Terumot, Chapter One, Mishnah Seven
As I stated in the introduction, the Torah does not establish a precise measure for how much terumah a person is supposed to give. The rabbis do set a measure, saying that 2 per cent is the standard measure, less is stingy and more is generous. Nevertheless, when one comes to give terumah, in order to retain some sense of the Torahs original meaning, one should not separate his terumah by measuring it in a precise fashion.
1) They may not give terumah according to measure, or weight, or number, but one may give it from that which has already been measured, weighed or counted.
2) They may not give terumah in a basket or a hamper of a measured capacity, but one may give in it when it is a half or a third filled.
3) He may not give a half of seah in a seah measuring vessel, for this half constitutes a known measure.
Section one: When one gives terumah, one should not give it by using a precise measure, such as a measuring cup, a scale or by the number of units he is giving. However, he may give the terumah from produce that has already been measured. He would just have to estimate exactly how much terumah he is giving, instead of giving it in by using a precise measure.
Section two: Although one shouldnt give terumah by putting the produce in a type of basket whose precise measure is known, he can give by filling up such a container to a half or a third full.
Section three: The one exception is giving half a seah in a vessel usually used to measure a seah. This is also a standard measure and thus one should not give terumah in such a manner.