Why do I owe more tax on RSUs than was withheld at vesting?

Most employers withhold federal tax on RSU vesting at the flat supplemental wage rate of 22%, but if your marginal bracket is 32%, 35%, or 37%, that 22% leaves a gap you'll owe at tax time. On a $100,000 vest, withholding might cover $22,000 while your real liability is $35,000 – a $13,000 shortfall, before state tax. The shares look fully paid for, but the IRS sees only a partial deposit. High earners with large vests routinely get a nasty April bill plus an underpayment penalty. Fix it by making estimated payments, increasing W-4 withholding, or setting aside the difference yourself. Estimate your true marginal rate, multiply the vest value by the gap, and reserve that amount. The tool at wealthserene.com/tools/rsu-espp-calculator can size the shortfall.