Estimated Tax Safe Harbor Checker 2026 by ReadyStack

What you must have paid by each 2026 IRS installment date - and what the gap is already costing you at 7%.


Click Add to Firefox on this page. Nothing to unzip, no developer mode - the free version runs right away.

• 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.

| You type | |
|---|---|
| Installment date you are checking | default 2026-09-15 |
| 2025 total tax (Form 1040, line 24) | default 18000 |
| 2025 AGI (Form 1040, line 11) | default 165000 |
| Filing status | default single |
| Expected 2026 total tax | default 24000 |
| 2026 W-2 withholding, whole year | default 0 |
| Estimated tax paid by Apr 15, 2026 | default 3000 |
| Estimated tax paid by Jun 15, 2026 | default 3000 |
| Estimated tax paid by Sep 15, 2026 | default 0 |

| It answers | |
|---|---|
| Your safe harbor | |
| Safe harbor for the full year | |
| Per installment | |
| Must have been paid by this date | |
| Counted as paid | |
| Shortfall at this installment | |
| Penalty running at 7% to Apr 15, 2027 | |
| Pay by this date to be current | |
| Verdict | |

Permissions requested: storage, alarms, notifications, downloads


Free: the calculation in the popup, no key, no limit. Full version ($60): download the result as .csv and deadline reminders - one licence key per person or team seat. Get it: https://getreadystack.com/api/buy/cl/polar_cl_996QPkhf5OIojuD3Nwn3Ou1828xe3fDUVnoO91amT7Y


When you use the tool, the extension sends one anonymous count a day (the tool’s name only — no inputs, no personal data). Turn it off on the first page after install. Firefox asks first and sends nothing unless you allow it. A licence key is checked with the payment provider (Polar) only when you enter one.