The computations aren't quite mind-numbing --- I could introduce her to section 751(b) or the computation of the section 743 optional basis adjustment --- but they are a bit tedious. I will share my response to her, somewhat revised:
1. Do a pro forma tax return for 2008. In other words, figure out what your tax liability will be on your summer earnings (and other income, if any).As an example, assume a law student obtains a job paying $2,000 per week for 10 weeks during the summer. If the student claims zero exemptions and the employer withholds based on $2,000 of wages per week, the employer will withhold $433.71 per week ($2,000 minus $451.04, or $1,548.96, multiplied by .28). That is a total of $4,333.71.
2. Determine whether you are paid weekly, bi-weekly, monthly, etc.
3. From that determination, compute how many paychecks you expect.
4. Divide the tax liability by the number of paychecks to get the "need to withhold per paycheck" amount.
5. Divide the expected earnings by the number of paychecks to get the "wages per paycheck" amount.
6. Find the IRS publication that has the employer withholding tables in it. The current one appears to be this one.
7. Get to the correct chart (weekly, bi-weekly, etc) that reflects your filing status. These begin on page 28.
8. For each possible number of withholding allowances, find the line that contains the appropriate "wages per paycheck" amount from step 5, and compute the withholding that would occur if that number of allowances were claimed.
9. Find the withholding that is closest to the "need to withhold per paycheck" amount computed in step 4. Identify how many withholding allowances correlates with that amount.
Note that if you get 10 or more exemptions, the employer might balk because the claiming of that many exemptions has to be reported to the IRS so that they can spot tax protesters who claim 10, 20, 30, 100, etc.
The student, however, will have gross income of $20,000, a standard deduction of $5,450, and a personal exemption of $3,500. The taxable income will be $11,050. The tax will be $1,256.25 ($802.50 plus 453.75 (15% of $3,025 ($11,050 minus $8,025))). The student will be overwithheld by $3,077.47 ($4,333.71 minus $1,256.24). What the student needs is withholding of roughly $125.63 per paycheck. Claiming one exemption won't work. It would generate weekly withholding of $414.86 ($2,000 minus $518.35, or $1,481.65, multiplied by .28). Claiming 9 exemptions gets the student closer, by generating a weekly withholding of $268.25 ($2,000 minus $926.99, or $1,073.01, multiplied by .25).
The student does not qualify for exemption from withholding, nor would the student want to take that position, because the student will have a tax liability for 2008. Instead, the student needs to claim more exemptions. Each exemption over 9 has a value of $67.31 and thus reduces withholding by $16.83. To get from $268.25 down to $125.63, the withholding needs to be decreased an additional $142.62, which requires 8 additional exemptions ($142.62 divided by $16.83). So the student would need to claim 17 exemptions. Unfortunately, that makes the student appear to be a tax protester!
The problem is that the withholding tables assume that the student will earn $2,000 each week for the entire year, for a total salary of $104,000. Thus, the withholding is based on the rates applicable to a taxpayer in the tax bracket correlating to income of that level, whereas the student will end up in a lower tax bracket. This scenario plays out repeatedly, as corroborated by the number of students who approach me after I mention the issue in the basic tax class. By focusing their attention on the interest-free loan they are making to the government at a time when they are strapped for cash, I succeed, at least to some extent, in elevating their awareness about the relevance of taxation no matter the area of law in which they wish to practice.
It isn't a problem limited to law students. It affects a variety of people, chiefly those who work for part of the year or who experience an significant increase in income part of the way through a year. They can let things play out as the withholding tables dictate. Or they can go through the arithmetic gymnastics that I have described. Or, I suppose, they can pay or persuade someone to perform the gymnastics for them.