OmniCalcsHow to Calculate BMI (and What It Misses)
The world's most famous health number — where it comes from and where it falls short.
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.