Quick answer: 50 kg at 170 cm gives a BMI of 50 ÷ 1.70² = 17.3, which is mild thinness (17 to 18.49); BMI 18.5 at that height is 53.5 kg.
How it's calculated
BMI = weight (kg) ÷ height (m)². The WHO calls a BMI below 18.5 underweight and divides it into three grades of thinness: mild (17 to 18.49), moderate (16 to 16.99) and severe (below 16).
The calculator also shows the weight that gives a BMI of 18.5 at your height (18.5 × height²), the lower edge of the normal range.
Some people are naturally slim and healthy at a BMI just under 18.5, while a low BMI can also come with illness, an eating disorder or poor nutrition. BMI alone cannot tell these apart, and it says nothing about bone density, muscle or recent weight change, which a doctor or dietitian would look at. The healthy weight calculator shows the full range for your height.
Worked examples
- 50 kg, 170 cm: 50 ÷ 1.70² = 17.3 (mild thinness)
- 45 kg, 165 cm: 45 ÷ 1.65² = 16.5 (moderate thinness); BMI 18.5 = 50.4 kg
- 60 kg, 170 cm: 60 ÷ 1.70² = 20.8 (normal weight)
WHO thinness grades for adults
| BMI | Grade |
|---|---|
| 17 – 18.49 | Grade 1: mild thinness |
| 16 – 16.99 | Grade 2: moderate thinness |
| Below 16 | Grade 3: severe thinness |
Frequently asked questions
What BMI is underweight?
For adults, a BMI below 18.5 in the WHO classification.
Is being underweight a health concern?
It can be. A low BMI is linked with weaker immunity, bone loss and, in women, menstrual changes. A doctor can check whether there is an underlying cause.
Does it differ for men and women?
No. The adult BMI cut-offs are the same for both.
How can I gain weight in a healthy way?
Add regular meals and snacks with nutritious, energy-rich foods such as nuts, dairy, eggs, olive oil and whole grains. Strength training helps the gain include muscle.
What can cause being underweight?
Genes and a fast metabolism can play a part, but so can low appetite, stress, thyroid or digestive problems, or an eating disorder. Unexplained weight loss should be checked by a doctor.
How much do I need to gain to reach a healthy BMI?
Multiply 18.5 by your height in metres squared, then subtract your weight. At 170 cm that is 53.5 kg, so someone weighing 50 kg needs about 3.5 kg more.