Safe harbor (estimated tax)

The protection holds even if your tax for the year turns out higher. You pay the safe harbor amount in four equal installments. Any tax still owed is due by April 15, even if you extend your return. For 2026, the installments are due April 15, June 15 and September 15, 2026, and January 15, 2027. Tax withheld from your pay counts toward them.

Some people owe no penalty at all. That includes anyone whose tax after withholding is under $1,000. It also includes someone who owed no tax for last year, if that was a full 12-month year. That person must have been a US citizen or resident all year.

A C corporation has a safe harbor of its own: 100% of this year’s tax, or 100% of last year’s. Last year counts only if it was a full 12-month year that showed tax, and a large corporation faces tighter limits.

The four due dates are not evenly spaced: the second comes two months after the first, and the last falls in the next January. The 100% and 110% tests look back at 2025, so a jump in 2026 income does not change them. That jump can raise the 2027 test to 110%.