21.3 grams

Petri dish, glass tube and plastic bag — 2018 

21.3 grams refers to the study published in 1907 by Duncan MacDougall. Six human subjects were weight before and after death to determine if a soul has physical weight. One of the six patients lost three-quarters of an ounce (21.3 grams). Even flawed (due to the small sample size, the methods used, as well as the fact that only one of the six subjects met the hypothesis), this unscientific experiment popularized the concept that a soul had weight.