Your RSUs Vested. Now There's a September 15 Deadline.

There's a specific kind of email we get in April. It's always the same shape: someone's RSUs vested last year, their W-2 clearly shows federal tax was withheld, and they've just found out they owe another five figures anyway. The subject line is usually some version of "is this right?"

It usually is right. And the window to have done something about it closed months earlier.

So here's the version of that conversation we'd rather have in July.

When is the payment due?

Q3 estimated tax is due September 15, 2026, covering income you received from June through August. If your RSUs vested in that window and your withholding didn't cover the tax, that's the date on the calendar. But the deadline is the less interesting half of this — the W-4 fix below is usually the better move, and almost nobody explains why.

Why you're short in the first place

Your employer isn't doing anything wrong. When RSUs vest, that value is supplemental wages, and the payroll rule is to withhold federal income tax at a flat 22% (§3402; 37% on supplemental wages above $1 million). Flat. Regardless of what you actually earn.

That's fine if you're in the 22% bracket. The trouble is that RSUs stack on top of your salary, so the people receiving meaningful equity are almost never in the 22% bracket by the time the shares land.

Here's what that looks like with numbers. Hypothetical, but a very ordinary one:

Single filer. $180,000 salary, $120,000 of RSUs vesting this year. The federal tax on those RSUs is $36,200 — an effective 30.2% once they're stacked on the salary. Payroll withheld the flat 22%, or $26,400. You are $9,800 short, and nothing on your pay stub said so.

Eight points. On every vested dollar. Your W-2 will faithfully report the tax that was withheld — it has no way of mentioning the part that wasn't. You can run your own numbers with the RSU tax calculator; it uses the same 2026 brackets this example does.

The penalty is for timing, not for owing

This is the part that surprises people. The IRS doesn't mind that you owe money in April. It minds that you didn't pay it during the year — the tax system is pay-as-you-go, and §6654 charges interest when you fall behind. The rate floats with the federal short-term rate under §6621, so it moves, but it has been meaningfully worse than "a slap on the wrist" for a few years now.

Which means there are two separate problems and only one of them is urgent. The tax itself is owed either way. The penalty is optional, and it's the one with a deadline.

Safe harbor: how to switch the penalty off entirely

§6654 has an escape hatch, and it's generous. Hit any one of these and the underpayment penalty goes away — even if you still write a large check in April:

• 90% of what you'll owe this year, or
• 100% of what you owed last year, or
• 110% of last year's tax if your prior-year AGI was over $150,000 ($75,000 if married filing separately).

The prior-year number is the useful one, because you already know it. It's sitting on last year's return. You don't have to forecast anything, guess at a bonus, or predict what the stock does — you just have to clear a number you can look up.

This is worth saying plainly: safe harbor doesn't reduce your tax by a dollar. It ends the penalty. That's all it does, and that's enough.

The part almost nobody tells you: withholding beats an estimated payment

Here's the rule that makes this whole article worth reading.

Tax withheld from your paycheck is treated as if it were paid evenly across the entire year — no matter when it was actually withheld. That's §6654(g). Estimated payments get no such treatment; they count when you actually make them.

Sit with the implication for a second, because it's genuinely useful.

It's late July. You're $9,800 short, and you've been short since the shares vested. If you make an estimated payment on September 15, it fixes Q3 and forward. The earlier quarters stay underpaid, and the penalty on them still accrues.

But if you go to payroll and raise your Form W-4 withholding for the rest of the year — say by adding an extra amount per paycheck on Step 4(c) — the IRS treats that money as though it had been withheld evenly since January. The same dollars, routed differently, retroactively cure quarters that an estimated payment can't touch.

It is, as far as timing tricks in the tax code go, an unusually good one. (It is also entirely ordinary and correct. There is nothing clever being pulled here — the code simply says withholding is ratable, and we're taking it at its word.)

Same dollars. Same total. The only difference is which mechanism you use to send them — and one of them fixes the past while the other only fixes the future.

What to actually do, and by when

1. Find your number. What did the RSUs actually cost, versus what payroll withheld? The calculator gets you close in about a minute.
2. Look up last year's total tax (Form 1040, the total tax line — not the refund, not the balance due). Multiply by 1.1 if your prior-year AGI was over $150,000. That's your safe-harbor target.
3. Compare it to what's been withheld so far this year. The gap between those two figures is the entire problem, stated as one number.
4. Fix it with the W-4 if you still have paychecks left. Divide the gap by the number of pay periods remaining and put that on Step 4(c). This is the move, and it gets weaker every pay period you wait — by late November there aren't enough paychecks left to carry it.
5. Use an estimated payment for the rest, or if you're self-employed and have no W-2 to adjust. September 15 for Q3; January 15, 2027 for Q4.

What if I've already missed a quarter?

Then you reduce the damage rather than eliminate it, and it's still very much worth doing. The penalty is interest-like — it accrues on the amount you're behind, for the time you're behind. Every week earlier you close the gap is a week it stops running. There's no cliff you've already fallen off, and no reason to wait for January because you missed June.

Common questions

If I hit safe harbor, do I still owe tax in April?

Yes, quite possibly a lot. Safe harbor turns off the underpayment penalty; it doesn't change what you owe. If you're relying on the 110% prior-year number and your income jumped sharply this year, plan for a real balance due at filing — you've protected yourself from the penalty, not from the bill.

Is it better to adjust my W-4 or make an estimated payment?

If you still have paychecks coming, the W-4 is usually better, and it's not close. Withheld tax is treated as paid evenly across the whole year under §6654(g), so increasing it now retroactively covers earlier quarters. An estimated payment is credited when you make it and does nothing for quarters already past. Use estimated payments when you have no wages to withhold from, or when there isn't enough salary left in the year to carry the amount.

My RSUs vest in December. Am I already too late?

Not necessarily, but your options narrow fast. There may not be enough remaining paychecks to withhold a meaningful amount, which pushes you toward the January 15 estimated payment. If your vesting schedule is Q4-weighted, the planning conversation belongs in October, not the following April.

I also own a business. Same rules?

Same deadline and the same safe harbor, but a different problem underneath: nobody withholds anything from business income, so the entire burden is yours from the start rather than partially covered by a flat 22%. Self-employment tax is usually the piece that surprises people, and if you run an S-corp your reasonable-compensation W-2 is a lever most owners never use. See September 15 for business owners.

Does my state have the same problem?

Often, yes — and people miss it because they're focused on the federal side. California withholds stock and bonus income at a flat 10.23% while a high earner's California marginal rate runs 9.3% to 12.3%, so California under-withholds for exactly the same reason the IRS does. New York withholds at a flat 11.7%, which usually does cover it. Texas and Washington don't tax wage income at all, so it's a federal-only question there.

I sold the shares immediately. Does that change anything?

It changes what you do with the money, not what you owe. The tax at vesting is triggered by vesting, not by selling — selling immediately just means little or no additional gain on top. If you sold to cover, check what was actually withheld against what's owed; "sell to cover" typically covers the same flat 22%, which is the shortfall we've been discussing, not a solution to it. See how RSUs are taxed for the full lifecycle, and are RSUs taxed twice for the cost-basis trap at sale.

When it's worth a conversation

If your RSUs are a small slice of your income and you're comfortably inside the prior-year safe harbor, this is genuinely a fifteen-minute job you can do yourself with the calculator and a copy of last year's return.

Where it stops being a form-filling exercise: multi-state situations, a vesting schedule weighted into Q4, RSUs large enough relative to salary that there isn't enough withholding capacity left in the year, or a prior year that looks nothing like this one. Those need someone to run the actual numbers rather than apply a rule of thumb.

That's the conversation we'd rather have in August than in April.