The government doesn't withhold money from your check, your employer (or pension plan) does based upon how you completed your withholding authorization (form W-4). You can find an IRS withholding calculator at: IRS Withholding CalculatorTo come up with a dollar estimate for how much you'll need to have withheld, a simpler calculator can be found at: 2018 Tax Reform Calculator | Trump Tax CalculatorIf you're not having enough withheld, instead of adjusting the number of exemptions, calculate the additional dollar amount you'll need to withhold for the year (a), determine the number of pay days you have remaining in the calendar year (b), and divide (b) into (a). The number which results is the additional dollar amount you should have withheld from each of your checks for the rest of the year. See W-4: https://www.irs.gov/pub/irs-pdf/...To stay up on this, you'll need to run these numbers a couple of times a year, and over time, you'll get a good sense of the adjustments you'll need to make.At the beginning of last year, the tax reform calculator showed that if my income stayed the same, I should go from paying an additional $275 at year end to receiving a refund of around $1,000.To test my assumptions, I maintained all of my withholdings at the same dollar amount which required that I adjust my additional withholding to make up for the adjustment in the federal withholding tables. In 2018 my income (AGI) increased by $5,000 for the year, so my actual refund would be less than the tax reform calculator estimate.I have filed my taxes and found (1) the tax reform calculator refund calculator was remarkably accurate, (2) even though my AGI increased, the difference between my additional tax payment and my 2018 refund was approx. $640.I came out ahead even though I lost my deductions for my state income taxes (in a high tax state), my property taxes, and my vehicle taxes.I plan to move sometime this year, so I'm going to leave my withholding alone for now.