Tax an employer or payer takes out and remits on your behalf. RSU vesting is often withheld at a flat rate that undershoots the actual tax.
Withholding is prepayment. When it falls short, the difference is due in April with a possible underpayment penalty. The common gaps: RSU vesting withheld at a flat supplemental rate below your bracket, two-income households, and self-employment or crypto income with no withholding at all.
File a new W-4 with additional withholding per paycheck, or make quarterly estimated payments. Both count toward the safe harbor.
Last reviewed September 10, 2026. Tax rules change; confirm current law before acting.
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