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BMI Explained: What the Numbers Mean and How to Use Them

Body Mass Index is one of the most widely used health metrics. Here's what it measures, where it falls short, and how to calculate yours.

What Is BMI?

Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you're at a healthy weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it's now used worldwide by health organizations as a screening tool.

The formula is straightforward:

BMI = weight (kg) ÷ height² (m²)

Or in imperial units: BMI = (weight in pounds × 703) ÷ height in inches²

BMI Categories

BMI Range	Category	Health Implications
Below 18.5	Underweight	May indicate malnutrition or health issues
18.5 – 24.9	Normal weight	Generally associated with lowest health risk
25.0 – 29.9	Overweight	Increased risk for certain health conditions
30.0 – 34.9	Obese (Class I)	Elevated risk for heart disease, diabetes
35.0 – 39.9	Obese (Class II)	High risk for weight-related conditions
40.0 and above	Obese (Class III)	Very high risk — consult a healthcare provider

Example Calculations

Let's walk through a few examples:

Person A: 5'8" (173 cm), 155 lbs (70 kg)

BMI = 70 ÷ (1.73)² = 70 ÷ 2.99 = 23.4 — Normal weight

Person B: 5'4" (163 cm), 185 lbs (84 kg)

BMI = 84 ÷ (1.63)² = 84 ÷ 2.66 = 31.6 — Obese (Class I)

Person C: 6'0" (183 cm), 135 lbs (61 kg)

BMI = 61 ÷ (1.83)² = 61 ÷ 3.35 = 18.2 — Underweight

Limitations of BMI

While useful as a quick screening tool, BMI has significant limitations:

• Doesn't distinguish muscle from fat — Athletes often have high BMIs despite low body fat
• Doesn't account for body composition — Two people with the same BMI can have very different body types
• Doesn't consider age — Healthy BMI ranges may differ for older adults
• Doesn't factor in gender — Women naturally carry more body fat than men
• Doesn't measure fat distribution — Where you carry fat matters for health (belly fat is more dangerous)

BMI is best used as one data point among many. Waist circumference, body fat percentage, and overall fitness level provide a more complete picture.

Better Metrics to Track Alongside BMI

For a more complete health assessment, consider also tracking:

• Waist circumference — Over 40" (men) or 35" (women) signals higher risk
• Waist-to-hip ratio — Another indicator of fat distribution
• Body fat percentage — More accurate than BMI for measuring body composition
• Blood pressure, cholesterol, blood sugar — Direct health markers

Products to Support Your Health Journey

Want to calculate your BMI right now? Use our free BMI Calculator — just enter your height and weight for instant results.