What you must have paid by each 2026 IRS installment date - and what the gap is already costing you at 7%.
Your safe harbor
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Safe harbor for the full year
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Per installment
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Must have been paid by this date
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Counted as paid
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Shortfall at this installment
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Penalty running at 7% to Apr 15, 2027
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Pay by this date to be current
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Verdict
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Picks the safe harbor that actually applies to you Compares 90% of your expected 2026 tax against 100% of your 2025 tax - or 110% if your 2025 AGI was above $150,000 ($75,000 if married filing separately) - and uses the lower of the two.
Tests each installment period separately Reports the cumulative amount due by the installment date you pick, which is how IRS Form 2210 tests it. One annual total hides a Q1 shortfall completely.
Prices the gap in dollars Applies the 7% underpayment rate the IRS set for 2026 across the days from the missed deadline to April 15, 2027, so the shortfall stops being abstract.
Exports the schedule as .csv Full version: saves the four installments, what you paid, the shortfall and the penalty figure as a file you keep for your records or hand to your accountant.
Reminds you before the deadline Full version: browser reminders 30, 14 and 1 day before the installment date you entered, so the next period does not get missed the same way.