3,382. Guesstimating Tithes

Hilchos Maaser 1:13

Only the best produce should be taken as tithes as per Numbers 18:30: “When you take the choice portion of it, it will be considered to the Leviim like the produce of the granary and the produce of the wine vat.” Just like the Leviim separate the best part of the tithes, the Yisroelim who separate tithes from the granary and the vat must separate the best produce.

Hilchos Maaser 1:14

One may not take tithes by estimate; rather, one must do so by measuring, weighing or counting. Being exacting in the measuring of tithes is praiseworthy; if one gives extra as tithes, his tithes are defective because untithed produce is mixed with them. Nevertheless, the produce from which he took it has been rendered fit.