Adult healthy body mass index is 18.5-23.9, so yours is normal.
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Body mass index calculation method
The formula is: body mass index (BMI) = weight (kg)/height (m). For example, a person's height is 1.75m, his weight is 68kg, and his body mass index =68/( 1.75)? =22.2 (kg/m), normal when the index is between 18.5-23.9.
Body mass index value-Baidu Encyclopedia