When are RSUs taxed — at grant or at vesting?
At vesting, not at grant. RSUs have no tax consequence when granted, since you don't yet own the shares -- only when they vest (and you actually receive the shares) does their fair market value become taxable as ordinary income, the same as a cash bonus of that dollar amount.
How is RSU vesting income withheld?
The value of shares that vest is "supplemental wages" for federal withholding purposes, same as a cash bonus -- your employer (or your brokerage, on the employer's behalf) typically withholds a flat 22% for federal tax (37% on the portion of your combined supplemental wages for the year above $1,000,000), commonly by withholding ("selling to cover") some of the vesting shares themselves rather than from cash pay.
Is RSU income also subject to Social Security and Medicare tax?
Yes -- RSU vesting income is wages for FICA purposes too, subject to Social Security tax (up to the annual wage base) and Medicare tax, withheld the same way as the federal income tax portion.
What happens if I worked in a different state between grant and vesting?
This is a genuinely complex multi-state sourcing question this calculator does not model. States (California's FTB is a prominent example) can apportion RSU income across the states you worked in during the vesting period, not just tax the full amount in whatever state you live in when it vests -- if your grant and vesting happened in different states, consult your state's specific sourcing rules or a tax professional.
Does the flat withholding rate match what I'll actually owe?
Not necessarily. The flat rate is a withholding shortcut applied regardless of your tax bracket -- if RSU income pushes your marginal rate above the flat withholding rate, you'll owe more at filing; if your marginal rate is lower, the extra withholding comes back as part of your refund.
