Underweight BMI
A BMI below 18.5 is classified as underweight. While less discussed than overweight and obesity, being underweight carries its own health risks — weakened immune function, bone density loss, nutritional deficiencies, fertility issues, and increased mortality. Causes range from high metabolism and inadequate nutrition to underlying medical conditions and eating disorders. Check your BMI below and consult a healthcare provider if your result falls in this category.
BMI Calculator
Enter your height and weight to calculate your Body Mass Index.
BMI Categories
WHO classification of Body Mass Index ranges.
| Category | BMI Range | Status |
|---|---|---|
Underweight | < 18.5 | May indicate nutritional deficiency |
Normal | 18.5 – 24.9 | Lowest health risk |
Overweight | 25 – 29.9 | Moderate increased risk |
Obese I | 30 – 34.9 | High risk |
Obese II | 35 – 39.9 | Very high risk |
Obese III | 40+ | Extremely high risk |
Frequently Asked Questions
- What BMI is considered underweight?
- A BMI below 18.5 is classified as underweight by the WHO. This can indicate insufficient body mass for optimal health, though some naturally thin individuals may be healthy at a low BMI if their nutrition and metabolic markers are good.
- What are the health risks of being underweight?
- Weakened immune system, osteoporosis and fracture risk, anemia, fertility problems, hair loss, fatigue, and increased surgical risk. In older adults, low BMI is a strong predictor of frailty and mortality. The health risks of severe underweight (BMI <16) are comparable to those of severe obesity.
- How can I gain weight healthily?
- Focus on nutrient-dense, calorie-rich foods: nuts, avocados, whole grains, lean proteins, dairy, and healthy oils. Eat more frequently. Combine with strength training to build lean mass rather than just fat. Consult a dietitian for a personalized plan. If you've lost weight unintentionally, see a doctor to rule out underlying conditions.
Medical disclaimer: BMI is a screening tool, not a medical diagnosis. It does not measure body fat directly and does not account for muscle mass, bone density, age, sex, or ethnicity. Consult a healthcare provider for personalized health advice.
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