How do I plan estimated taxes for a big RSU vesting year?

When a large RSU vest or IPO settlement lands, employer withholding at the 22% supplemental rate often falls far short of your true bracket, so you may owe a big balance plus an underpayment penalty. To avoid that, estimate your real marginal rate, calculate the gap on the vested value, and cover it through quarterly estimated payments or by bumping your W-4 withholding. The safe-harbor rule helps: generally you avoid penalties if you pay at least 110% of last year's tax (for higher incomes) or 90% of this year's. In a spike year, paying based on last year's tax can be the cheaper safe harbor. Set the extra cash aside the day shares vest so it isn't spent. Size the shortfall and quarterly payments with wealthserene.com/tools/rsu-espp-calculator.