Body Mass Index (BMI) is the most widely used screening tool for weight classification worldwide. Over 1 billion people are classified as obese by BMI standards. Here's everything you need to know about calculating and interpreting your BMI.
The BMI Formula
BMI is calculated using a simple formula:
BMI Formula
Metric: BMI = weight (kg) ÷ height² (m²)
Imperial: BMI = weight (lbs) × 703 ÷ height² (inches²)
Or skip the math — use our free BMI calculator for instant results.
BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Moderate |
| 18.5 – 24.9 | Normal weight | Low |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese (Class I) | High |
| 35.0 – 39.9 | Obese (Class II) | Very High |
| 40.0+ | Obese (Class III) | Extremely High |
BMI by Age and Gender
While the standard BMI chart applies to all adults equally, research suggests optimal BMI ranges may vary:
- Young adults (18-25): 18.5-24.9 is ideal
- Middle age (25-65): 22-27 may be optimal
- Seniors (65+): Slightly higher BMI (25-27) is associated with better outcomes
- Athletes: BMI is often inaccurate due to muscle mass
Limitations of BMI
BMI is a useful screening tool, but it has significant limitations:
- Doesn't measure body fat directly — a muscular athlete can have an "overweight" BMI
- Doesn't account for fat distribution — belly fat is more dangerous than hip fat
- Age and gender blind — same scale for everyone
- Racial differences — health risks may occur at different BMIs for different populations
For a more accurate picture, combine BMI with our body fat calculator and waist-to-hip ratio.
How to Improve Your BMI
If Overweight (BMI 25-29.9)
- Create a modest calorie deficit of 250-500 calories/day (use our calorie calculator)
- Increase daily movement — aim for 10,000 steps
- Focus on protein and fiber for satiety
- Aim for 1-2 lbs of weight loss per week
If Underweight (BMI below 18.5)
- Increase calorie intake by 300-500 calories/day
- Eat nutrient-dense foods (nuts, avocados, whole grains)
- Add strength training to build muscle mass
- Consult a doctor to rule out underlying conditions