The Bible does not specify how much dough is required for challah, but this issue is discussed in the Talmud. The rabbis said that 1 part in 24 was allocated to the priest in the case of private individuals, and 1 part in 48 in the case of a baker. If the baker forgets to set aside challah, it is permissible to set aside the same portion of bread.

According to the Talmud, the requirement to separate challah from the dough was imposed on the owner of the dough, not on the person who kneaded it; hence if the owner was not Jewish, even if the kneader was, hafrashat challah was not mandatory. The requirement did not apply to quantities of less than one omer in size, to bread prepared as animal feed; to dough prepared from a flour derived from anything other than wheat, barley, oats, spelt, or rye. Although the Biblical expression when you eat of the bread of the land might be understood as applying only to bread eaten in the Land of Israel, classical rabbinical sources argue that hafrashat challah should be observed in the Diaspora.

Since the destruction of the Temple, no one is considered ritually pure. The idea of "priestly descent" still exists, and the title of "cohen" is passed down from father to son, but there are no rites comparable to those practiced in the Temple. Hence the custom of separating "challah" is a symbolic act, with a blessing recited before the dough is separated and thrown into the fire or discarded.

Challah was a means of sustenance for the kohanim, who had no income of their own. This is a point upon which rabbinical sources and modern scholars agree. The Priestly Code, containing the law of challah, is believed by textual scholars to be a series of accretions to the earlier priestly source, and to postdate the law codes in the Torah, Thus the instruction concerning challah is believed to be a later development, perhaps reflecting the emergence of a full-time professional priesthood.

Other insights on the symbolism of challah appear in Midrashic and Kabbalistic literature. The mitzvah of separating challah is traditionally regarded as one of the three mitzvot performed especially by women (the others are lighting the Shabbat candles and family purity).