Quarterly estimated payments and safe harbor rules
The US taxes income pay-as-you-go. If your withholding doesn't keep up — the classic RSU situation — the IRS expects you to make up the difference yourself, four times a year. Miss by enough and you owe an underpayment penalty on top of the tax.
Who has to pay estimated tax?
Generally: anyone who expects to owe $1,000 or more at filing after withholding. If your RSU withholding gap alone is a few thousand dollars, you're squarely in estimated-payment territory.
The safe harbor: your get-out-of-penalty-free card
You avoid the underpayment penalty if your total payments (withholding + estimated) hit at least one of these:
- 100% of last year's total tax (Form 1040, line 24) — or
- 110% of last year's total tax, if your AGI last year exceeded $150,000 — or
- 90% of this year's total tax (harder to aim at mid-year, since this year's tax is still unknown).
Safe harbor #1 is the simplest target in the tax code: take last year's line 24, multiply by 1.0 or 1.1, subtract what your employer will withhold this year, divide by four. The estimated-payment planner does exactly this.
2026 due dates
| Quarter | Due date |
|---|---|
| Q1 2026 | April 15, 2026 |
| Q2 2026 | June 15, 2026 |
| Q3 2026 | September 15, 2026 |
| Q4 2026 | January 15, 2027 |
Pay online at IRS Direct Pay (free, from your bank account) and apply each payment to the correct year and quarter. A useful quirk: withholding is treated as paid evenly throughout the year no matter when it was withheld — so bumping W-4 withholding late in the year can retroactively cover early quarters, while a late estimated payment only covers from its date forward.
How equity comp creates the penalty
The pattern repeats every year: salary withholding is roughly right, RSU vests are withheld at 22%, your marginal rate is 32%, and by December you're $8,000 short. If that shortfall drops you below safe harbor, the IRS charges an underpayment penalty — an interest-based charge set quarterly (check the current rate at IRS.gov; it moves with market rates).
The fix is boring and effective: quantify the gap with the RSU calculator, then either add W-4 withholding or schedule quarterly estimates to close it.
State estimated payments too
Most states with income taxes have their own quarterly estimated requirements and their own underpayment penalties. The federal planner on this site doesn't cover them — check your state's tax site (for California, that's the Franchise Tax Board) and mirror the federal schedule.