How to Calculate BMI: Formula, Chart & Healthy Ranges

March 31, 2026 · 12 min read

Body Mass Index (BMI) is one of the most widely used health screening tools in the world. Whether you're visiting your doctor, applying for life insurance, or simply tracking your fitness journey, understanding how to calculate and interpret your BMI is essential. This comprehensive guide walks you through everything you need to know about BMI calculation, interpretation, and its practical applications.

What Is BMI and Why Does It Matter?

Body Mass Index (BMI) is a numerical value derived from your weight and height that provides a rough estimate of body fat. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was originally called the Quetelet Index before being renamed BMI in 1972.

Healthcare providers use BMI as a quick screening tool to identify potential weight-related health risks. While it doesn't directly measure body fat percentage, research shows strong correlations between BMI ranges and conditions like heart disease, type 2 diabetes, and certain cancers.

The formula is intentionally simple—requiring only a scale and measuring tape—making it accessible worldwide. This simplicity is both BMI's greatest strength and its most significant limitation, which we'll explore in detail later.

BMI Formulas: Metric and Imperial Systems

BMI can be calculated using either the metric system (kilograms and meters) or the imperial system (pounds and inches). Both formulas produce the same result when measurements are accurate.

Metric Formula

The metric formula is the international standard and the simplest to use:

BMI = weight (kg) ÷ height (m)²

For example, if you weigh 70 kilograms and are 1.75 meters tall, you would square your height (1.75 × 1.75 = 3.0625) and then divide your weight by that number (70 ÷ 3.0625 = 22.9).

Imperial Formula

The imperial formula includes a conversion factor of 703 to account for the different units:

BMI = (weight (lbs) × 703) ÷ height (in)²

The number 703 is a conversion constant that adjusts for the difference between metric and imperial measurements. Without it, the imperial calculation would produce an incorrect result.

Why the 703 Multiplier?

The conversion factor comes from the relationship between pounds and kilograms (1 kg = 2.20462 lbs) and inches and meters (1 m = 39.3701 in). When you work through the unit conversions mathematically, you arrive at approximately 703 as the necessary multiplier.

You can verify this yourself with our BMI Calculator, which handles both metric and imperial inputs automatically.

Step-by-Step Calculation Examples

Let's walk through several real-world examples to demonstrate how BMI calculations work in practice. Understanding the process helps you verify results and catch potential measurement errors.

Example 1: Metric Calculation

Person: 70 kg, 1.75 m tall

1. Square the height: 1.75 × 1.75 = 3.0625 m²
2. Divide weight by height squared: 70 ÷ 3.0625 = 22.86
3. Result: BMI of 22.9 (rounded)

This falls within the "normal weight" category (18.5-24.9), suggesting a healthy weight for this height.

Example 2: Imperial Calculation

Person: 154 lbs, 5'9" (69 inches) tall

1. Square the height: 69 × 69 = 4,761 in²
2. Multiply weight by 703: 154 × 703 = 108,262
3. Divide: 108,262 ÷ 4,761 = 22.74
4. Result: BMI of 22.7 (rounded)

Notice this is essentially the same person as Example 1 (70 kg ≈ 154 lbs, 1.75 m ≈ 69 inches), and both formulas produce nearly identical results.

Example 3: Higher BMI Range

Person: 95 kg, 1.70 m tall

1. Square the height: 1.70 × 1.70 = 2.89 m²
2. Divide weight by height squared: 95 ÷ 2.89 = 32.87
3. Result: BMI of 32.9 (rounded)

This falls into the "Obese Class I" category (30.0-34.9), which healthcare providers consider a moderate health risk requiring attention.

Example 4: Lower BMI Range

Person: 110 lbs, 5'6" (66 inches) tall

1. Square the height: 66 × 66 = 4,356 in²
2. Multiply weight by 703: 110 × 703 = 77,330
3. Divide: 77,330 ÷ 4,356 = 17.75
4. Result: BMI of 17.8 (rounded)

This falls into the "underweight" category (below 18.5), which may indicate nutritional deficiencies or other health concerns.

Understanding BMI Categories and Health Risks

The World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) have established standard BMI categories that correlate with different health risk levels. These categories help healthcare providers identify patients who may benefit from lifestyle interventions or further health screenings.

What These Categories Mean

Underweight (BMI < 18.5): Being underweight can be just as concerning as being overweight. It may indicate inadequate nutrition, eating disorders, or underlying medical conditions. Underweight individuals often have weakened immune systems and are at higher risk for osteoporosis and fertility issues.

Normal Weight (BMI 18.5-24.9): This range is associated with the lowest risk of weight-related health problems in population studies. However, individual health depends on many factors beyond BMI, including diet quality, physical activity, and genetics.

Overweight (BMI 25.0-29.9): People in this category have an elevated risk of developing chronic diseases, though the risk increase is moderate. Many people in this range are metabolically healthy, especially if they're physically active and have good cardiovascular fitness.

Obese Classes I-III (BMI ≥ 30.0): Obesity significantly increases the risk of serious health conditions. Class I obesity warrants lifestyle interventions, while Classes II and III often require more intensive medical management and may qualify for bariatric surgery.

Healthy Weight Ranges by Height

Rather than memorizing BMI formulas, many people find it helpful to reference weight ranges that correspond to healthy BMI values (18.5-24.9). The table below shows healthy weight ranges for common heights.

These ranges provide a quick reference point, but remember that healthy weight varies based on individual factors like muscle mass, bone density, and body composition. Use our Ideal Weight Calculator for personalized recommendations based on multiple formulas.

Important Limitations of BMI

While BMI is useful for population-level health screening, it has significant limitations when applied to individuals. Understanding these limitations helps you interpret your BMI results more accurately and avoid over-reliance on a single metric.

Doesn't Distinguish Between Muscle and Fat

BMI only considers total body weight, not body composition. A bodybuilder with 8% body fat and a sedentary person with 30% body fat could have identical BMIs if they're the same height and weight. Muscle tissue is denser than fat, so athletes and physically active individuals often have "overweight" or even "obese" BMIs despite being very healthy.

Professional athletes like NFL running backs frequently have BMIs over 30, which would classify them as obese, despite having body fat percentages in the single digits.

Ignores Fat Distribution

Where you carry fat matters more than how much you carry. Visceral fat (around internal organs) is metabolically active and strongly linked to heart disease, diabetes, and other conditions. Subcutaneous fat (under the skin) is relatively benign. BMI can't distinguish between these fat types.

Waist circumference and waist-to-hip ratio are better indicators of visceral fat and metabolic health risk than BMI alone.

Doesn't Account for Age

Body composition changes with age. Older adults naturally lose muscle mass (sarcopenia) and gain fat, even if their weight stays constant. A 70-year-old and a 25-year-old with the same BMI likely have very different body compositions and health profiles.

Some researchers suggest that slightly higher BMIs (25-27) may actually be protective for older adults, though this remains controversial.

Sex Differences Ignored

Women naturally have higher body fat percentages than men at the same BMI due to biological differences. A woman and man with a BMI of 24 will have different body fat percentages, yet BMI treats them identically.

Women typically have 6-11% more body fat than men at equivalent BMI values, which is normal and healthy.

Ethnicity Variations

BMI thresholds were developed primarily from European populations. Research shows that Asian populations develop obesity-related health risks at lower BMIs, while some other ethnic groups may tolerate higher BMIs without increased risk.

The WHO recommends lower BMI cutoffs for Asian populations: overweight at 23 and obese at 27.5, rather than 25 and 30.

BMI for Children and Teens

BMI calculations for children and adolescents use the same formula as adults, but interpretation is completely different. Children's bodies change rapidly as they grow, so BMI must be compared to age- and sex-specific growth charts rather than fixed categories.

BMI Percentiles for Children

Pediatric BMI is expressed as a percentile that shows how a child's BMI compares to other children of the same age and sex. For example, a child at the 65th percentile has a higher BMI than 65% of children their age and sex.

The CDC provides the following categories for children ages 2-19:

• Underweight: Below the 5th percentile
• Healthy weight: 5th to 84th percentile
• Overweight: 85th to 94th percentile
• Obese: 95th percentile or above

Why Percentiles Instead of Fixed Numbers?

Children's bodies change dramatically during growth spurts. A BMI of 18 might be healthy for a 10-year-old but concerning for a 16-year-old. Percentiles account for these normal developmental changes.

Growth charts are based on data from millions of children and show typical BMI patterns throughout childhood and adolescence.

Special Considerations for Teens

Adolescence brings unique challenges for BMI interpretation. Puberty timing varies widely—some teens experience growth spurts at 11, others at 16. This variation means two teens of the same age can have very different healthy BMI ranges.

Healthcare providers also consider pubertal development stage (Tanner stages) when evaluating teen BMI, not just age and sex.

Use our Child BMI Calculator to calculate and interpret BMI percentiles for children and teens ages 2-19.

BMI Adjustments for Different Ethnicities

Emerging research shows that BMI thresholds should vary by ethnicity because different populations have different body compositions and disease risk profiles at the same BMI values. This is an evolving area of medicine that's reshaping how we interpret BMI.

Asian Populations

Studies consistently show that Asian individuals develop type 2 diabetes, cardiovascular disease, and other obesity-related conditions at lower BMIs than European populations. The WHO recommends modified BMI cutoffs for Asian populations:

• Underweight: Below 18.5 (same as standard)
• Normal weight: 18.5 to 22.9 (vs. 24.9 standard)
• Overweight: 23.0 to 27.4 (vs. 25.0-29.9 standard)
• Obese: 27.5 and above (vs. 30.0 standard)

These lower thresholds reflect the fact that Asian populations tend to have higher body fat percentages and more visceral fat at the same BMI compared to European populations.

Black Populations

Research suggests that Black individuals may have lower health risks at higher BMIs compared to White individuals. Some studies show that Black adults have greater bone density and muscle mass, which increases BMI without increasing health risk.

However, this doesn't mean obesity is harmless in Black populations—it simply means the relationship between BMI and health risk may differ slightly from the standard thresholds.

Hispanic/Latino Populations

Hispanic and Latino populations show varied patterns depending on specific ancestry. Some research suggests slightly higher diabetes risk at lower BMIs, similar to Asian populations, but more studies are needed to establish clear guidelines.

The Bottom Line on Ethnicity

While ethnicity-specific BMI thresholds are gaining recognition, most healthcare systems still use standard cutoffs. If you're from a population known to have different risk profiles, discuss this with your healthcare provider when interpreting your BMI.

Better Alternatives to BMI

Given BMI's limitations, several alternative measurements provide more accurate assessments of body composition and health risk. Many healthcare providers now use multiple metrics together for a comprehensive evaluation.

Waist Circumference

Measuring your waist is one of the simplest and most effective alternatives to BMI. Waist circumference directly indicates abdominal fat, which is strongly linked to metabolic disease risk.

High-risk thresholds:

• Men: Waist circumference over 40 inches (102 cm)
• Women: Waist circumference over 35 inches (88 cm)

To measure correctly, place a tape measure around your bare abdomen just above your hip bones, at the level of your belly button. Breathe normally and measure after exhaling.

Waist-to-Hip Ratio

This ratio compares waist circumference to hip circumference and indicates fat distribution patterns. "Apple-shaped" bodies (more abdominal fat) have higher health risks than "pear-shaped" bodies (more hip and thigh fat).

High-risk thresholds:

• Men: Ratio above 0.90
• Women: Ratio above 0.85

Calculate yours with our Waist-to-Hip Ratio Calculator.

Body Fat Percentage

Directly measuring body fat percentage provides the most accurate picture of body composition. Methods include:

• DEXA scans: Most accurate, uses X-rays to measure