One Good Year: Why wasn't enough tax withheld on my RSUs?

The year your equity finally vests is the year your taxes stop behaving.

Nothing changes at work. You don't file a new W-4, payroll doesn't make a mistake, and every pay stub looks the way it always did. Then April arrives with a five-figure bill you did not plan for, and the next April arrives with the same bill plus a penalty on top.

The shortfall is the same both years. What changes is the bar it gets measured against — and that bar moves a year behind your income. It's worth knowing which year you're in.

Why wasn't my RSU withholding enough?

When your employer hands you something that is not a regular paycheck — a bonus, a commission, an RSU vest — the tax code calls it a supplemental wage and lets them withhold at a flat rate: 22% up to $1 million a year, 37% above it.

The rate is set by the size of the payment, not by your income. A vest is withheld at 22% whether it lands on top of a $60,000 salary or a $600,000 one.

Pranav is a staff engineer filing single, on a $250,000 salary with $600,000 of RSUs vesting through the year. His payroll does two separate, entirely correct things: it withholds on the salary as though the salary were his whole year, and it withholds 22% on every vest.

Those RSUs are stacked on top of a $250,000 salary, so the real federal rate on them is 35.5%, not 22. Every vest is under-withheld by roughly thirteen cents on the dollar, quietly, all year, and the total lands as an $81,200 surprise in April.

When shares vest, your company usually sells a slice of them to raise the withholding. That is sell-to-cover, and what it raises is the 22% — the number that was already too small.

Why did I owe a penalty this year but not last year?

Because the shortfall and the penalty are two different things, and only one of them showed up the first year.

Federal income tax isn't due once in April. It's due in four installments across the year, and the safe harbor is the minimum you have to pay across the four. Come in under it and the IRS charges interest on every installment you were short, running from that installment's own deadline.

The harbor is the lower of two numbers: 90% of what you owe this year, or 110% of what you owed last year (100%, if last year's income was under $150,000). Last year's is the one that moves.

The first year your income jumps, last year's tax is still the small one, so the bar you have to clear is set at the height of your old life.

Pranav's first big RSU year is protected for exactly this reason. Last year he earned an ordinary salary and owed $120,600, so his bar is 110% of that, $132,700. His paychecks send in $51,300 on the salary and $132,000 on the vests, $183,300 in all, clearing the bar with $50,000 to spare. He writes an $81,200 check in April and owes no penalty at all.

By year two, the safe-harbor bar has caught up:

In the second year, last year's tax is the first big year's total: the $183,300 his paychecks sent plus the $81,200 he wrote in April, $264,500 in all. So 110% of it is $290,950, and 90% of this year is $238,050. He is measured against the smaller one, and the same withholding that sailed over the bar last year now lands $54,750 short. The penalty — really just interest on the late installments — is roughly $2,620, and it repeats every year he does nothing, because the bar has permanently caught up with his income.

Same salary. Same vests. Same payroll settings. The only thing that moved was the number he was being measured against.

Can I still fix this in December?

Yes, and the calendar is on your side for once.

Withholding is treated as paid evenly across all four installments no matter when it happened. So $40,000 withheld from a December 28 paycheck counts as $10,000 arriving in April, June, September, and January, and it erases the shortfalls in all four retroactively. A $40,000 estimated payment made the same day counts on December 28 and cures nothing before it.

Which gives you a genuine year-end repair kit:

• File a new W-4 with extra withholding on the remaining paychecks. Pranav's $54,750 gap is about $2,100 a paycheck, or one heavily withheld December bonus.
• Ask payroll which withholding method it uses for future vests. The flat method is locked at 22%, but federal rules also let payroll combine the vest with regular wages and calculate withholding through the normal brackets.
• Take an IRA distribution with the withholding dialed up, if you have one, then put the full amount back within 60 days from other cash. The withholding still counts as spread across the year, but the IRS allows this move once per 12 months.
• Show the IRS your timeline on Schedule AI if the income genuinely arrived late in the year. The form matches each installment to what you had actually earned by its deadline, so a December tender offer does not owe an April installment. You just have to lay out the quarterly detail to prove it.

One warning for anyone in California. Everything above is federal. The state runs its own schedule on top of it, 30/40/0/30 instead of four equal payments, and once your AGI reaches $1 million the prior-year escape disappears completely. Above that line you are on 90% of the current year, estimated in real time, in the exact year your income became impossible to predict.

What's your next move?

Which year you're in is printed on last year's return. If the bar is still set at your old life, you have an April to save for and no penalty coming. If it has already caught up, it doesn't move back, and the fix is a W-4 rather than a checkbook.

Against a tax bill that size, the $2,620 penalty is a rounding error. It's the surprise that's expensive — not the interest. The safe harbor gives you exactly one good year.

Know which one you're in before April tells you.