Estimated Tax Payments & Safe Harbor

The US operates a pay-as-you-go tax system. Taxpayers whose income is not subject to adequate withholding must make quarterly estimated tax payments, and failure to pay enough during the year triggers an underpayment penalty computed as interest on each quarter's shortfall, at the federal short-term rate plus three percentage points.

Individuals owing $1,000 or more at filing generally face the penalty unless they satisfy a safe harbor. Two safe harbors apply: paying at least 90 percent of the current year's tax liability, or paying 100 percent of the prior year's total tax - increased to 110 percent for taxpayers whose prior year adjusted gross income exceeded $150,000, or $75,000 for married filing separately. The prior-year safe harbor is the practitioner's preferred approach because it is a known, fixed figure requiring no forecast of an uncertain current year.

Individual due dates fall on April 15, June 15, September 15, and January 15 of the following year. Corporations follow the fifteenth day of the fourth, sixth, ninth, and twelfth months of the tax year, and large corporations - those with taxable income of $1 million or more in any of the three preceding years - cannot use the prior-year safe harbor except for the first installment.

Two mechanics deserve attention. Because the penalty is computed quarterly, an underpayment early in the year is not cured by overpaying later; each period stands on its own. And the annualized income installment method, elected on Form 2210 for individuals or Form 2220 for corporations, allows taxpayers with uneven income - a fourth-quarter transaction gain, a seasonal business - to match required payments to when income was actually earned rather than assuming even distribution.