The unit of volume used to measure the amount of Zakat al-Fitr is the sa'a, also written sāʿ or saʿ.
Classical Islamic law traditionally specifies one sa'a of staple food for each eligible person. Reports associated with the Prophet Muhammad describe the payment as a sa'a of foods such as dates, barley, raisins or dried cheese. The unit was a volumetric measure rather than a fixed modern weight, so its exact mass depends on the food being measured.
A sa'a is commonly described as four mudds, with a mudd representing a smaller measure associated with the amount held by two cupped hands. Modern conversions vary: estimates for a prophetic sa'a often range from roughly 2.6 to 3 kilograms of grain, while volume estimates may be around 3 to 4.2 liters depending on the historical standard used.
The sa'a is not the same as a dinar or dirham, which were historical monetary units. Mudd is related to the calculation but is not the standard total amount named in the question. Some legal schools and contemporary authorities allow cash equivalent payments, but the classical measure remains a sa'a of food.