Q3 Estimated Tax Payments for High Earners: Closing the RSU Underwithholding Gap by September 15

Disclaimer: This article is for educational purposes only and does not constitute tax, legal, or financial advice. Tax rules are complex and subject to change. Consult a qualified CPA or tax attorney before implementing any tax strategy.

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Why Q3 Is the Make-or-Break Deadline for High Earners

September 15 is the deadline most high earners get wrong. By the time the third estimated tax payment is due, eight and a half months of the year are already behind you. Your bonus has been paid. Your spring and mid-year RSU vestings have settled. Your W-2 withholding is on track for whatever percentage of your salary it has covered all year. The gap between what you owe and what has been collected is now measurable, and so is the cost of ignoring it.

The IRS underpayment penalty rate for Q2 and Q3 of 2026 is 8% APR, accruing daily on the shortfall. That rate has been at or above 7% since late 2022, well above the 3-5% range that high earners learned to ignore in the 2010s. At today’s rate, the penalty on a $30,000 underpayment carried for ten months is roughly $2,000. Most high earners would rather not write that check to the IRS for the privilege of bad cash management.

The other reason Q3 matters: it is your last reasonable chance to fix the year. The Q4 estimated payment is not due until January 15 of the following year, after the calendar has already closed and most of the damage is done. If you correct the shortfall by September 15, you stop the penalty from compounding through the heaviest quarter of the tax year.

Key Facts: Q3 estimated tax payments for high earners

• The Q3 2026 estimated tax payment is due September 15, 2026
• The 2026 underpayment penalty rate is approximately 8% APR, the highest in over a decade
• Safe harbor for high earners (prior-year AGI above $150,000) is 110% of last year’s total tax or 90% of this year’s total tax, whichever is less
• Employer RSU withholding at the 22% supplemental rate systematically underwithholds high earners by 10-15 percentage points relative to a 32-37% marginal rate
• Bumping W-2 withholding via Form W-4 Step 4(c) retroactively covers earlier quarters because withholding is treated as paid evenly across the year
• For most high earners with W-2 plus RSU income, a Form W-4 bump in September beats a Q3 estimated payment

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The RSU Underwithholding Trap

The most common reason a high earner ends up owing estimated tax has nothing to do with side income or capital gains. It is RSU vesting.

When your RSUs vest, your employer treats the vested value as supplemental wages. IRS regulations require the supplemental withholding rate of 22% for any single supplemental payment under $1 million per year. Anything over $1 million in a single year gets 37% on the excess. For most high earners, every RSU vest hits at the 22% rate.

The math gets ugly once your marginal rate climbs above 22%. A $200,000 RSU vesting year for someone in the 35% federal bracket withholds $44,000 (22% of $200,000), but the actual federal tax owed on that income is around $70,000 (35% of $200,000). The shortfall is $26,000 before state taxes. Add California’s 11.3% and you are short another $22,600, for a combined federal-plus-state shortfall close to $50,000.

This is not theoretical. Most tech, finance, and pharma compensation packages front-load RSU value into years three and four of vesting, which is also typically when total comp pushes the recipient into the 32%, 35%, or 37% federal bracket. The withholding stays at 22%, the marginal rate climbs, and the shortfall grows.

Brokerages do not fix this. Schwab, Morgan Stanley at Work, Fidelity, and E*Trade all default to selling enough shares at vest to cover the 22% supplemental rate plus whatever state withholding the employer specifies. None of them automatically increase withholding when you cross into a higher bracket. The responsibility falls entirely on you.

For a deeper breakdown of the RSU withholding math by federal bracket and state, see our RSU Tax Strategies for High Earners guide.

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How Safe Harbor Math Actually Works for $300K+ Earners

Safe harbor is the IRS rule that says you do not owe an underpayment penalty if you have paid enough tax through withholding and estimated payments, regardless of what you ultimately owe. The exact threshold depends on your prior-year AGI.

Prior-Year AGI	Safe Harbor Threshold
$150,000 or less	90% of current-year tax OR 100% of prior-year tax
Over $150,000	90% of current-year tax OR 110% of prior-year tax

For anyone reading this article, the relevant number is the second row. You owe the lesser of 110% of last year’s total tax or 90% of this year’s total tax. Pay at least that through some combination of W-2 withholding, employer withholding on RSU vestings, and estimated payments, and the IRS will not assess an underpayment penalty even if your final April bill is enormous.

The two calculations matter for different reasons:

• 110% of prior-year tax is the predictable number. You already know what your 2025 total tax was; look at line 24 of your 2025 Form 1040 and multiply by 1.10.
• 90% of current-year tax is harder to pin down because your income changes through the year. But if your 2026 income will be materially lower than 2025 (you got laid off, lost a bonus, or had a smaller RSU vest), the current-year calculation is your friend.

Most high earners with rising income target the prior-year safe harbor because it is fixed and known. Most high earners with falling income target the current-year safe harbor because it lets them pay less.

Why “100%” Does Not Apply to You

A lot of generic tax content cites the 100%-of-prior-year safe harbor. That rule only applies if your prior-year AGI was $150,000 or less. If you are reading the High Earner Playbook, you almost certainly cleared that threshold long ago. Use 110%.

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How to Calculate Your Q3 Payment

The right number to pay on September 15 depends on what you have already paid year-to-date and which safe harbor you are targeting. Walk through this in order:

1. Pull your year-to-date W-2 federal tax withheld. Look at your most recent pay stub or your employer’s payroll portal. Multiply by 12 divided by months elapsed to project full-year withholding.
2. Add RSU and bonus federal tax withheld. Pull this from your equity broker portal (Schwab, Morgan Stanley at Work, Fidelity, E*Trade) or your bonus pay stub.
3. Compare to your safe-harbor target. This is either 110% of last year’s Form 1040 line 24, or 90% of your projected current-year tax.
4. Take the safe-harbor target minus projected full-year withholding. This is your total estimated tax need for the year.
5. Divide by four for the per-quarter amount. If you have already paid Q1 and Q2 evenly, your Q3 payment is the quarterly amount. If you have paid nothing through estimated payments, send three quarters’ worth on September 15.

Worked Example

A senior software engineer at $450,000 total comp ($300K salary plus $150K of RSU vesting expected in 2026):

• 2025 total tax (Form 1040 line 24): $122,000
• 110% of 2025 tax = $134,200 (the safe-harbor target)
• Projected 2026 W-2 withholding: $72,000 (24% effective on $300K salary)
• Projected 2026 RSU withholding at 22%: $33,000 (22% of $150K)
• Total projected withholding: $105,000
• Required estimated tax for the year: $134,200 minus $105,000 = $29,200
• Per-quarter estimated payment: $7,300

If this engineer has paid nothing through estimated taxes by September 15, the catch-up payment is $7,300 multiplied by 3, or $21,900.

This is the worst-case scenario where the engineer has been paying nothing and now owes three quarters’ worth. In practice, most high earners are better off bumping W-2 withholding instead.

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The Withholding Bump Alternative (and Why It Is Usually Better)

The IRS treats W-2 withholding as paid evenly through the year, regardless of when it is actually withheld. This is a useful quirk. If you increase your W-2 withholding via Form W-4 in September, the IRS counts that increased withholding as if it had been spread across all four quarters. Estimated payments do not work this way; they are credited to the quarter in which they are paid.

This means a Form W-4 Step 4(c) bump in September can cure underpayments from Q1 and Q2. A direct estimated payment cannot.

How to Bump Withholding

1. Pull a fresh Form W-4 from irs.gov.
2. Complete Steps 1 (personal info) and 5 (signature).
3. In Step 4(c), Extra withholding, enter the per-paycheck amount you want added.
4. Calculate the per-paycheck amount by dividing the total catch-up by your remaining paychecks. If you are paid biweekly and have eight paychecks left in the year, divide by eight.
5. Submit to your employer’s payroll team.

Using the example above: $21,900 catch-up divided by 8 remaining biweekly paychecks is $2,738 per paycheck. That is a noticeable hit to take-home pay, but it eliminates the September 15 estimated payment and the penalty risk.

Withholding vs. Estimated Payment: Quick Comparison

Method	Treated as paid evenly all year	Adjusts mid-year	Best for
W-2 withholding bump (Form W-4)	Yes	Yes	Catching up after underpayment, regular salary income
Estimated tax payment (Form 1040-ES)	No, credited to actual quarter paid	No	Self-employment, bonuses already received, RSU vestings already settled

For most high earners with primarily W-2 plus RSU income, the W-4 bump is faster, easier, and retroactively covers earlier quarters. Estimated payments make more sense if you are self-employed, have large one-time capital gains, or are early in the year and want to spread payments evenly.

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What to Do If You Already Missed Q1 or Q2

The penalty for Q1 and Q2 underpayment is already accruing. There is no way to retroactively eliminate that penalty by writing a check now. The only mechanism that retroactively cures earlier quarters is bumping W-2 withholding, because withholding is deemed paid evenly across the year.

If you have W-2 income, do the withholding bump. If you do not (self-employed, recently separated, retired), make the Q3 estimated payment for the full year-to-date catch-up amount. This stops the meter at September 15 and prevents the Q3 and Q4 portion from accruing further.

The penalty itself is calculated on Form 2210 when you file your 2026 return in spring 2027. The IRS will compute it for you if you do not. Either way, it is a one-time line item, not an ongoing collection.

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How to Actually Pay

For the September 15 payment itself, your options:

Method	Speed	Fee	Best for
IRS Direct Pay	Immediate	$0	Most high earners
EFTPS	1-2 business days	$0	Recurring quarterly payers (requires enrollment)
Form 1040-ES voucher by check	5-10 days	$0	Last-resort or amounts over $25K
Pay1040 credit card	Immediate	1.75%	Earning a welcome bonus or category multiplier that exceeds 1.75%

The credit card path is a high-earner-specific play. If you are paying $30,000 in estimated tax and have just opened a card with a $1,500 welcome bonus that requires $6,000 in spend, you can clear the spend, earn the bonus, and pay the 1.75% Pay1040 fee. Net result is roughly $1,000 of value created from a tax payment you owed anyway. See our Best Credit Cards for High Earners guide for cards with current welcome bonuses.

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What This Looks Like in Practice

For a typical high earner with $400K-700K total comp and a meaningful RSU vesting schedule, the September 15 calculation usually lands in one of three buckets:

• Under-withheld by $5K-$20K: Bump Form W-4 Step 4(c) for the rest of the year. Skip the estimated payment.
• Under-withheld by $20K-$50K: Bump W-4 hard, then send a partial estimated payment via IRS Direct Pay to cover the gap. Splitting reduces the per-paycheck hit.
• Under-withheld by $50K+: Send the full catch-up via Pay1040 with a high-bonus credit card, then start a W-4 bump for the new year.

The Q3 deadline is one of the few times during the year when 30 minutes of attention saves a meaningful four-figure penalty. Pay attention to it.

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Related reading:

• RSU Tax Strategies for High Earners
• The High Earner’s Tax Playbook
• Estimated Tax Payments for $300K+ Earners
• 2026 Tax Brackets for High Earners
• Best Credit Cards for High Earners