BMI is the most widely used health metric on the planet. It’s also one of the most misunderstood. Here’s what it actually tells you, where it fails, and what to pair it with.

What BMI Is

Body Mass Index = weight (kg) / height (m) squared.

It was invented in the 1830s by a Belgian mathematician named Adolphe Quetelet. He wasn’t a doctor. He was studying population statistics. BMI was designed to categorize populations, not diagnose individuals.

Where BMI Works

For the average person who doesn’t exercise intensely, BMI is a reasonable proxy for body fat. Large population studies consistently show that BMI correlates with health risks:

  • BMI 18.5-24.9: lowest risk for most diseases
  • BMI 25-29.9: elevated risk of heart disease, diabetes
  • BMI 30+: significantly elevated risk

For a doctor screening thousands of patients, BMI is quick, cheap, and “good enough” to flag who needs closer examination.

Where BMI Fails

1. It ignores body composition

A muscular person and an overweight person at the same height and weight have identical BMIs. Dwayne “The Rock” Johnson’s BMI is around 34 (obese). He’s clearly not.

2. It doesn’t account for fat distribution

Where you carry fat matters more than how much. Visceral fat around your organs is far more dangerous than subcutaneous fat under your skin. Two people with BMI 27 can have very different health profiles.

3. Different populations, different thresholds

The standard BMI categories were developed using data from European populations. For South Asians, health risks rise at lower BMI values. Many researchers suggest using 23 (not 25) as the overweight threshold for Asian populations.

4. Age and gender blindness

BMI doesn’t adjust for the fact that body composition naturally changes with age, or that women carry more essential fat than men.

What to Track Instead (or In Addition)

Waist circumference

The single best addition to BMI. Measure around your navel.

  • Men: above 90 cm signals elevated risk (for South Asians; 102 cm for Europeans)
  • Women: above 80 cm signals elevated risk

Waist-to-hip ratio

Waist measurement divided by hip measurement.

  • Men: above 0.9 is high risk
  • Women: above 0.85 is high risk

Waist-to-height ratio

Your waist should be less than half your height. Simple, effective, works across populations.

Body fat percentage

The gold standard if you can measure it. Healthy ranges:

  • Men: 10-20%
  • Women: 18-28%

DEXA scans and bioimpedance scales can estimate this, with varying accuracy.

The Practical Take

Don’t ignore BMI entirely. It’s free, instant, and gives you a starting point. But don’t let a number from an 1830s Belgian statistician be your only health metric. Pair it with waist circumference at minimum. If your BMI says you’re fine but your waist says otherwise, trust the waist.