I'll admit it: I filed my taxes on Monday. Every year I resolve to be more prompt, but I have a long track record of filing in April. I suppose I can at least claim some redeemable amount of dignity for doing them on my own (without the aid of a parent, which apparently is a trend, according to the Wall Street Journal), and I am definitely thankful I filed on Monday, mere hours before the Internal Revenue Service e-File system went down on the final day to submit.

Phew, another year done.

But before you forget about federal taxes for a while, it's worth visiting the IRS 2018 Withholding Calculator. Your taxes are likely to be very different next year. It's better to check now than wait for a surprise (possibly a nasty one) next April.

President Trump and Republicans in Congress passed the largest overhaul to the U.S. tax code in three decades in December. (Here's a good overview of what changed.) Whether you love or hate the new tax law (more people dislike it than like it, according to the latest NBC News/Wall Street Journal poll), it's now the system in place.

It took me only five minutes to complete the IRS 2018 Withholding Calculator. No, it's not a glamorous activity, but filling it out will tell you: (1) whether your taxes are likely to go up or down for 2018 and (2) what your tax refund (or amount due) is likely to be.

Here's what I found: Yes, my 2018 taxes are going down, but my refund is going to be less than what I've become accustomed to in recent years. It was shocking to see how low it's going to go.

I realized I need to adjust my savings and budgeting, because I can't rely on my tax refund to be an extra “bonus” next spring. (For those who argue it's a bad idea to get a large refund every year, I hear you. Washington Post personal finance columnist Michelle Singletary has a great explainer here on why refunds aren't a great idea.)

What will happen to you? The big fear is that some Americans are going to be in for an ugly surprise on their 2018 taxes (either suddenly owing money or else getting a much smaller refund), but they don't know it yet.

I talked to tax experts at the Tax Foundation, a more right-leaning think tank, and the Center on Budget and Policy Priorities, a more left-leaning think tank. Both sides agree that the IRS crafted the 2018 withholding tables as well as it could. They also agreed there was not an attempt to try to create a situation in which Americans get fatter paychecks now, then end up owing the IRS money during tax season next year.

But here's the catch: Yes, the IRS tried to craft the withholding tables so that roughly the same percent of U.S. tax filers — more than 70 percent — get refunds on their 2018 taxes, as happened in prior years. But that doesn't mean every individual will be in the same situation and, as I learned, it doesn't mean you will get the same refund amount.

The average refund last year was $2,763. If you are someone who typically gets that much, you should definitely check the calculator.

Overall, 65 percent of Americans are getting a tax cut, according to the nonpartisan Tax Policy Center. Another 6 percent will face a tax hike, and everyone else will stay about the same.

But even if a person gets a tax cut, someone could still end up owing money in tax season next year if their company (or companies) withheld the wrong amount, if their situation changes a lot (e.g., new house, kid or job), or if they own a home in an expensive city such as Washington, New York or San Francisco.

One of the biggest changes in the new tax code is that the state and local tax deduction (including for real estate taxes) is capped at $10,000. Many of the people who will pay more because of the new tax law are homeowners in pricey coastal cities, according to the Tax Policy Center.

The IRS 2018 Withholding Calculator is far more comprehensive than any of the tax estimators on various news and financial advisory websites. It gives you a to-the-dollar estimate of what your federal tax bill is likely to be for 2018.

A few pro-tips on the calculator:

First, don't even bother starting until you have your latest pay stub in front of you. It's impossible to fill out without that. It's also helpful to have your 2017 federal tax form (the 1040 or 1040EZ).

Second, be extra careful on the second page of the calculator. Whether you check some of the boxes could have a big impact on the final results.

Third, when they talk about a “tax-deferred retirement plan,” they mean: “Did you contribute to a 401(k)?”

Fourth, when they talk about “cafeteria or other pre-tax plan,” they mean: “Do you get company health insurance?”

Fifth, when they ask for your tax withholding so far this year, they mean just your federal income tax withholding (so don't include your Medicare or Social Security tax withholding or your state or local withholding).

Finally, if you get a really wacky number at the end, you probably did something wrong. I asked several friends I consider very good with numbers to test out the calculator and let me know what they thought. Almost all of them goofed the first time around. I'll admit I did something wrong, too. But the final page of the calculator gives you a summary of what you entered and you can usually figure out where you messed up.

Tax Day 2017 is coming to a close, but do yourself a favor and run the calculation now to ensure Tax Day 2018 isn't a shocker.

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