a better predictor of metabolic risk than BMI.
• Ethnicity. Some populations have higher metabolic risk at lower BMI thresholds.

A Better Picture

Pair BMI with measurements like waist circumference, body-fat percentage or your personal history. Our BMI Calculator and Body Fat Calculator give you a fuller starting point — but a professional's assessment matters most for real decisions.