The tax law passed late last year included slews of changes that will have wide impacts across the U.S. Among them is a likely jump in the numbers of Americans who'll owe taxes when they file 2018 returns. The Government Accountability Office (GAO) recently issued a report warning that more than 4.5 million taxpayers will come up short next April, unless they act now to adjust their withholding amounts.

This is because the tax law limited or even eliminated many itemized deductions claimed by millions of taxpayers -- nearly 28 million of them for 2017. The biggest contributors to this are the new limits on state and local tax deductions (the SALT deductions), a restriction on the amount you can deduct for home mortgage interest and the elimination of the deduction for job-related expenses.

Those most likely to owe tax for 2018 are those who itemize deductions. The GAO looked closely at this group and provided more clarity as to who it will include. Specifically, married taxpayers who itemize deductions, with two children under age 17, income exceeding $180,000 from one or more jobs and who have $20,000 or more in nonwage income (dividends, interest or capital gains) are highly likely to have to pay additional tax when they file next year.

The IRS recommends that taxpayers who itemize should do some planning now to avoid the sticker shock of a large tax bill for 2018.

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They're urged to use the Withholding Calculator on the IRS website to perform a "paycheck checkup" now. This free tool will ask you to enter estimated values for your income in 2018, the number of children you would claim, an estimate of your itemized deductions and the amount of federal tax withheld on your last paycheck. It only takes a few minutes -- all you'll need is your most recent pay statement. But having your 2017 tax return handy can help to speed up the process.

It's important to note that the calculator won't allow you to enter an amount that exceeds $10,000 as a deduction for state and local taxes you paid this year to reflect the new limit on them.

If your results indicate you'll owe tax when you file, you can do several things now to limit the damage.

You can save money toward your tax bill each month, for instance, in a money market fund, which now pay around 2 percent in interest.

You can increase the amount of tax withheld from your pay for the rest of this year by revising your W-4 form. As a rule, when you reduce the number of allowances you claim, your employer will withhold more federal income tax.

Also review how much of your income isn't subject to tax withholding. This could be income from interest, dividends and capital gains. It could also be income you've earned from services such as Airbnb, Uber or Lyft. The more of this kind of income you have, the more you'll need to reduce the allowances you claim on the W-4 or the more you'll need to save to pay your coming tax bill.

It's important to know what your tax situation is now because having too little money withheld could result in an unexpected tax bill or even a penalty when you file your 2018 return. By doing this checkup now, you'll still have time to make any necessary adjustments.