The Bible reveals that we are to tithe on the increase we receive, that is, the result of our productive effort (Leviticus 27:30-33; Deuteronomy 14:22). Therefore, a person working for wages would tithe one tenth of the total amount of his pay—before income tax, social security, or other deductions are removed.
Tithable income also includes such sources as capital gains from investment property, dividends from stocks, and interest from bank accounts. It does not include social security benefits, welfare pensions, gifts, or loans that must be repaid.
A farmer or a person in business should tithe on the income from his business or crops after operating expenses (but not personal living expenses or personal taxes) are deducted. Also, the value of products used out of the garden or field should be included in figuring total income, because we must not omit paying tithes on that part (Matthew 23:23). The value of garden produce should be figured according to the wholesale price, the price one would receive if he sold it.