Anorexic BMI Calculator — Free Underweight Assessment Tool

How to Use the Anorexic BMI Calculator

1. Select your preferred unit system: Click the toggle at the top to choose between Metric (kilograms and centimeters) or Imperial (pounds, feet, and inches). The calculator adjusts all input fields to match your selection. Both systems produce identical BMI results after conversion.
2. Enter your weight: Type your current body weight into the weight field. For the most consistent readings, weigh yourself at the same time each day, preferably in the morning before eating. The calculator accepts values from 20 kg (44 lbs) upward.
3. Enter your height: Input your height in the appropriate fields. In Metric mode, enter centimeters directly. In Imperial mode, enter feet and inches separately. Stand barefoot on a flat surface for the most accurate height measurement.
4. Review your assessment: The results panel immediately shows your BMI value, WHO underweight severity classification (Mild, Moderate, or Severe Thinness), and a personalized recommendation. If your BMI indicates underweight status, the calculator also displays how much weight you would need to gain to reach the lower boundary of the healthy BMI range (18.5). Crisis helpline numbers are displayed alongside the calculator for immediate access to support.

This calculator is designed as a health awareness and screening tool. It does not diagnose eating disorders, which require comprehensive clinical evaluation. If your results concern you, please reach out to a healthcare provider or one of the helpline numbers provided.

BMI Formula and Underweight Classification

WHO Underweight Severity Levels

• BMI 17.0 - 18.4 (Mild Thinness): Slightly below the normal range. May indicate insufficient caloric intake or a naturally lean build. A healthcare provider can help determine whether intervention is needed.
• BMI 16.0 - 16.9 (Moderate Thinness): Significantly below normal. Associated with increased risk of nutritional deficiencies, weakened immunity, and bone density loss. Medical consultation is recommended.
• BMI below 16.0 (Severe Thinness): Critically low BMI associated with serious health risks including organ damage, cardiac complications, and severe malnutrition. Immediate medical attention is strongly recommended.

Step-by-Step Calculation Example

Suppose someone weighs 48 kilograms and is 168 centimeters tall:

1. Convert height to meters: 168 cm / 100 = 1.68 m
2. Square the height: 1.68 x 1.68 = 2.8224
3. Divide weight by height squared: 48 / 2.8224 = 17.0
4. Classify: BMI 17.0 falls at the boundary of Mild Thinness (17.0-18.4)
5. Weight needed for BMI 18.5: 18.5 x 2.8224 = 52.2 kg, so 52.2 - 48 = 4.2 kg to gain

This individual would need to gain approximately 4.2 kg to reach the lower end of the healthy BMI range. A registered dietitian can create a safe, gradual weight gain plan tailored to individual needs and preferences.

Practical Examples

Example 1: Maria's Health Awareness Check

Maria is a 24-year-old college student who has been feeling fatigued and frequently catching colds. She weighs 50 kg and is 165 cm tall. Using the calculator:

• BMI: 50 / (1.65)² = 50 / 2.7225 = 18.4
• Classification: Mild Thinness (17.0-18.4)
• Weight to reach BMI 18.5: 0.3 kg

Maria's BMI of 18.4 is just below the healthy range. Her doctor recommends a nutritional evaluation to ensure she is getting adequate calories and nutrients. Adding 200-300 daily calories through nutrient-dense snacks could help her reach and maintain a healthier weight.

Example 2: Tom's Recovery Monitoring

Tom is a 30-year-old working with a treatment team as part of his recovery journey. He currently weighs 55 kg at 180 cm tall:

• BMI: 55 / (1.80)² = 55 / 3.24 = 17.0
• Classification: Mild Thinness (borderline Moderate)
• Healthy range weight: 59.9 kg - 80.7 kg
• Weight to healthy BMI: 4.9 kg

Tom uses the calculator to track his progress alongside his treatment team. His dietitian has created a structured meal plan adding 400 calories per day, targeting a gain of approximately 0.5 kg per week. Regular monitoring helps Tom and his care team celebrate gradual progress toward a healthy weight.

Example 3: Aisha's Postpartum Assessment

Aisha is a 28-year-old new mother who experienced significant weight loss after childbirth due to the demands of breastfeeding and disrupted eating patterns. She weighs 45 kg at 160 cm tall:

• BMI: 45 / (1.60)² = 45 / 2.56 = 17.6
• Classification: Mild Thinness
• Healthy weight range: 47.4 kg - 63.7 kg
• Weight to healthy BMI: 2.4 kg

Aisha's doctor identifies that her caloric needs are elevated due to breastfeeding and recommends working with a dietitian to increase intake by approximately 500 extra calories per day. The calculator helps Aisha set a concrete, measurable goal for her weight restoration.

Underweight BMI Reference Table

Tips and Complete Guide to Understanding Underweight BMI

Recognizing Signs of Underweight

Beyond the BMI number, several physical and psychological signs may indicate that your body weight is too low. Physical signs include persistent fatigue despite adequate sleep, feeling cold when others are comfortable, hair thinning or loss, brittle nails, dry skin, dizziness upon standing, and frequent illness. Psychological signs may include difficulty concentrating, irritability, anxiety around food, and social withdrawal. If you recognize several of these signs in yourself or someone you care about, a comprehensive health evaluation can help identify the underlying cause and guide appropriate support.

Support Resources and Getting Help

If you or someone you know is struggling with low body weight or disordered eating, compassionate, confidential help is available. The National Eating Disorders Association (NEDA) helpline at 1-800-931-2237 connects callers with trained volunteers who can provide support and treatment referrals. The Crisis Text Line (text HOME to 741741) offers 24/7 text-based crisis counseling. SAMHSA's National Helpline at 1-800-662-4357 provides free, confidential referrals. Your primary care physician can also serve as a starting point for referrals to specialists including dietitians, therapists, and eating disorder treatment programs. Recovery is a process, and seeking help is a courageous first step.

Nutrition Strategies for Healthy Weight Gain

Gaining weight in a healthy way focuses on nutrient-dense foods rather than empty calories. Helpful strategies include eating smaller, more frequent meals throughout the day rather than three large meals, adding healthy fats like avocado, nuts, olive oil, and nut butters to meals, choosing calorie-dense whole foods such as whole grains, lean proteins, and dairy products, drinking calories through smoothies made with fruits, yogurt, and protein powder, and timing snacks between meals and before bed. A registered dietitian can tailor these strategies to your specific needs, preferences, and any underlying medical conditions.

The Role of Mental Health in Weight

Weight and mental health are deeply interconnected. Conditions such as depression, anxiety, obsessive-compulsive disorder, and post-traumatic stress disorder can all affect appetite and eating behaviors. Addressing mental health alongside nutritional needs is essential for sustainable weight restoration. Cognitive behavioral therapy (CBT), dialectical behavior therapy (DBT), and other evidence-based approaches have shown effectiveness in treating both eating disorders and co-occurring mental health conditions. Many treatment programs take an integrated approach that addresses both physical and psychological aspects of recovery simultaneously.

Common Mistakes to Avoid

• Using BMI as the sole measure of health: BMI is a screening tool that uses only height and weight. Body composition, blood work, functional capacity, and mental health all matter. A comprehensive health assessment provides a far more complete picture.
• Attempting rapid weight gain without medical guidance: Refeeding syndrome is a potentially fatal condition that can occur when severely malnourished individuals increase caloric intake too quickly. Always work with a healthcare provider when addressing significant underweight.
• Comparing your body to others: Everyone has a unique body composition influenced by genetics, age, activity level, and health conditions. What constitutes a healthy weight varies significantly between individuals.
• Relying on processed junk food for weight gain: While these foods are calorie-dense, they lack essential nutrients needed for recovery and long-term health. Focus on whole, nutrient-rich foods for sustainable, healthy weight gain.
• Ignoring emotional and psychological factors: If low weight is related to disordered eating, anxiety, or depression, nutritional changes alone are unlikely to be sufficient. Integrated treatment addressing both physical and mental health is most effective.

Frequently Asked Questions