One of my friends recently asked the following question about adjusting the withholding on his paycheck in order to minimize overpaying his income tax:
We recently just bought a house. We’ve never itemized my taxes before because we didn’t have enough to come close to the standard deduction. Now, however, clearly we will. We’ll have approximately $30k in write-off’s between the property tax and mortgage interest. The difference between this and the standard deduction (~$10k) is ~$20k. Using the 25% tax bracket, this means we’ll get back $5k in taxes (compared to last year). How much should I adjust my w4 exemptions so that we can get this money in our pay checks instead of a big check from the fed’s at the end of the year?
The gist of the scenario is that the difference between this year and last year is that this year he will be able to include $30,000 in itemized deductions rather than take the $10,000 from the standard deduction. How should he adjust his Form W-4 exemptions so that he doesn’t unnecessarily overpay his taxes through withholding?
The simple, for me, answer is to turn to the IRS’s 2008 Withholding Calculator  because that’s the official answer (but that would be too easy!). The “rule of thumb” is that each of the exemptions on your W-4 represents a dependent deduction and each dependent deduction is worth approximately $3,400. The purpose of the W-4 was so that you could indicate how many dependent children you had and so you can repurpose this to cover all the other deductions you have on a basis of ~$3,400 per exemption. Based on the rule of thumb, the answer appears to be 9 exemptions.
Now comes the question of whether the couple both puts 9 or if one person puts 9 and the other puts 1. Since you’re adjusting your total tax liability between two people, I believe the answer is that one person puts 9 and the other puts in 1 but I’m not 100% sure. If both were to claim 9 exemptions (total of 18!), you would double counting and thus underpaying by 9 exemption’s worth. I’m not a tax expert but a few readers are, so hopefully someone can chime in on whether this is correct.
Ultimately, to be 100% sure, I’d check the IRS 2008 Withholding Calculator  and then speak to a professional tax accountant.