Sell to cover pays your withholding, not your tax. Our RSU withholding shortfall calculator works out the difference for a specific vest.
What actually happens on vest day
Under sell to cover, your employer takes the shares that vested, sells enough of them to raise the withholding due, remits that to the tax authorities, and deposits the remaining shares in your account. You did not choose the sale price and you had no window to act; it happens as part of the vest.
The number of shares sold is set by the withholding calculation, not by your tax bill. That distinction is the whole of it.
Why the proceeds miss the bill
Withholding on a vest is computed at the flat supplemental rate. Your tax on the vest is computed at your marginal rate, on top of everything else you earn. When the second is higher than the first, selling enough shares to cover the first leaves you short on the second, and no part of the sell-to-cover process is looking at the difference.
It is easy to read the transaction as “the tax was handled”, because something was sold and something was paid. What was handled was withholding.
The part people miss twice
There is a second, smaller effect. The shares you keep have a cost basis equal to the value on the vest date, and the shares that were sold to cover were sold at roughly that same price, so the sale itself usually produces little or no capital gain. That is why it does not show up as a meaningful line on your return, and why it is easy to forget the vest happened at all when the bill arrives months later.
Working out where you stand
The RSU withholding shortfall calculator compares what was withheld against what the vest actually costs, which is the comparison sell to cover does not make for you.