Whether you're a business owner, an employee with a side hustle or an investor with a hefty capital gain, a key tax deadline is coming up. Taxpayers who pay the IRS every quarter must make their third payment for this year by Sept. 16. For the 2019 tax year, the quarterly deadlines are April 15, June 17, Sept. 16 and Jan. 15, 2020. To avoid a penalty from the IRS, you must pay at least 90% of the income taxes you owe for a given year, or 100% of the tax liability from the prior year, to avoid an underpayment penalty on your tax return. The threshold is 110% if your adjusted gross income on that year's return exceeded $150,000.

Though employees generally have their federal income taxes withheld from each paycheck, self-employed individuals tend to pay what they owe every quarter. This means individuals who pay quarterly taxes need to plan those payments well in advance. "Typically you're projecting out your income for the whole year," said Lisa Featherngill, CPA and member of the American Institute of CPAs' personal financial planning committee.

Determine your liability

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Taxpayers who write a check to the IRS every quarter need to estimate their income and then work with a CPA to calculate the taxes they are likely to owe for that year, said Featherngill. It can be difficult for filers to accurately account for their income and taxes owed. For instance, mutual funds tend to distribute capital gains from selling underlying investments during the fourth quarter. This can result in taxes if an investor holds the fund in a taxable account. Investors need to account for that tax liability as they figure out how much they'll owe the IRS each quarter. "You may need to go back to last year's tax returns and see how much income you recognized from this last year," said Featherngill.

Year-end bonuses are another tax surprise. While your wages — and your tax withholding — may be level throughout the year, bonuses may not be withheld at the appropriate rate. For instance, if your employer currently withholds taxes from your pay, the IRS gives it two options to withhold on bonuses: The employer can withhold at a flat rate of 22% or it can fold the bonus into your regular wages and withhold from your full compensation. "If the withholding on the bonus isn't at the full rate, and you're in a higher tax bracket, you will most likely owe taxes on that," said Featherngill. You need to account for all these income sources as you hash out your tax liability for the year and determine how much to set aside for quarterly payments.

A lighter touch