Paying enough to clear either of two IRS safe-harbor thresholds avoids an underpayment penalty entirely, regardless of how much tax is ultimately owed on the return.
How it works
The safe harbor is the smaller of 90% of this year’s projected tax, or 100% of last year’s tax (110% if last year’s AGI exceeded $150,000). Comparing that required amount against what’s already been paid shows any shortfall.
What this does not include
This does not include the $1,000 de minimis exception (no penalty if the amount owed after withholding is under $1,000), or the fact that withholding is treated as paid evenly across the year even if concentrated late, both of which can cure an apparent shortfall.
How to use this calculator
- Enter prior year tax and AGI, this year’s projected tax, and payments made so far.
Frequently asked questions
Why does prior-year AGI matter here?
Higher-income taxpayers (AGI over $150,000) face a stricter 110% threshold instead of 100%, since Congress designed the safe harbor to be somewhat less generous at higher income levels.
Does withholding count toward the safe harbor the same as estimated payments?
Yes — the IRS treats withholding and timely estimated payments the same way when checking against the safe-harbor thresholds.
What’s the penalty rate if I fall short?
It floats with market rates — the federal short-term rate plus 3 percentage points, compounded daily, recalculated quarterly.
