According to halacha one must give a tenth of his income to tzedaka. How does one calculate what amount the tenth is calculated on?

Giving 10% of one’s income to Tzedaka according to many Poskim is only a minhag (Maharam MiRotenburg Prague 74) and according to others it is a rabbinical commandment. Since there are many poskim who say the 10% should be calculated after necessary household expenses are deducted (among them the Shulchan Aruch himself in Shut Avkat Rachel 3), their opinion should be accepted. Also expenses on your children over the age of six may be considered Tzedaka(Rabbi Ovadia Yosef Shut Y’Chave Daat 3, 76; Shut Tzitz Eliezer 10, 6).