Tax Safe Harbor

The safe harbor (IRC Section 6654) answers the anxious question behind quarterly taxes: how much is enough? Hit either target through withholding plus timely estimated payments and the IRS charges no underpayment penalty, even if a large balance remains due at filing. The prior-year option is the planner’s favorite because it is a fixed, known number the whole year, while the 90 percent rule requires forecasting income that may still be arriving.

The high-earner uplift is the common trap: once last year’s adjusted gross income passes $150,000, the prior-year harbor requires 110 percent, not 100. And the harbor is a penalty shield, not the bill; in a year when income jumps, prepaying 110 percent of a smaller prior-year tax can leave a five-figure amount due in April, penalty-free but very real. The quarterly estimated tax calculator picks your smaller harbor and splits it into the four payments.