The Internal Revenue Services offices in Washington, D.C. Adam Jeffery | CNBC

At this point, if you haven't adjusted your tax withholding at work, you should prepare yourself to owe the IRS or get a smaller refund in 2019. Though Uncle Sam has repeatedly warned filers with high incomes and dependents, as well as retirees, to ensure that they are withholding sufficient income tax in 2018 in light of the new tax law, many people may not be making the appropriate adjustments. About 1 in 5 participants surveyed by H&R Block said that they updated their Form W-4 — a document that helps you figure out how much income tax you should withhold from your pay — in response to the tax overhaul.

The form itself can confuse people as they complete it. Nathan Rigney lead tax analyst at H&R Block

The tax prep service polled 2,002 individuals online in November. Three out of 10 participants said they would feel unprepared if they had to adjust their W-4 on their own. If you haven't tweaked your withholding yet, it's probably too late to head off that tax bill next spring. "Adjusting it this late in the year won't have much impact on your 2018 withholding," said Nathan Rigney, lead tax analyst at H&R Block. "It won't correct the error that's existed this entire year," he said. However, if you make the appropriate updates now, you should be set for the 2019. Here's what to know.

Tax overhaul

This year, the Tax Cuts and Jobs Act revamped the tax code, roughly doubling the standard deduction, tossing personal exemptions and placing sharp limits on itemized deductions. Individual tax rates have also been changed. See below for next year's brackets.

As a result, the IRS and Treasury updated the tax withholding tables, guidelines employers use along with Form W-4 to determine how much income tax workers should withhold from each paycheck. Uncle Sam kicked off an outreach effort to remind taxpayers to check their withholding in 2018 in response to all of the changes. Nevertheless, the Government Accountability Office estimates that about 2 in 10 taxpayers will owe the IRS in 2019. That's equal to about 30 million people.

Most affected

MartinPrescott | E+ | Getty Images

These people should pay especially close attention to their withholding: Filers who itemize: Prior to the new tax law, it may have made sense to withhold less from your pay if you itemized deductions. That may no longer be the case now that the standard deduction has nearly doubled to $12,000 for singles and $24,000 for married couples filing jointly. Those who itemized in the past may no longer do so going forward, so they may need to revisit their withholding. Households with dependents: Under the old law, it may have made sense to withhold less in taxes if you had dependents. Now, personal and dependent exemptions are out, so these filers should review their paystubs to ensure that they're not underwithheld. Retirees: Just because you're no longer working, doesn't mean that you still won't owe. You can use Form W-4V to withhold a flat rate from your Social Security check or Form W-4P to withhold from your pension.

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