In late 2017, Congress passed one of the biggest tax overhauls in generation. Now, come April, you could find out you’ve been paying too much — or too little — in taxes.

Want to find out now? One of the easiest ways is to use a free online tax calculator.

The New Tax Law

Tax Cuts and Jobs Act, which went into effect for the 2018 tax year, doubled the size of the standard deduction, while also eliminating the $4,000 personal exemption and curtailing other popular perks, like deductions for mortgages and state and local taxes, among other changes.

While the new law delivered a tax cut to many Americans, that’s not true across the board. A report by the Government Accountability Office (GAO) warns that more than 30 million Americans will owe more, and and could be underpaying unless they have adjusted their withholding. If you pay too little, you are likely to owe back taxes and possibly penalties too.

How to use an Online Tax Calculator

While it may be too late to change what you paid last year, it’s better to fix any discrepancies as soon as possible — and avoid them for 2019. The IRS’ online income tax calculator is an especially useful resource, according to tax planners, since it makes it easier to navigate rule changes from the Tax Cuts and Jobs Act.

If you find that you’re withholding too much, it’s time to update your W-4 form. With an average tax refund of almost $2,800, according to the GAO, adjusting your withholding could boost your paycheck by approximately $230 per month.

What if you aren’t withholding enough? Daniel Kenny, a financial planner and accountant in Sterling, Va., suggests the best way to increase your withholding is to keep your exemptions as they are and instead use the ‘additional withholding’ box to add the extra dollar amount you need to your withholding figure. That’s because changing the exemptions on your W-4 form with your employer will often result in a less than exact change to your withholdings.

Tax Calculators for State and Local Taxes

One drawback of the IRS tax calculator, however, is that it won’t help you estimate state and local taxes. That’s not necessarily as critical, unless your state has, like the federal government, also overhauled its taxes laws for this year. If you want to be sure, Denver, Colo. financial adviser Joseph Clemens recommends TurboTax’s TaxCaster tax calculator, which allows you to estimate your tax liability to the federal government and state. Although the calculator is free to use, you can’t actually file your taxes with it unless you pay to use the service.