Why BMI is Broken (and 3 Better Things to Track Instead)

2026-05-16 · ~7 min read · By the FitLife coaching team

Body Mass Index — weight in kilograms divided by height in metres squared — was invented by a Belgian astronomer named Adolphe Quetelet in 1832. He was studying populations, not individuals. He was clear about this. His index was a way to compare average weights across populations, not a diagnostic tool for any one person.

Two centuries later, doctors weigh you, measure your height, divide one by the other squared, and tell you whether you're "normal" or "overweight." This is using a population statistic for an individual decision. And it goes wrong constantly.

Where BMI fails as an individual measure

Three failure modes show up routinely:

1. Muscular people register as overweight or obese

Most rugby players, weightlifters, and powerlifters score "overweight" on BMI. Many score "obese." Their actual body fat is in single digits. The number is meaningless for them.

2. Skinny-fat people register as healthy

Someone at 65 kg and 170 cm has a BMI of 22.5 — squarely "normal." If they have low muscle and high body fat, they may have metabolic risk factors (high blood sugar, high triglycerides, fatty liver) despite a fine BMI.

3. Older people are systematically misjudged

BMI doesn't account for the muscle loss and fat redistribution that happens with aging. An elderly person with sarcopenia (low muscle) may show "normal" BMI but be at significant health risk.

Three better measurements

1. Waist-to-height ratio (WHtR)

Measure your waist at the navel. Divide by your height (same units). The target: under 0.5.

Why this works: visceral fat (the fat around organs in the abdominal cavity) is the metabolically dangerous fat. Waist circumference is a direct proxy. Multiple large studies have shown WHtR is a stronger predictor of cardiovascular events than BMI, particularly in adults.

2. Body fat percentage (estimated)

Methods, in order of accuracy:

1. DEXA scan (clinical): most accurate, ~$50-100, takes 10 minutes
2. BodPod (air displacement): accurate, available at some gyms
3. Skinfold calipers (with experienced operator): ±3-5%
4. Navy method (tape measure formula): ±3-4% accuracy, free, can do at home
5. Bioelectrical impedance scales: poor accuracy, varies by hydration; only useful for tracking your own trend

The Navy method using a tape measure is the most accessible — measure waist, neck, and (for women) hips, plug into the formula. FitLife's Tools tab includes this calculator.

3. Strength relative to body weight

This is rarely on a doctor's chart but is one of the strongest predictors of long-term health and longevity. Can you:

• Squat your bodyweight for 5 reps?
• Bench press 0.75× your bodyweight for 5 reps?
• Deadlift 1.25× your bodyweight for 5 reps?
• Walk 10,000 steps without unusual fatigue?

If yes to all, you're in the top 25% of physical fitness for your age. None of those metrics show up on BMI. All matter more for healthspan.

What BMI is still useful for

Two narrow uses:

1. Population-level public health statistics (its original purpose).
2. Identifying severe obesity (BMI > 35) where the assumption of high body fat is reliable.

Outside these, individual decisions about diet, exercise and health should use better tools.

The simple practical replacement

If you currently use BMI to track your fitness, replace it with two numbers:

1. Waist circumference, measured monthly. Target: trending down (if losing fat) or holding steady.
2. One strength benchmark (e.g., max push-ups in 60 seconds, or weight you can squat for 5 reps). Target: trending up.

If both are moving in the right direction, you're getting healthier — regardless of what BMI says.

FitLife's Tools tab includes WHtR, BMI (for reference), Navy body fat, and ideal weight calculators side by side — so you can see how they compare for your specific body.