I was unclear in my post and sadly the edit timeout has passed. You pay taxes on RSUs not at vesting, but when they settle into shares. This might be shortly a

I was unclear in my post and sadly the edit timeout has passed. You pay taxes on RSUs not at vesting, but when they settle into shares. This might be shortly after vesting; it might be delayed until an acquisition/IPO. (generally it is delayed for companies far away from IPO). The delay causes multi-year income to be batched into a single year. With a progressive tax system, that results in your money being taxed at a rather high marginal tax rate: If you have a substantial amount you vest a year, it's easiest to use the highest marginal bracket as a conservative guess of what you'll be taking home. In California, that's somewhere on the order of 48% combined state + federal.