What if I can't afford to pay my taxes?

It’s the surprise you don’t want when preparing your tax return — instead of the anticipated refund, you owe money. Millions of Americans are getting hit with unexpected tax bills this year.

“If you’re stuck with a tax bill that you weren’t expecting, you are not alone,” said Andrea Coombes, tax specialist at NerdWallet, the personal finance website. “Our 2019 Tax Study found that only 16 percent of Americans had changed their federal tax withholding as a result of the new tax law.”

In a report to Congress last year, the Government Accountability Office (GAO) estimated that about 30 million people — 21 percent of U.S. taxpayers — would owe the IRS money this tax season because their employers didn’t withhold enough from their paychecks during the year.

“Most people are paying less tax in 2018, but their income tax withholdings didn't always match up with that decrease,” explained Alison Flores, principal tax research analyst at The Tax Institute at H&R Block. “Sometimes, the amount of tax an employer withheld decreased more than the actual tax liability decreased, and people are just now finding out about this under-withholding.”

Maybe you have enough in savings to cover the higher tax bill. If not, don’t panic. Here are some things you can do:

Three companies are currently authorized by the IRS to handle credit card payments. They take Visa, Mastercard, Discover and American Express. Each company has a service fee, about 2 percent of the amount you’re charging (with a minimum fee of about $2.50). So, if you owe the IRS $2,000, you’d pay the payment processor about $40 to use your credit card.

Keep in mind: If you can’t pay off that bill in full when it shows up on your credit card payment, you’ll be stuck paying interest. Right now, the average interest rate is 17.84 percent, according to Bankrate.com.

If you qualify, you can charge the tax payment (or some of it) and potentially have up to 18 months to pay off the bill with no interest.

“Look at your total debt and if you can pay it down [the tax bill] by the time that introductory zero percent interest rate period ends, then that's actually a pretty smart move,” said Coombes. “Yes, you are paying that 2 percent processing fee, but you're paying off this loan with zero interest. So, that could be a great way to do it, if you have that option available to you.”

You might be able to get a personal loan from your bank or credit union to cover the payment and pay significantly less than carrying that balance on a traditional credit card. You can compare personal loan rates at Bankrate.

Yes, Uncle Sam will let you make pay over time. You can apply on the IRS website. Your specific tax situation will determine which payment options are available to you. There is a short-term plan (paying in 120 days or less) with no set-up fee and a long-term plan (paying in more than 120 days). You will pay interest on the unpaid balance.

“I recommend that people look into these IRS installment plans, because they really can be a great way to manage this unexpected debt,” Coombes told NBC News BETTER.

For those who would suffer serious financial hardship by paying their tax debt, the IRS might be willing to extend the time frame for payment or accept an “offer in compromise” that settles the debt for less than the full amount owed.

The one thing you don’t want to do is raid your retirement accounts, Coombes said. That could have tax consequences and involve early withdrawal penalties.

Even if you can’t come up with all the money by April 15, you still need to file on time.

"If you owe money to the IRS and can't pay it, the worst thing you can do is not file your return on time,” said Matthew Frankel, a certified financial planner who writes for The Motley Fool. “The IRS penalty for failure to file is 5 percent of your unpaid balance per month, as opposed to just one-tenth of that percentage for underpayment.” (IRS website has information on the common penalties for individual tax payers.)

And pay as much as you can by tax day because you’re going to be charged interest and penalties on the unpaid balance.

In January, the IRS said it is generally waiving the estimated tax penalty for “any taxpayer who paid at least 85 percent of their total tax liability during the year through federal income tax withholding, quarterly estimated tax payments or a combination of the two.” The usual percentage threshold is 90 percent to avoid a penalty.

You can avoid this unpleasant tax surprise next year by adjusting your W-4 form with your employer to withhold more from each paycheck. Tax experts recommend filling out a new W-4 anytime your personal situation changes, such as marriage, divorce, birth of a baby or buying a house.

“Make a visit to your payroll department ASAP and ask to update your withholding going forward,” Frankel said. “Your goal should be to make sure you're not having too much withheld, but at least enough to cover your federal taxes.”

The IRS has a withholding calculator on its website to help you make your withholding as accurate as possible.

• How the 2018 tax law will affect your refund
• 11 smart ways to spend your tax refund, according to personal finance experts
• Ask a tax expert: Is it better to file your taxes jointly or separately?
• Freelancing? Here's how to prepare for tax season all year long

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