Charity (tzedakah) is an integral part of both Jewish life and Jewish law. And while giving charity is most certainly a value that goes beyond the bounds of any specific religion, in Jewish law charity is mandated via maaser (tithing one’s income).
So great is the act of giving charity that the sages even noted that it protects a person from death (Talmud Shabbat 156b). Knowing that tzedakah has such incredible powers of protection, one might be tempted to give away everything except for one’s most basic necessities. The sages recognized humankind’s tendency toward zealousness and therefore quotes Rabbi Elai in Talmud Ketubot (50a): “If a man desires to spend liberally [for charity], he should not spend more than a fifth [of his wealth], lest he might himself come to be in need of people....What [is the proof from] the Torah? ‘And of all that You will give me, I will surely give the tenth to You’” (Genesis 28:22).
In Hebrew, “I will surely give the tenth to You” is written aser ah’ahsrenu lach, repeating the root word for tenth, which led the sages to conclude that one may give two tenths, or 20% (with the second equal to the first).
From this discussion it is understood that at a bare minimum, one should give ten percent to charity. One who wishes to be generous should give another 5 percent to charity, and even possibly an additional 5%. Twenty percent, however, is the line that is drawn between true charity and irresponsibility. Even then, giving twenty percent to charity is reserved for truly wealthy folks for whom such generosity will not endanger their own financial well-being.