April 10, 2018 Following tax law changes, you should do a paycheck checkup using the IRS’s Withholding Estimator and, if necessary, complete a new W-4 form. The estimator helps determine the right amount of withholding. You should check if you: Are a two-income family. Have two or more jobs at the same time or only work part of the year. Claim credits like the child tax credit. Have dependents age 17 or older. Itemized deductions in 2017. Have high income or a complex tax return. Have a large tax refund or tax bill for 2017. See IRS.gov/taxreform for more information.