Avoiding the underpayment penalty doesn’t require guessing this year’s tax exactly — it requires meeting one of two safe harbor thresholds.
How it works
The safe harbor is the smaller of 90% of this year’s expected tax, or 100% of last year’s tax (110% if last year’s AGI was over $150,000, or $75,000 married filing separately). Paying at least that much through withholding and quarterly payments avoids the penalty even if more is owed at filing.
What this does not include
This computes the annual safe harbor total divided evenly across four quarters — real quarterly due dates aren’t evenly spaced, and income earned unevenly through the year can require the annualized income installment method instead, which this calculator doesn’t model.
How to use this calculator
- Enter this year’s expected total tax and last year’s total tax.
- Indicate whether last year’s AGI was over the $150,000 / $75,000 threshold.
Frequently asked questions
Why would I use last year’s tax instead of this year’s?
Because this year’s exact tax isn’t known until the year ends — the prior-year safe harbor lets you pay a known, fixed amount and avoid the penalty regardless of how this year turns out.
What happens if I pay less than the safe harbor?
You may owe an underpayment penalty on the shortfall, even if you pay the full balance due by the filing deadline.
Why does the multiplier jump to 110%?
Higher-income taxpayers face a stricter safe harbor — the 110% figure applies once last year’s AGI crossed $150,000 ($75,000 if married filing separately).
