Quantity of Zakathul Fitr

بِسۡمِ ٱللَّهِ ٱلرَّحۡمَٰنِ ٱلرَّحِيمِ

Zakât al-Fitr becomes due at the end of Ramadan. It is obligatory upon every person who has enough wealth to suffice him for the day and night of `Id.

The Messenger of Allah (peace be upon him) enjoined the payment of one Sa‘ of dates or one Sa‘ of barley as Zakat-ul-Fitr on every Muslim slave or free, male or female, young or old, and he ordered that it be paid before the people go out to offer ‘Eid Prayer.

Al-Muwatta*, vol. 1, p. 284; Ahmad, vol. 2, pp. 5, 55, 63, 66, 102, 114, and 137; Al-Bukhari, vol. 2, pp. 138-140; Muslim, vol. 2, pp. 677-678, no. 984; Abu Dawud, vol. 2, pp. 263-265, nos. 1611 and 1612; Al-Tirmidhy, vol. 3, p. 61, nos. 675 and 676; Al-Nasa*y, vol. 5, pp. 47-49, nos. 2500-2505; Ibn Majah, vol. 1, p. 584, nos. 1825 and 1826; Al-Darimy, vol. 1, p. 392; Al-Daraqutny, vol. 2, pp. 139-141, no. 143; Ibn Hibban, vol. 8, pp. 94-97, nos. 3300-3304; Ibn Khuzaymah, vol. 2, pp. 80-84 and 87, nos. 2392, 2393, 2395, 2397, 2399, 2403, and 2411; Ibn Abu Shaybah, vol. 3, p. 172; Ibn Al-Jarud, vol. 2, p. 19, no. 356; and Al-Bayhaqy, vol. 4, pp. 162-166.

Abu Sa‘id Al-Khudry (may Allah be pleased with him) is reported to have said:  We used to pay Zakat-ul-Fitr as one Sa‘ of food, or one Sa‘ of dried dates, or one Sa‘ of barley, or one Sa‘ of raisins, or one Sa‘ of curd during the lifetime of the Messenger of Allah (peace be upon him). (Related by both Al-Bukhari and Muslim)

         
The amount that should be paid is one sâ` of a staple foodstuff. That means food such as rice, corn, dates, wheat, etc. It should be given on behalf of oneself and every member of the family that one is liable to support.

This sâ` is a measure of capacity equivalent to four double-handfuls.

Sheikh Muhammad Al-Jibâlî writes in Celebrations in Islam:

حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا ابْنُ دُكَيْنٍ، حَدَّثَنَا سُفْيَانُ، عَنْ حَنْظَلَةَ، عَنْ طَاوُسٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ الْوَزْنُ وَزْنُ أَهْلِ مَكَّةَ وَالْمِكْيَالُ مِكْيَالُ أَهْلِ الْمَدِينَةِ ‏"‏

Narrated Abdullah ibn Umar:

The Prophet (ﷺ) said: (The standard) weight is the weight of the people of Mecca, and the (standard) measure is the measure of the people of Medina.

Abu Dawud said: Al-Firyabi and Abu Ahmad have also transmitted from Sufyan in a similar way, and he (Ibn Dukain) agreed with them on the text. The version of Abu Ahmad has: "from Ibn 'Abbas" instead of Ibn 'Umar. It has also been transmitted by al-Walid b. Muslim from Hanzalah. This version has: "the weight of Medina and the measure of Mecca."

Abu Dawud said: There is a variation in the text of the version of this tradition narrated by Malik b. Dinar from 'Ata' from the Prophet (ﷺ).   [Sunan Abi Dawud 3340,  Sunan an-Nasa'i 2520]

    Therefore, the amount should be measured according to the sâ` of Madinah, which is still available to this time. However, this measure is not accessible to most people. Because of this, many scholars have estimated it approximately to four full double-handfuls (i.e. with the hands put together) of an average man. [Refer to al-Nawawî in al-Majmu’]

    This demonstrates that it is wrong to measure Zakat al-Fitr by weight, because the weight of one sâ` varies for different materials. However, if the relationship between weight and capacity is determined for a specific foodstuff, then the equivalent weight of one sâ` of that material may then be used for that particular food.

Al-`Uthaymîn, in his fatwâ on the matter mentions that the sâ` is four full double-handfuls and then goes on to say that scholars give an approximate weight value for rice to be roughly 2.1 kilograms and for dates to be roughly 2.04 kilograms.

And Allah knows best.
http://en.islamtoday.net/quesshow-182-637.htm

The estimation of Shaykh Ibn Baaz (may Allaah have mercy on him), who reckoned the weight of zakaat al-fitr as being approximately three kilograms.
This was also the estimate of the scholars of the Standing Committee (9/371).

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) estimated it in grams as being 2100 grams, as it says in Fataawa al-Zakaah, p. 274-276.

This discrepancy occurred because a saa’ is a measure of volume, not weight.

 The scholars worked it out by weight so that it would be easier to work out. It is well known that the weight of grains varies; some are light and some are heavy, and some are in between. In fact the weight of a saa’ of the same kind of grain may vary, and new may weigh more than old. Hence if people err on the side of caution and give more, that will be better.
https://islamqa.info/en/49793

1 Sa‘ = 2.172 kg

http://alifta.com/Fatawa/FatawaChapters.aspx?languagename=en&View=Page&PageID=57&PageNo=1&BookID=10

http://www.bakkah.net/en/zakat-fitr-measurements-saa-three-litres-mudd.htm