BMI Guide: What Your Number Really Means

Body Mass Index (BMI) is one of the most widely used health metrics in the world. But what does your BMI actually mean, and should you rely on it? This guide explains everything you need to know about BMI.

What Is BMI?

BMI is a simple calculation that uses your height and weight to estimate whether you're at a healthy weight. It was developed in the early 19th century by Belgian mathematician Adolphe Quetelet as a quick way to measure the degree of obesity in populations.

The BMI Formula

BMI = weight (kg) / height (m)²

Or in imperial units:

BMI = [weight (lb) / height (in)²] × 703

BMI Categories

The World Health Organization defines these BMI categories:

What Each Category Means

Underweight (BMI < 18.5)

Being underweight can indicate malnutrition, eating disorders, or underlying health conditions. Health risks include weakened immune system, osteoporosis, and fertility issues.

Normal Weight (BMI 18.5-24.9)

This range is associated with the lowest health risks for most people. However, health is multifaceted—fitness, diet, and other factors also matter.

Overweight (BMI 25-29.9)

Increased risk of heart disease, type 2 diabetes, and high blood pressure. However, some people in this range are metabolically healthy.

Obese (BMI 30+)

Significantly elevated health risks. The higher the BMI, the greater the risk. Medical intervention and lifestyle changes are typically recommended.

Limitations of BMI

BMI is a useful screening tool, but it has significant limitations:

1. Doesn't Distinguish Fat from Muscle

A muscular athlete might have a high BMI despite having low body fat. A bodybuilder with 8% body fat could be classified as "obese" by BMI.

2. Doesn't Account for Fat Distribution

Where you carry fat matters. Belly fat (visceral fat) is more dangerous than fat in other areas. Someone with a "normal" BMI but high belly fat may have elevated health risks.

3. Age and Gender Differences

Older adults tend to have more body fat and less muscle. Women naturally carry more fat than men. BMI doesn't account for these differences.

4. Ethnic Variations

Health risks at certain BMI levels vary by ethnicity. For example, Asian populations may have higher health risks at lower BMI levels.

5. Doesn't Measure Health

You can have a "healthy" BMI but be unfit, eat poorly, and have high blood pressure. Conversely, someone "overweight" by BMI could be metabolically healthy.

Better Health Metrics

Consider using these alongside BMI:

Waist Circumference: Men over 40 inches and women over 35 inches have increased health risks

Waist-to-Hip Ratio: Measures fat distribution; higher ratios indicate more belly fat

Body Fat Percentage: More accurate measure of body composition

Blood Pressure, Cholesterol, Blood Sugar: Direct health indicators

When to Use BMI

BMI is most useful for:

• Quick population-level health assessments

• Initial screening in medical settings

• Tracking weight changes over time

• General guidance when combined with other metrics

The Bottom Line

BMI is a simple, free screening tool—not a definitive measure of health. If you're concerned about your weight or health, consult a healthcare provider who can consider your complete picture: BMI, body composition, fitness level, diet, family history, and other factors.

Calculate Your BMI

Use our free BMI calculator to find your score, see where you fall on the categories, and get personalized recommendations for your health goals.