Aruch HaShulchan, Laws of Tzedakah 249:7

It is a simple matter that yearly income is calculated each and every year from Rosh haShanah to Rosh haShanah. If, in this year, a person had been doing business in such a way as to have both income and loss, do a general calculation, and whatever remains of that person’s income, take a tenth for tzedakah. In general, income is considered only the actual income. All business expenses, even food and drink while traveling, are considered to be business expenses and all are deductible. The remainder of the income, after the deduction of all the expenses, is what is considered “income” (by income, they mean profit.) However, household expenses may not be deducted; therefore, one whose income is a thousand gold pieces in a year gives a hundred or two hundred to tzedakah even if the household expenses are more than a thousand gold pieces. In any case, that person is obligated to give a tenth of their income, and only those household expenses which they give to tzedakah can be considered a deductible. For example, if that person had divided bread on Monday and Thursday, or had given some change to the poor, or food one day to Talmud students, or welcomed a poor guest on Shabbat or for a festive meal as is the way of the Jews, then one could deduct this from a portion the tenth.... [Translation by Rabbi Bruce Elder. Edited for gender neutrality]