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Shows the dollar due by each 2026 IRS installment date and the penalty already running on the gap. Full version: .csv + reminders. Estimated Tax Safe Harbor Checker 2026 What you must have paid by each 2026 IRS installment date - and what the gap is already costing you at 7%. Install (no store account needed) 1. Unzip this folder somewhere permanent. 2. Open chrome://extensions and turn on Developer mode (top right). 3. Click Load unpacked and pick the unzipped folder. 4. Pin the icon and click it. Works in Chrome, Edge, Brave and any Chromium browser (Manifest V3). What it does - 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. The numbers it works out | 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_ Full version 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://buy.polar.sh/polar_cl_996QPkhf5OIojuD3Nwn3Ou1828xe3fDUVnoO91amT7Y