Guide
How to Calculate BMI (Formula, Chart & Examples)
Body Mass Index is a free screening tool—but it's not a medical diagnosis.
What Is BMI?
BMI Formula: Metric and Imperial
- Metric formula: BMI = weight (kg) / height (m)²
- Imperial formula: BMI = [weight (lbs) / height (in)²] × 703
- Example (metric): 70 kg person, 1.75 m tall = 70 / (1.75)² = 70 / 3.0625 = 22.9 BMI
- Example (imperial): 155 lbs person, 5'5" (65 inches) = [155 / (65)²] × 703 = [155 / 4225] × 703 = 22.8 BMI
WHO BMI Categories
- Underweight: BMI < 18.5 (may indicate insufficient nutrition or underlying health issues)
- Normal weight: BMI 18.5–24.9 (lowest risk of weight-related health conditions)
- Overweight: BMI 25.0–29.9 (increased risk of chronic diseases)
- Obese Class I: BMI 30.0–34.9 (substantially increased health risk)
- Obese Class II: BMI 35.0–39.9 (high health risk)
- Obese Class III (Severe): BMI ≥ 40.0 (very high health risk)
Step-by-Step Worked Examples
- Person A (metric): 85 kg, 1.80 m tall. BMI = 85 / (1.80)² = 85 / 3.24 = 26.2 (Overweight)
- Person B (metric): 62 kg, 1.65 m tall. BMI = 62 / (1.65)² = 62 / 2.7225 = 22.8 (Normal weight)
- Person C (imperial): 200 lbs, 5'10" (70 inches). BMI = [200 / (70)²] × 703 = [200 / 4900] × 703 = 28.7 (Overweight)
- Person D (imperial): 140 lbs, 5'6" (66 inches). BMI = [140 / (66)²] × 703 = [140 / 4356] × 703 = 22.5 (Normal weight)
What's a Healthy Weight Range for Your Height?
Important Limitations of BMI
BMI and Health Risk
When to Seek Professional Advice
Frequently asked questions
Is BMI the same as body fat percentage?
No. BMI is calculated from height and weight only, so it cannot distinguish between muscle and fat. Body fat percentage is measured separately and gives a more direct assessment of how much of your body is fat tissue. A muscular person can have a high BMI but low body fat percentage.
Why do doctors still use BMI if it has limitations?
BMI is quick, inexpensive, and requires no equipment. For large populations, it correlates reasonably well with health outcomes and identifies people who may need further assessment. Doctors use it as a screening tool alongside other tests and clinical judgment, not as a final diagnosis.
Is there a different BMI chart for men and women?
No. The WHO BMI categories are the same for both men and women. However, women tend to naturally carry more body fat than men at the same BMI, and the health implications may differ slightly by sex. Your doctor can help interpret your BMI in context.
At what age does BMI become relevant?
BMI categories apply to adults aged 18 and over. Children and teenagers have different growth patterns and should use age- and sex-specific BMI percentiles. If you're calculating BMI for a young person, consult a pediatrician.
Can I lose weight and still have a high BMI?
Yes. If you lose weight but gain muscle (through strength training), your weight might not change much even though your body composition improves. Conversely, losing weight purely through crash dieting can result in muscle loss and less metabolic benefit than a balanced approach. Focus on how you feel and your fitness metrics, not the scale alone.