US federal tax is pay-as-you-go: the IRS expects money during the year you earn it, via paycheck withholding or quarterly estimated payments. Pay too little along the way and each quarterly shortfall is charged interest (currently 7% a year) until you catch up. The escape hatches are called safe harbors: thresholds that, once met, make the penalty zero even if a big balance is due at filing.
The three individual safe harbors
| Safe harbor | You owe no penalty if you paid in… |
|---|---|
| Current-year | at least 90% of this year's total tax |
| Prior-year | at least 100% of last year's total tax |
| Prior-year, high income | 110% of last year's tax, if last year's AGI was over $150,000 ($75,000 married filing separately) |
You automatically get the most favorable one. Your "required annual payment" is the smallest of the applicable numbers. The prior-year harbor is the planner's favorite because last year's tax is a number you already know. A freelancer having a breakout year can simply pay 100% (or 110%) of last year's liability in four equal installments and owe zero penalty on the windfall until April.
The floors: when no penalty applies at all
- Individuals: no penalty if the balance after withholding is under $1,000.
- Corporations: no penalty if the year's tax is under $500. (Corporate safe harbors are 100% of current or 100% of prior year, with no 90% discount, and the prior-year option requires that year to have shown tax due. Large corporations face extra limits.)
- Zero prior year: an individual whose prior-year tax was $0 (and who was a US citizen or resident for that full 12-month year) owes no estimated-tax penalty at all.
The withholding time machine
That makes year-end withholding the only way to retroactively cure missed quarters. Our calculator's "withholding rescue" number tells you exactly how much extra withholding before December 31 would zero out your penalty.
When you can't reach a harbor
Then the penalty is simple interest on each quarterly shortfall from its due date until paid (or until the April deadline, whichever comes first). It isn't a flat fine, so paying even one day sooner genuinely costs less. Two more softeners exist: the annualized-income method (Form 2210 Schedule AI) can shrink the penalty when income arrived unevenly, and penalty waivers exist for casualty, disaster, retirement, or disability situations.
Check your own numbers
The calculator applies every rule on this page automatically: harbors, floors, the evenly spread withholding rule, dated payments, and the exact quarterly interest rates.