BMI Formula Explained: How to Calculate Body Mass Index

Body Mass Index (BMI) is one of the most widely used screening tools for assessing whether a person's weight falls within a healthy range. Understanding the BMI formula helps you interpret your results with confidence.

This guide walks you through the body mass index formula in both metric and imperial systems, explains BMI categories, and highlights important limitations.

What Is BMI?

BMI stands for Body Mass Index. It is a numerical value derived from a person's weight and height, used as a quick screening tool to categorize individuals into weight classes. The World Health Organization (WHO) uses BMI as a population-level indicator for overweight and obesity.

BMI is not a diagnostic tool. It does not measure body fat directly and should not be used as the sole indicator of health status.

The BMI Formula

Metric Formula

The metric formula uses kilograms and meters:

BMI = weight (kg) ÷ height² (m²)

Example: A person weighing 70 kg at 1.75 m tall:

BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9

Imperial Formula

The imperial formula uses pounds and inches, with a conversion factor of 703:

BMI = weight (lbs) ÷ height² (in²) × 703

Example: A person weighing 154 lbs at 69 inches tall:

BMI = 154 ÷ (69)² × 703 = 154 ÷ 4,761 × 703 = 22.7

The factor 703 converts between imperial and metric systems to produce the same BMI value.

Metric Calculation: Step by Step

1. Measure your weight in kilograms (kg).
2. Measure your height in meters (m). If in centimeters, divide by 100.
3. Square your height — multiply it by itself.
4. Divide your weight by the squared height.
5. Round to one decimal place.

Example: Weight: 80 kg, Height: 180 cm → 1.80 m

• Height squared: 1.80 × 1.80 = 3.24
• BMI: 80 ÷ 3.24 = 24.7

Imperial Calculation: Step by Step

1. Measure your weight in pounds (lbs).
2. Measure your height in inches. Convert feet and inches to total inches.
3. Square your height.
4. Divide your weight by the squared height.
5. Multiply by 703.

Example: Weight: 180 lbs, Height: 5'10" → 70 inches

• Height squared: 70 × 70 = 4,900
• BMI: (180 ÷ 4,900) × 703 = 25.8

BMI Categories

The WHO defines these standard categories:

Category	BMI Range (kg/m²)
Underweight	Below 18.5
Normal weight	18.5 – 24.9
Overweight	25.0 – 29.9
Obese (Class I)	30.0 – 34.9
Obese (Class II)	35.0 – 39.9
Obese (Class III)	40.0 and above

A healthy BMI range is generally 18.5 to 24.9. Values outside this range may indicate increased health risks, but BMI alone does not provide a complete picture.

BMI Limitations

The BMI formula has significant limitations:

• Does not distinguish muscle from fat. Athletes may have a high BMI despite low body fat, since muscle is denser than fat.
• Ignores age. Older adults tend to have more body fat at the same BMI.
• Ignores gender. Women generally have more body fat than men at the same BMI.
• Ignores ethnicity. Some Asian populations face health risks at lower BMI values.
• Ignores fat distribution. Abdominal fat carries higher risks, but BMI doesn't capture this.

BMI should be used as a general screening tool, not a definitive health assessment.

BMI and Different Populations

Athletes

BMI often overestimates body fat for athletes. A professional rugby player may have a BMI of 30+ while maintaining very low body fat. Additional measurements like body fat percentage or waist circumference provide better context.

Elderly Adults

Older adults often experience muscle loss (sarcopenia) and increased fat storage. Some researchers suggest a slightly higher BMI range (25–27) may be optimal for this population.

Children and Adolescents

For children, BMI is expressed as a percentile relative to age and sex using CDC or WHO growth charts, not standard adult categories.

Try Our Free BMI Calculator

Ready to calculate your BMI? Our BMI Calculator supports both metric and imperial units with instant results and category classification.

Explore our What Is BMI? guide for more context, or use the Unit Converter to switch between measurement systems.

Frequently Asked Questions

What is the BMI formula?

Weight divided by height squared. Metric: BMI = kg ÷ m². Imperial: BMI = (lbs ÷ in²) × 703.

Is BMI accurate for everyone?

No. BMI doesn't distinguish muscle from fat and ignores age, gender, and ethnicity. It works best as a general population screening tool.

What is a healthy BMI range?

Between 18.5 and 24.9 according to the WHO.

How is BMI calculated for children?

The same formula is used, but results are interpreted using age- and sex-specific percentile charts rather than adult categories.

What are alternatives to BMI?

Body fat percentage, waist-to-hip ratio, waist circumference, DEXA scans, and skinfold measurements provide more detailed body composition data.

Can BMI be used with both metric and imperial units?

Yes. Metric uses kg and meters (BMI = kg ÷ m²). Imperial uses lbs and inches with a conversion factor (BMI = lbs ÷ in² × 703).

What are the main limitations of BMI?

It doesn't measure body fat directly, ignores muscle mass, age, gender, and ethnic differences, and doesn't account for fat distribution.

Conclusion

The BMI formula provides a quick way to screen weight categories across populations. Understanding how BMI is calculated in both metric and imperial systems helps you interpret your results with context.

However, BMI is a starting point, not a complete health assessment. Always consider it alongside other indicators and consult a healthcare professional for personalized guidance.

Use our BMI Calculator to check your BMI instantly, or explore the Age Calculator and Unit Converter for more utilities.