Will you owe an underpayment penalty?
The IRS wants tax paid as you earn, not in one April lump sum. If you didn't, §6654 adds a non-deductible interest charge on the gap. Two safe harbors make the penalty go away — this tool checks both and estimates the damage if neither applies.
How this is computed
A penalty applies under IRC §6654 when withholding plus estimated payments fall short of the lesser of two targets:
- 90% of the current year's total tax, or
- 100% of the prior year's total tax (110% if prior-year AGI exceeded $150,000).
If you clear either, no penalty. The tool reports which harbor you cleared (if any) and, if neither applies, the shortfall — the amount you underpaid relative to the binding target.
Related reading: the 110% safe-harbor rule, explained →
The estimated penalty is a single-period proxy: shortfall × §6621 rate × fraction of year outstanding. Form 2210 computes it per quarter on actual payment dates; the front-end estimate assumes the shortfall persisted through the filing deadline (~day 105). Use it to size the risk, not to file.