What the Bible says about Tithe on Increase
(From Forerunner Commentary)
God's Word reveals we are to tithe on the increase (profit) we receive as a result of our productive effort (Leviticus 27:30-33; Deuteronomy 14:22). Therefore, a person working for wages—the vast majority of us—would tithe one tenth of the total amount of his pay before income tax, Social Security, or other deductions are removed. Other tithable income would include capital gains from investments, dividends from stock, and interest from bank accounts. The value of products used out of the garden or field should be included in figuring total income also (Matthew 23:23).
Income that is not tithable include Social Security benefits, welfare, gifts, unemployment or disability compensation, and loans that must be repaid. A general rule of thumb is that any income not earned is not tithable.
A farmer or person in business for himself should tithe on the income from his crops or business after operating expenses but before deducting personal living expenses or taxes.
John O. Reid
Tithing: God's Financial System