The Prophet (may Allaah raise his rank and grant him peace) ordered that a fasting person give out Zakaat al-Fitr at the end of Ramadhan – one saa’ of staple food on behalf of yourself and each person you are responsible for. 
What is a Saa’?
A Prophetic saa’ ( صاع ) is not a weight measurement. It was and still is a measurement of volume, similar to the size of a large salad bowl. It is made up of four mudds, and a mudd ( مد ) is a smaller container, close to the size of a small salad bowl.
To be precise, a Prophetic mudd in modern volume measurements is .75L (or 750mL), which means that a saa’ is three litres.
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