After second-quarter taxes are over and I’m caught up on extended tax returns, I plan to reach out to clients who got surprised with a 2018 tax bill and do a “Paycheck Checkup” with them, to make sure the same thing doesn’t happen for 2019. I’ve had a couple clients already meet to go over their tax returns to figure out “what happened?” The results have most commonly stemmed from the limitation on state/local/property tax deductions to the elimination of the unreimbursed employee expense deduction. However, in some cases it was simply that the new TCJA tax law is too complex to use a regular W-4 to calculate withholding.

So, if you haven’t done a Paycheck Checkup, consider now a good time for it, before it gets too deep into the calendar year to make up the difference via paycheck withholdings.

Source: Done with taxes this year? Use 2018 return to get 2019 withholding right | Internal Revenue Service