exemptions, you literally put dollar amounts for dependent credits, deductions, additional withholding etc. The numbers, though dollar amounts, are really just input variables into a fairly complicated formula so I'm not sure it's really all that helpful that they changed it. Essentially what amounts to is, the W4 is like a predicted tax return. You're basically doing your taxes ahead of time. Which is why if you only have one income and you put in the correct values per the instructions it should be fairly accurate. If you itemize deductions then you may want to add additional dollars into the deductions line.