The dough must contain 43.2 beitzah of flour [the gematria of חל"ה ] to require hafrashas challah (שו"ע סי' שכ"ד ס"א ). The foremost poskim differ as to the modern-day measurements. As a result, there is an amount from which challah is separated as a chumra without a bracha and an amount from which challah is separated with a bracha.
The poskim differ about this too. There are primarily two shiurim of wheat flour [see table in Issue 333 for other types of flour]. The opinion of Rav Chaim Na’eh, which is the minhag Yeruhalayim, is to separate challah without a bracha from dough containing 1.25 kg of flour, and with a bracha from 1.67 kg (מו"ר הגרי"י בלויא בלקט העומר פ"ה ס"ב ).
According to the Chazon Ish, the dough must contain 1.2 kg of flour to require hafrashas challah without a bracha, and 2.25 kg with a bracha.
Thus, to satisfy all opinions, many people do hafrasha without a bracha from 1.2 kg [approx. 10 US cups] of flour or more. They only do hafrasha with a bracha from more than 2.25 kg [approx.. 19 US cups], in accordance with the rule of ספק ברכות להקל .
There is no need at all to separate challah from less than 1.2 kg of wheat flour, and if one did, it is not considered challah (שם ס"ג ).
Occasionally, a dough will have less than the minimum amount of flour, but while kneading, a woman adds some flour here and there. She should take care not to add enough flour to require hafrashas challah. If she is uncertain, she should separate challah without a bracha.
