Is the measure of two Qullahs a precise determination (Tahdid) or an approximation (Taqrib)?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
The sounder position is that the measure of two Qullahs is an approximation (Taqrib). This is because the narrators who described the capacity of the Qilal did not fix a precise boundary, stating one Qullah holds 'two Qirbs or something more' or 'I suppose two Qirbs.' Since the reports indicate approximation and uncertainty regarding any excess, and because the Qirb itself is not uniformly measured across regions, fixing a precise number is impossible. The intent was that one should perform Wudu from water if one judges it approximates two Qullahs and resists impurity, but refrain if it falls clearly short.
Supporting text
The opposing view considers it a precise determination because establishing a measure is often taken as a necessary precaution (Wajib), similar to counting the strokes in wiping the head during Wudu. If the measure is taken as precise, even a slight shortfall below the limit renders the water impure upon contact with filth.