TurboTax uses the same calculators that the IRS has to determine the withholding, but it is still a moving target unfortunately. Has your income changed year to year? This is usually the biggest factor to an extent, especially if you are straddling tax brackets. Also, if you have income other than just W2s, the formulas get difficult. I would recommend reviewing your withholding at the midpoint of the year, with the focus on the total income. Then you can make an adjustment if needed.
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