Underweight Calculator – Low BMI
Check whether BMI is below 18.5, the degree of underweight according to the WHO and how many kg are missing for the minimum healthy weight.
Tip: Healthy weight gain is done with more frequent meals, calorie-dense foods (nuts, olive oil, dairy) and strength training — not with processed food.
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About this digital tool
Underweight means a BMI below 18.5. The World Health Organization divides it into three degrees: mild (17–18.49), moderate (16–16.99) and severe (below 16). This calculator places your BMI in one of them and shows you how many kilograms are missing to reach the minimum healthy weight for your height.
Being underweight can come from a fast metabolism, illness, stress or problems with food. BMI alone cannot diagnose eating disorders such as anorexia — if you are afraid of gaining weight, restrict food or are worried about your body, talk to a doctor or a psychologist; help works and starts with a conversation.
How to use
- Enter your current weight in kg
- Enter your height in cm
- See your BMI and the degree of underweight
- Read how many kg are missing for BMI 18.5
- If you come out underweight, talk to a doctor about the cause and the plan
Frequently asked questions
Which BMI is considered underweight?
Any BMI below 18.5. The mild degree is 17–18.49, moderate 16–16.99, severe below 16. Severe underweight requires a medical assessment.
Does a low BMI mean I have anorexia?
No. Anorexia is a medical disorder diagnosed by specialists from behaviour and thoughts, not just from the number on the scale. Many people have a low BMI without having a disorder, and anorexia can exist even with a normal BMI.
How many kg should I gain to reach a normal weight?
The calculator shows the difference up to BMI 18.5. Safe gain is usually 0.25–0.5 kg a week, preferably under a doctor's supervision.
What are the risks of being underweight?
A lack of vitamins and iron, a weak immune system, fragile bones (osteoporosis), menstrual cycle disorders in women, fatigue and slower recovery after illness.
Who should I turn to if I'm worried?
Your family doctor, who can refer you to a dietitian or psychologist. In urgent cases or thoughts of self-harm, contact the emergency service.