The First Presidency has answered this question in this way: “The simplest statement we know of is the statement of the Lord himself, namely, that the members of the Church should pay ‘one-tenth of all their interest annually,’ which is understood to mean income. No one is justified in making any other statement than this” (First Presidency letter, Mar. 19, 1970).
In other words, the way you define your income, and consequently your tithing, is a matter between you and the Lord. Prayerfully seek the Lord’s guidance on issues like taxes, gifts, scholarships, and other matters to determine what qualifies as a full tithe.