Tithing, Net or Gross?
What I'd like to briefly present here is my own opinion on the topic. Again, this is my own opinion, and I won't be backing it up with any scriptural or General Authority references. "Gross income" is simple. What is the top line of your paycheck? What is your total salary? If someone is paid $10 an hour, and they worked 80 hours in a 2-week period, then their gross income is $800. If one were to pay tithing based on their gross income, they would pay $80.
However, there those who believe that tithing is to be paid on one's "net income". This one is not so simple. Net income is usually taken to mean Gross income minus withholding taxes. With our above example of a gross income of $800, we'll pretend the total withholding amounts for Federal Income Tax, Social Security Tax, Medicare Tax, and possibly State Income Tax is $100. $800 minus $100 equals $700. Tithing on net income is then paid in the amount of $70.
My biggest problem with "net income" tithe paying is knowing how to define "net". Why stop at withholdings? Withholdings are supposedly a fee for service, so why don't we also subtract other fees for service in our net income calculation? Why only fees for services? Why not fees for goods? Why don't we calculate net income by taking our gross income, subtracting withholdings, utility payments, food budget, housing, health-care expenses, etc.? In other words, why can't net income be described as whatever's left in our savings account?
Thankfully, I don't concern myself with such questions. I believe tithing should be paid on the amount of money earned by one's labor. Our ability to labor and other talents are a gift from God. All he asks in return (right now) is 10% of what that labor creates. If my employer pays me $10 an hour for my labor, then I will pay $1 an hour in tithing. In my view, paying tithing on gross income is the only option that makes sense from a principle point of view.
Update 12/13/12: I am currently of a different opinion than the above. I've been pondering lately the historical meanings of the words "increase," "gain," and "surplus" and am finding it very difficult not to conclude that they mean "income after all paid expenses." It's very likely that tradition has developed in a way that most Saints pay too much in tithing and not enough fast offerings and private giving. I have decided to experiment with the amount I give officially and unofficially in the coming year and to gauge accordingly the consequences.