How does the weight of commodities affect the calculation of Zakat?
Chapter on Zakat on Crops and Fruits
Al-Mughni
Book of Zakat
Primary text
Volumetric goods differ in weight; some are heavy, such as wheat and lentils, some are light, such as barley and millet, and others are intermediate. Some scholars stated that the people of the Two Sanctuaries agreed that the Prophet's Mudd (a smaller measure) is one ratl and one-third of a ratl of medium wheat. Therefore, if the total wheat reaches sixteen hundred ratls, Zakat becomes obligatory. This implies that they assessed the Sa' using the heavy standard (wheat).
Supporting text
For light commodities, Zakat becomes obligatory when their amount approaches this weight standard, even if it does not fully reach it. If doubt exists regarding the obligation of Zakat for a commodity and no measure exists to determine it precisely, the precautionary action is to pay Zakat. However, if one does not pay, there is no blame because the default ruling is the non-obligation of Zakat, and obligation cannot be established merely by doubt.