BMI Calculator — Body Mass Index

What Is BMI?

Body Mass Index (BMI) is a simple ratio of weight to height: weight(kg) / height(m)². Developed by Adolphe Quetelet in the 1830s, it was designed as a population-level screening tool — not an individual health diagnosis.

BMI works well for large-scale health research and as a quick check for the general population. For individuals who train with weights, it has a well-known blind spot: it can't tell muscle from fat. A 5'10" lifter at 200lbs and 12% body fat has the same BMI as someone at 200lbs and 30% body fat.

WHO BMI Categories

The World Health Organization classifies BMI into these ranges:

• Underweight: Below 18.5. Associated with nutrient deficiency, weakened immune system, and bone loss.
• Healthy weight: 18.5-24.9. Lowest risk for weight-related health conditions.
• Overweight: 25.0-29.9. Mildly increased health risk. For muscular individuals, this range often indicates a healthy body composition.
• Obese Class I: 30.0-34.9. Significantly increased risk for type 2 diabetes, cardiovascular disease, and sleep apnea.
• Obese Class II: 35.0-39.9. High risk. Medical intervention often recommended.
• Obese Class III: 40.0+. Very high risk. Also called "severe obesity."

What Is BMI Prime?

BMI Prime is your BMI divided by 25 — the upper limit of the healthy range. It gives you a single number to understand your position relative to healthy:

• BMI Prime < 0.74: Underweight
• BMI Prime 0.74-1.00: Healthy weight
• BMI Prime > 1.00: Overweight or obese

A BMI Prime of 0.92 means you're at 92% of the upper healthy limit. It's a more intuitive way to track weight changes relative to the healthy range than raw BMI.

What Is the Ponderal Index?

The Ponderal Index (PI) divides weight by height cubed instead of height squared: weight(kg) / height(m)³. This makes it more accurate for people at the extremes of height.

Standard BMI systematically overestimates fatness in tall people and underestimates it in short people — a known limitation identified by Quetelet himself. The Ponderal Index corrects for this. A normal PI ranges from 11-15 kg/m³.

Limitations of BMI

BMI has several well-documented limitations:

• Muscle vs. fat: BMI doesn't distinguish between lean mass and fat mass. Most people who weight train will have "overweight" BMIs while being perfectly healthy.
• Age: Older adults lose muscle and gain fat — their BMI can stay the same while health deteriorates.
• Ethnicity: Research shows South Asian and East Asian populations have higher metabolic risk at lower BMIs. The WHO suggests using 23 (not 25) as the overweight threshold for these groups.
• Fat distribution: BMI doesn't tell you where fat is stored. Visceral fat (around organs) is far more dangerous than subcutaneous fat (under the skin), but BMI treats them the same.

When BMI Is Useful

Despite its limitations, BMI remains useful in several contexts:

• Population health studies — comparing disease risk across large groups
• Initial screening — flagging potential weight-related health concerns
• Tracking trends — monitoring weight changes over time when body fat measurement isn't available
• Insurance and medical guidelines — still widely used as a risk criterion

For training purposes, RepStack's Strength Score (0-999) is more actionable than BMI — it measures how much you lift relative to your bodyweight across five compound lifts.

BMI vs. Body Fat Percentage

If you lift weights regularly, body fat percentage gives you a far more accurate picture of your composition. The US Navy method requires only a tape measure and is accurate within 3-4% of DEXA scans.

The practical rule: use BMI for a quick baseline, then track body fat percentage and strength metrics for real progress insight. RepStack calculates your Strength Score from your training data — no manual measurements needed.