You know the Christmas present that President Trump promised us in the form of a tax overhaul? Under the law that has passed both houses of Congress and is awaiting his signature, millions of Americans will find that their pockets have been picked instead. If you’re one of them, it’s important to understand that now, and to plan accordingly. Otherwise, you may be surprised to find yourself short of funds about a year down the line.

The steps below are designed to give you a preview of what you’ll owe for 2018 – the first year that most provisions take effect. That tax return isn’t due until April of 2019. Conveniently enough for members of Congress who put their own interests before those of their constituents, the midterm congressional election will be over by then. And the very legislators who voted for this heist may have already been reelected. If you would rather not have that happen, take stock of the law’s impact, adjust your budget and start working to unseat those who endorsed it.

You don’t need software or an online calculator to run the numbers. In fact, it’s easier to visualize how the law affects you by using your 2016 federal income tax return as a scratch pad. Print out the first two pages – that’s Form 1040, plus the next page (Schedule A), if you itemized deductions. Sharpen a pencil that you can use to mark them up as you follow along.

Assume, for these purposes, that various aspects of your situation remain the same, including: your salary, investment earnings, property taxes, marital status, and number and ages of children. (Otherwise you could wind up comparing apples and oranges.) The new rules discussed here are set to expire December 31, 2025, at which time the law will go back to what it is now.

Meanwhile, you can get a rough idea of how the overhaul could change your tax bill. Included here are a couple of suggestions about how to reduce what you’ll owe this April. But after that you may start feeling the pain. My main goal is to help you guesstimate whether – and, if so, by how much – your after-tax income might decline, so you can factor that into your financial planning.

Start with the amount on line 38 of Form 1040 – your adjusted gross income. Currently it’s reduced by the personal exemption: $4,050 for yourself, your spouse and each of your dependents. That will no longer be the case going forward. The overhaul repeals the personal exemption, so cross out lines 6a through 6d, where you enter the number of exemptions you are claiming, as well as line 42, which shows how much they trim from your taxable income. An increase in the child tax credit (for children under 17 – see line 52), from $1,000 to $2,000, does not come close to making up for it.

The changes that are most likely to affect what you owe in 2018 are: 1) The deductions that apply to your situation; and 2) The tax rates. In three easy steps, here’s how to guesstimate how much better – or worse – off you will be.

Step 1. Figure your deductions. This amount, entered on line 40 of Form 1040, gets subtracted from the number on line 38 before you compute your tax.

The choice each year is whether to take what’s called the standard deduction or to itemize, meaning that you list each deduction to which you are entitled – something done on Schedule A of your return. For decades, a popular (and legal) way to minimize what you owed has been to take as many deductions as possible, and thereby reduce the amount that was subject to tax.

Two aspects of the overhaul make it likely that many more people will take the standard deduction, rather than itemize.

First, the new law roughly doubles the standard deduction, which in 2016 was $6,300 for single people and $12,600 for married couples. Starting in 2018, the standard deduction will be $12,000 for individuals and $24,000 for married couples filing jointly.

What’s a deduction worth? Its approximate value to you, in pocket, is the amount you are deducting, multiplied by the tax rate that applies to you in a given year. That’s true whether you take the standard deduction or itemize.

(If you took the standard deduction in 2016, assume you will do that again in 2018, and go to Step 2.)

By capping – or eliminating – certain deductions, the tax overhaul also leaves much less room for the strategies that made itemizing attractive to some folks. People who own their homes or live in a state or city that has an income tax could be especially hard hit under the overhaul.

Consider homeowners in Chappaqua, N.Y., the community where former President Bill Clinton and former Secretary of State Hillary Rodham Clinton live. A four-bedroom house there, currently on the market for about $1.4 million, has annual real estate taxes of $38,643. Assuming the owners are in the 25 percent tax bracket, they can save about $9,660.75 ($38,643 times 25 percent) by deducting those taxes this year.

Next year, as a result of the tax overhaul, this couple (I’m assuming for this example that the owners are married) will be much worse off. That’s because the new tax law sets a $10,000 cap on the total deduction for real estate taxes; state and local income taxes; and sales tax.

With this in mind, the Chappaqua couple might want to consider prepaying their real estate taxes for 2018. If they do that before December 31, they can deduct whatever they paid in 2017 on this year’s tax return. After that, their deduction will be limited to $10,000 and won’t count at all unless they itemize.

Note: How much of next year’s real estate taxes can be prepaid will depend on what your community allows. For example, if you live in a jurisdiction where the fiscal year ends on June 30, you may be able to prepay your real estate taxes for only the first two quarters of 2018.

In contrast, the tax overhaul makes it clear that you can’t deduct 2018 state and local income taxes that you prepay this year. However, it behooves you to pay all the state and local taxes you owe for 2017 by December 31, even though your fourth-quarter payment (if you pay estimated tax) is not due until January 16. If, instead, you wait until next year, it counts as a payment in 2018 and can no longer be deducted on your 2017 return.

How does your situation compare with that of the Chappaqua couple?

Turn to Schedule A of your federal income tax return and use it as a worksheet to figure out your deductions for 2018.

Start by checking the amounts on line 5 (state and local income taxes and sales tax) and line 6 (real estate taxes). If they total more than $10,000, you’ll need to shave off the difference (the total of these two lines, minus $10,000) for purposes of this calculation. Reduce the number on Schedule A, line 9 by that amount.

Other itemized deductions that are no longer allowed are those previously listed under the heading “Job Expenses and Certain Miscellaneous Deductions.” Examples include unreimbursed job travel, training or union dues; and investment fees. Previously you could claim them if they totaled more than 2 percent of your adjusted gross income. Now you can’t deduct them at all, so they shouldn’t factor into your estimate. If you took these deductions in 2017, strike out the number on line 27 of Schedule A.

Once you have penciled in the changes, add up lines 4 through 28 on Schedule A and enter the total on line 29. Use this number or the standard deduction – whichever is higher – as you move on to Step 2.

Step 2. Subtract your deduction from the amount on line 38 of Form 1040. The difference between your Form 1040, line 38 amount and your deduction is your taxable income. It gets entered on line 43 of Form 1040.

Step 3. Figure your tax based on the new tables. Though there’s been much talk about the seven new tax brackets (which replaced the seven old ones), those who benefit most from the changes are people whose taxable income is more than $500,000 per year. And as you can see from Step 2, with fewer deductions now available, you might have more income subject to tax.

To see how this would affect you in 2018, go to the tables that download here as a PDF, and which can be found on pages 12 to 13 of the Conference Agreement. Use the number you entered on line 43 to determine how much tax you owe.

Based on these three steps, does the new law leave you worse off in 2018 than you were in 2016? If you are a salaried employee, look for an occasion to ask your boss for a raise. To justify the overhaul, with its outsize advantages for corporations, proponents argued that the benefits would “trickle down” to the middle and lower classes. Your boss’s reaction might offer a sneak peek at whether or not this controversial theory actually works.

Deborah L. Jacobs, a lawyer and journalist, is the author of Four Seasons in a Day: Travel, Transitions and Letting Go of the Place We Call Home and Estate Planning Smarts: A Practical, User-Friendly, Action-Oriented Guide. Follow her on Twitter at @djworking and join her on Facebook here. You can subscribe to future blog posts by using the sign-up box on her website’s homepage.

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