Terumos chapter 1, mishnah 7. The mishnah establishes that one who separates Terumah takes its measure by estimate, and he is not required to measure out precisely two percent of what he separates.
Two approaches to the reason for this:
For the benefit of the kohen: A person who is not sure of the exact amount tends to round upward, and this works out to the kohen's advantage.
For the benefit of the one separating: All the mitzvos are meant for a person's own good. The essence of giving Terumah is setting aside a portion for the Holy One, Blessed is He, or for His representative, the kohen, and therefore it is fitting that it come from the giver's own will. Although the obligation to separate rests upon him, the amount is left to his estimate and no precise quantity is forced upon him. This discretion makes it easier for him, encourages him to give more, instills in him the quality of giving, and awakens within him gratitude to the Holy One, Blessed is He, through the giving of the Terumah.
Either way, the separation is done by estimate, and therefore we learned "ein torimin" - one does not take the Terumah in a precise fashion:
"lo b'midah" - not by an exact measure of volume, such as exactly six cups.
"lo b'mishkal" - not by an exact weight, such as exactly six kilograms.
"lo b'minyan" - not by counting items, such as exactly six items.
A person estimates roughly two percent of what he is separating, and if he goes above that amount or falls below it, it makes no difference. In the words of the Tiferes Yisrael, there is no concern here: his best estimate is enough.
It is worth remembering that by Torah law any amount of Terumah suffices to exempt the produce and to be considered Terumah. It emerges that anything given beyond the measure of a single kernel is not required by the Torah but by rabbinic law, and it was the Sages who established that the separation be made by estimate. Although a verse is cited as a support for giving by estimate, it seems that this is only an asmachta, in the way that a verse is brought to support a rabbinic obligation.
"Aval torem hu es hamadud v'es hashakul v'es haminuy":
While the first part of the mishnah taught that the Terumah itself may not be measured precisely, the latter part teaches that the estimate itself may be based on a known and precise quantity. For example, one first measured and found that there are a hundred gallons of wine here, and on that basis he separates an estimated amount of about two gallons for the kohen.
The Orach Chaim holds that this is even the preferable way: it is better to know the total quantity of the produce, since this knowledge allows for a reasonable estimate of two percent. Someone who does not know how much grain is lying on the threshing floor has almost no way of correctly estimating what two percent of the pile is; but once he has measured, he knows the target, and he estimates what appears to him to be two percent of it.
"Ein torimin b'sal uv'kupah shehem shel midah":
One may not use a vessel whose volume is known and whose capacity is fixed at the time of separating Terumah. For example, a person who does not know how much grain he has, and estimates that about two percent is roughly ten gallons, should not measure it out with vessels that hold a gallon each. True, this is still an estimate, since his starting point is unknown to him and he only judged by eye that there are about five hundred gallons here and therefore gives ten, and perhaps he should have given more or less; but someone who sees him measuring out exactly ten gallons will think that he is measuring precisely. Because of the mistaken impression of onlookers, it was established not to use measuring vessels of fixed and known volume.
"Aval torem hu bahen chatza'in u'shelishin" - but one may separate Terumah with them in halves and thirds:
One may use vessels whose capacity is known, provided that he does not fill them to the top. For example, one may fill a vessel that holds a gallon to about halfway, and that is his estimate of roughly half a gallon. Since this is not a precise half gallon, an onlooker will not come to think that an exact measurement was taken here.
"Lo yitrom b'se'ah chetzyah, shechetzyah middah" - he may not separate Terumah with a se'ah vessel filled to half, because its half is itself a measure:
The se'ah served as the standard measure for large quantities of volume, and it can be compared to a case of six two-liter bottles - about twelve liters. The standard se'ah measuring vessel had a mark at half its height, indicating half a se'ah.
Therefore, filling the se'ah vessel up to the mark at its midpoint is like using a measuring vessel in full. Although the earlier words of the Mishnah taught that one may fill a measuring vessel to half, since that is still only an estimate, in the se'ah vessel - which has a fixed mark at half its height - this is considered a precise measurement. This is what the Mishnah means when it says that one may not use the full se'ah vessel in order to measure out half of it, since the half in this vessel is a precise measure in its own right.
In summary: In this Mishnah we learned that Terumah is separated by estimate and not by measure, weight or count - whether this works to the advantage of the kohen or to the advantage of the one separating; that one may base his estimate on knowing the total quantity, and according to the 'Orach Chaim' this is even the preferable way; and that one does not separate Terumah with vessels whose capacity is known, because of the appearance it creates, unless he fills them in halves and thirds - excluding the se'ah vessel, whose half is a precise measure in its own right.