[Chicago, IL] [income tax] I'm technically full-time at Company A but I spend one day a week at Company B so I had to fill out two w-4s.

Uh, I don't think this is correct. The "0" you're filling in is the number of exemptions you're entitled to (if you're talking about the additional withholding section, my apologies). Claiming "0" is the lowest number of exemptions, meaning they take out the greatest amount of taxes in your check. If you put in "1", they should take out less money than if you put in 0. Check the IRS publication here: "Each allowance [on your W4] reduces the amount withheld."

Neither of these things actually changes the amount of taxes you pay. You either get your money now or you get it with your tax return--the math is the same. I'm just explaining that you should fill out the number appropriately for your filing status based on the instructions in your form (which should also cover multiple employer situations, if I recall). If you end up having small amounts of money from other sources here and there that you did not have taxes withheld on, it makes sense that you might keep the number of exemptions as low as possible (0) so that the taxes taken out of your check will offset the taxes owed on the additional income without having to file additional paperwork (1040-ES, etc.).

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