How to Calculate BMI: Formula, Categories, and Limitations

Introduction

Body Mass Index (BMI) is a numerical measure of body weight relative to height. Developed in the 19th century and adopted by the WHO as a population-level screening tool, it provides a quick, cost-free indicator of weight categories that may signal increased health risk.

BMI is not a direct measure of body fat, and it has well-documented limitations for individuals. However, it remains a useful starting point for weight management discussions and personal health tracking.

When to Use This Calculator

Use the BMI calculator when tracking body composition changes over time, comparing your result against standard weight classification ranges, preparing for a healthcare appointment where weight-related health risk will be discussed, or setting specific weight management goals with clear numerical benchmarks.

How the Math Works

BMI is calculated differently depending on your unit system:

Results are interpreted using WHO standard categories: Underweight (< 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (≥ 30.0).

Metric example: A person weighing 75 kg at 1.75 m tall: BMI = 75 ÷ (1.75)² = 24.5 → Normal weight.

Metric:   BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lbs) ÷ height (in)²

Practical Example

Alex is 5 ft 10 in (70 inches) tall and weighs 195 lbs. Applying the imperial formula: BMI = 703 × 195 ÷ (70²) = 137,085 ÷ 4,900 = 27.98 → Overweight category.

Alex's doctor considers this alongside waist circumference (38 inches), blood pressure (128/82), and activity level. BMI confirms a screening flag; the full clinical picture guides any recommendations.

Common Mistakes

Treating BMI as the sole health indicator: Athletes with high muscle mass frequently fall in the Overweight category despite excellent metabolic health. BMI is one data point among many.

Not adjusting for age: Clinicians often use 25–27 as the overweight threshold for adults over 65, where slightly higher BMI may be associated with better survival outcomes.

Comparing across ethnic groups without adjustment: Health risks associated with elevated BMI can occur at lower thresholds in some Asian populations, prompting alternate clinical cut-offs.

Making dietary or medical decisions based on BMI alone: Always consult a licensed healthcare professional before beginning a weight management program.

Use the Calculator

Enter your height and weight in either metric or imperial units to instantly calculate your BMI and see your WHO weight category. Use the result as a starting point for a broader conversation about your health goals.