How to Calculate BMI: Complete Guide
Updated: March 2026 · Category: Health & Wellness
Body Mass Index (BMI) is the most widely used screening tool in the world for assessing whether a person has a healthy weight relative to their height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI remains the standard recommended by the World Health Organization (WHO) for population-level weight classification. In this guide you will learn the formulas, see worked examples, understand the WHO classification table, and discover the limitations of BMI.
What Is BMI?
BMI is a simple numeric value derived from your weight and height. It does not measure body fat directly, but research shows that BMI correlates reasonably well with more direct measures of body fat, such as underwater weighing and dual-energy X-ray absorptiometry (DEXA). Health professionals use it as a quick first-pass screening tool to identify potential weight-related health risks.
A BMI value places you into one of several categories—from underweight through normal weight, overweight, and three grades of obesity. Each category carries different statistical risks for conditions like cardiovascular disease, type 2 diabetes, and certain cancers.
The BMI Formula (Metric)
BMI = Weight (kg) ÷ Height (m)²
Step-by-step example: Suppose you weigh 75 kg and your height is 1.75 m.
- Square your height: 1.75 × 1.75 = 3.0625
- Divide your weight by the result: 75 ÷ 3.0625 = 24.5
- Look up the result in the WHO table below → Normal weight
The BMI Formula (Imperial)
BMI = (Weight (lb) × 703) ÷ Height (in)²
Step-by-step example: Suppose you weigh 165 lb and your height is 5′9″ (69 inches).
- Square your height in inches: 69 × 69 = 4,761
- Multiply your weight by 703: 165 × 703 = 115,995
- Divide: 115,995 ÷ 4,761 = 24.4
- Result → Normal weight
WHO BMI Classification Table
| Classification | BMI Range | Health Risk |
|---|---|---|
| Underweight | < 18.5 | Increased |
| Normal weight | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Increased |
| Obesity Class I | 30.0 – 34.9 | High |
| Obesity Class II | 35.0 – 39.9 | Very high |
| Obesity Class III | ≥ 40.0 | Extremely high |
Quick-Reference Examples
| Weight | Height | BMI | Category |
|---|---|---|---|
| 50 kg (110 lb) | 1.70 m (5′7″) | 17.3 | Underweight |
| 68 kg (150 lb) | 1.73 m (5′8″) | 22.7 | Normal |
| 90 kg (198 lb) | 1.75 m (5′9″) | 29.4 | Overweight |
| 110 kg (242 lb) | 1.80 m (5′11″) | 33.9 | Obesity I |
Limitations of BMI
- Does not distinguish muscle from fat. Athletes or people with high muscle mass may register as “overweight” despite having low body fat.
- Ignores fat distribution. Abdominal (visceral) fat is more dangerous than subcutaneous fat, and BMI does not account for where fat is stored.
- Less accurate for certain groups. Elderly individuals tend to have more body fat than younger adults at the same BMI. Children and adolescents use age- and sex-specific percentile charts instead.
- Does not reflect bone density or body composition. Two people with the same BMI can have very different health profiles.
- Ethnic variations. Research suggests that some Asian populations face higher health risks at lower BMI thresholds, while some Black populations may have lower risks at the same BMI.
When to See a Doctor
BMI is a screening tool, not a diagnostic instrument. You should consult a healthcare professional if:
- Your BMI falls outside the normal range (below 18.5 or above 24.9)
- You have risk factors such as high blood pressure, high cholesterol, or a family history of heart disease or diabetes
- You have experienced unexplained weight changes
- You are an athlete or very muscular and want a more accurate body composition assessment
Your doctor may recommend additional tests such as waist circumference measurement, skin-fold thickness testing, or a DEXA scan for a more complete picture of your health.
Calculate Your BMI Now
Use our free BMI Calculator to find out your Body Mass Index in seconds. Just enter your weight and height and get your result with the corresponding WHO classification instantly.