Anduril RSU Tax Withholding: What to Fix Before January

By Joe Forish, CFA, CFP®|Published On: Oct 9, 2026|Categories: Financial Planning, RSUs|

Your RSUs settled. Shares disappeared. Anduril RSU tax withholding showed up on your statement as a substantial figure, and a reasonable person would conclude the tax was handled.

Two things are wrong with that conclusion — and only one gets discussed.

The first is the size of the gap. Anduril RSU tax withholding follows the federal supplemental wage rules: a flat 22% of settlement value up to $1 million of supplemental wages, then 37% above that. It is a statutory default, not a calculation of what you owe. If your marginal rate is 35% or higher — and at Anduril compensation levels, it very likely is — withholding covered roughly two-thirds of your federal liability. The rest is yours to pay.

The second problem is the one almost nobody raises. That remaining third was not due in April. It was due in quarterly installments. And one of those installments has already passed.

This article is about the second problem and the one lever that still works in October. For what the conversion to single-trigger vesting actually changed about your equity situation, start with the Anduril employee equity guide.

Why Anduril RSU Tax Withholding Is Not the Payment

When your RSUs settled, Anduril withheld federal tax at the IRS supplemental wage rate: 22% on the first $1 million of supplemental wages in a calendar year, then 37% on anything above that threshold. That rate is set by rule. It does not change based on your salary, your filing status, or your total income for the year.

Here is what that looks like in practice. Take $250,000 in salary and $800,000 in RSU settlement income. Withholding at 22% produces $176,000. The actual federal tax attributable to that settlement, stacked on top of the salary, runs closer to $296,000. The gap is roughly $120,000 — before state tax.

Furthermore, the state layer adds to that shortfall in a real way. New York State withholds supplemental wages at its own flat rate, which carries the same structural flaw as the federal default: it is a statutory rate, not a calculation of your liability. New York City residents owe city income tax on top of that, and it is not separately withheld on supplemental income. If you are a New York resident, your combined gap is materially larger than the federal figure alone.

Get the Anduril RSU Tax Withholding Rate Right Before You Calculate Anything

Before calculating your Anduril RSU tax withholding gap, one correction is worth making because a wrong rate produces a wrong plan.

Some coverage of the 2026 Anduril settlement includes the 3.8% net investment income tax (NIIT) in all-in rate estimates. That figure does not belong there. NIIT under IRC §1411 applies to net investment income: interest, dividends, capital gains, and passive income. Wages are specifically excluded. RSU settlement income shows up on your W-2 as compensation, and it is unambiguously not investment income.

On $800,000 of settlement income, incorrectly including NIIT inflates your estimate by roughly $30,000. That is not a rounding error — it is a planning error that leads people to over-reserve in ways that hurt other decisions.

The wage-side surtax that does apply is Additional Medicare Tax at 0.9% on wages above $200,000 per employer, under IRC 3101. Your employer generally withholds this automatically once wages with them cross the threshold, so for most employees it is already covered rather than a separate gap.

State income taxes are the additional layer that is most often underestimated. Calculate your state exposure explicitly — not as an afterthought to the federal math.

The September Payment Date You Already Missed and What It Means for Your Anduril RSU Estimated Tax

Here is the part that changes what to do this month.

Federal income tax operates on a pay-as-you-go basis. The IRS does not expect you to settle up in April — it expects you to have paid as the income arrived, through withholding, estimated payments, or some combination of both. Income is assigned to a payment period based on when you received it, and each period carries its own due date.

For 2026 calendar-year taxpayers, as defined in IRS Form 1040-ES, those periods run as follows:

Period 1: January 1 – March 31

Due April 15, 2026.

Period 2: April 1 – May 31

Due June 15, 2026.

Period 3: June 1 – August 31

Due September 15, 2026.

Period 4: September 1 – December 31

Due January 15, 2027.

An August settlement falls squarely in Period 3. The tax on it was due September 15 — a date that has passed. Under IRS Topic No. 306, underpayment interest accrues per installment from each due date forward, not as a single charge at filing. So if withholding did not cover the shortfall and no estimated payment was made, that charge has been running since mid-September.

The amount is modest so far. Underpayment is computed as interest on the shortfall, not a flat penalty, and only a few weeks have elapsed. The useful part is this: one payment date is behind you and one is ahead, and there is still a quarter of payroll left to work with. The same reader looking at this in March will have neither.

The Three Safe Harbors for Anduril RSU Withholding, and Which One to Aim At

You avoid the underpayment charge entirely if your total 2026 payments reach any one of three safe harbors, as described in IRS Publication 505.

Safe Harbor 1: 90% of Your Current-Year Tax

Pay at least 90% of your actual 2026 liability. This is accurate in principle, but it requires forecasting a return you have not filed yet. In a year with a large unexpected income event, that estimate carries real uncertainty.

Safe Harbor 2: 100% of Prior-Year Tax

Pay an amount equal to your total tax from 2025. You already have that number — it is on the return you filed earlier this year.

Safe Harbor 3: 110% of Prior-Year Tax

For taxpayers whose prior-year adjusted gross income exceeded $150,000, the harbor shifts to 110% of prior-year tax. At Anduril compensation levels, this is almost certainly the version that applies to you.

There is also a de minimis threshold: no charge if you owe less than $1,000 after withholding and credits. On a settlement of this size, that will not help.

Aim at Safe Harbor 3. It is the only one you can calculate exactly today, using a form you already have. In a year where income spiked sharply, it is almost always the cheaper target — because 110% of a normal year is far less than 90% of an exceptional one.

Pull your 2025 return. Find total tax. Multiply by 1.1. That number is your target for total 2026 payments. Everything else is arithmetic from there.

Why a W-4 Change May Beat a January Check for Anduril RSU Tax Withholding

When it comes to Anduril RSU tax withholding, estimated tax payments and additional W-4 withholding are not equivalent. The difference matters more than most people realize.

A January 15 estimated payment is credited as of January 15. It covers the fourth installment period only and does nothing for the September shortfall already accruing.

Withholding works differently. Under Form 2210 and IRC 6654, withholding is treated as paid in equal parts across all four periods regardless of when it was actually withheld. Additional withholding on your remaining 2026 paychecks can therefore reach the September shortfall in a way a January payment cannot. Ask your preparer to run both instruments against your remaining pay schedule before deciding.

Filing a revised W-4 takes about ten minutes. Every week of delay reduces the pay periods it can reach, so the window is real and closing.

What the Anduril RSU 409A Settlement Sets Up for Later

Two details from the August settlement deserve recording now, because both matter when the next liquidity window opens.

Your Basis Is the 409A Settlement Value

The price used at your Anduril RSU 409A settlement becomes your cost basis in the shares you received. Appreciation above that figure is a capital gain rather than compensation income, and gains on shares held for more than twelve months qualify for long-term capital gains rates. That is a meaningful improvement over the double-trigger structure, where the entire position arrived as ordinary income in a single moment.

Take the per-share figure from your own settlement statement. Anduril has not published the settlement price. The one firm anchor in the public record is the Series H preferred round: $68.95 per share on May 13, 2026, at a $61 billion post-money valuation. Common stock is priced at a discount to preferred, so your 409A basis sits below that figure. By how much is a question your statement answers and no blog post reliably does.

Your Holding Period Started at Settlement

Hold shares for more than twelve months from the settlement date and gains above basis qualify for long-term rates. Sell inside twelve months and gains are taxed as ordinary income — the same rate you just paid at settlement. For context on how tender windows affect this calculation and why the buyer’s identity also matters, see Anduril tender offers.

The downside scenario is also worth naming. If the valuation falls below your basis, you hold a capital loss on shares you have already paid ordinary income tax on. Capital losses offset capital gains first, and only $3,000 per year against ordinary income. The tax paid at settlement and any future loss on the shares sit in permanently separate buckets — the loss cannot recover the earlier tax payment. Accordingly, this asymmetry is the strongest argument for keeping your tax reserve in cash rather than in additional Anduril stock.

The New York Anduril RSU Tax Withholding Calculation Is a Separate Number

Most coverage of Anduril RSU tax withholding in 2026 has been written from a California frame of reference. If you are based in New York, the mechanics differ in ways that change the total materially.

New York State withholds supplemental wages at its own flat rate, a default rather than a calibrated calculation. New York City residents also owe city income tax on the settlement income, and that obligation is not separately withheld at a supplemental rate. Moreover, New York maintains its own estimated tax installment regime, with safe harbor thresholds that do not automatically align with the federal ones.

If you relocated during the vesting period, a multi-state allocation question may also arise. Equity compensation is generally sourced to where services were performed, which means more than one state could have a valid claim on the same income. That allocation is worth getting right before you file rather than after a notice arrives.

Calculate the state side alongside the federal figure, not as a follow-up task. For New York City residents, the combined gap is substantially larger than the federal shortfall in isolation.

A Note for Cleared Employees on Anduril RSU Tax Withholding

One consideration specific to employees with a security clearance, because it rarely comes up in tax planning conversations.

A six-figure tax liability is a financial obligation. Covering it through borrowing with a margin loan or a line of credit creates debt. Under Guideline F of the adjudicative guidelines, delinquency on financial obligations is the relevant concern, not the dollar amount itself. An obligation that becomes difficult to service is a material issue; an obligation funded from proceeds or cash generally is not.

In practice, funding this tax bill from settlement proceeds or existing cash is cleaner than funding it from credit, even in scenarios where borrowing looks better on the surface. The full analysis of how the common funding routes compare for cleared employees is in Anduril options and security clearance reporting.

What to Do Before January 15 to Cover Your Anduril RSU Withholding Gap

If you have read this far, you probably already know whether you have a shortfall. Here is the sequence that turns that recognition into action.

Step 1: Pull Your Prior-Year Return

Find your 2025 total tax on line 24. Multiply by 1.1. That is your target for total 2026 payments. This takes ten minutes and produces the number everything else is measured against.

Step 2: Add Up What You Have Actually Paid

Total your year-to-date federal withholding from every employer, plus any estimated payments already made. Your most recent pay stub and any Form 1040-ES vouchers you submitted will have these figures.

Step 3: Find the Gap

Subtract total payments from your target. That difference is the shortfall you are solving for. A zero or negative result may mean you are already covered, but confirm the installment-level detail with your preparer, because annual totals can look fine while a specific period is still short.

Step 4: Decide Between Withholding and an Estimated Payment

This is the decision with the most leverage and the narrowest window. Ask your preparer specifically whether additional withholding or a January estimated payment does more work for your situation, given your remaining pay schedule and the September shortfall you may be carrying.

Step 5: File a Revised W-4 If Withholding Is the Answer

If withholding is the right instrument, file the revised W-4 with your employer this week. It only works across pay periods that remain in the calendar year, and every week of delay reduces the number of periods it can reach. Ask payroll how many periods remain and what additional withholding per period is required to close the gap.

Step 6: Calculate the State Side Separately

Federal and state are separate calculations with separate due dates. New York City residents should add the city layer explicitly. If you relocated during the vesting period, raise the multi-state allocation question with your preparer before filing.

Step 7: Record Your Basis and Settlement Date Per Lot

Pull your settlement statement — the actual document, not a third-party figure — and record the per-share 409A price and the settlement date for each grant lot. Store both somewhere you will find in 2028, because you will need them at every future liquidity event.

Step 8: Model the Next Settlement Before It Arrives

Single-trigger vesting means this recurs. Another settlement may arrive before year-end. The September installment that passed cannot be undone, but the next one can be covered on the day it happens — if you have already run the calculation and know what your target is.

Frequently Asked Questions about Anduril RSU Tax Withholding

How much tax is withheld on Anduril RSUs?

Anduril RSU tax withholding follows the federal supplemental wage rules: a flat 22% on settlement value up to $1 million of supplemental wages in a calendar year, then 37% on amounts above that threshold. State withholding applies separately. Additional Medicare Tax at 0.9% applies to wages above $200,000, though employers generally withhold this automatically. Because 22% is a statutory default rather than a rate calibrated to your bracket, employees at a 35% or higher marginal rate will find that withholding covered roughly two-thirds of their federal liability. The remainder is due in quarterly installments, not at filing.

Do I need to make estimated tax payments on my Anduril RSUs?

If withholding did not cover enough of your 2026 liability to reach a safe harbor, then yes — or you need to increase withholding instead. An August 2026 settlement falls in the payment period running June 1 through August 31, with a due date of September 15. That date has passed. However, you can still avoid the underpayment charge on the fourth installment by reaching a safe harbor before January 15, 2027. The most actionable target for most Anduril employees is 110% of prior-year tax.

Will I owe an underpayment penalty on my Anduril RSU tax withholding shortfall?

Possibly, and the charge may already be accruing. Under IRS Topic No. 306, underpayment interest is computed per installment from each due date — not as a flat penalty at filing. An employee who settled in August and made no additional payment has likely been short since September 15. The amount to date is modest because only weeks have elapsed. Form 2210 is where the full computation is made, and your preparer can run it against your actual pattern.

Should I increase my withholding or make an estimated payment?

They are credited under different timing rules, and the difference is consequential. An estimated payment is credited when you make it, so a January 15 payment counts only toward the fourth installment. Withholding is generally treated as paid in equal parts across all four periods under Form 2210, regardless of when it was actually withheld. Consequently, additional withholding on remaining 2026 paychecks can reach the September shortfall in a way a January payment cannot. An alternative treatment credits withholding on the actual dates withheld, available when it produces a lower charge. Ask your preparer to run both before deciding.

Does the 3.8% net investment income tax apply to Anduril RSU income?

No. NIIT under IRC 1411 applies to net investment income — interest, dividends, capital gains, and passive income. Wages are specifically excluded, and RSU settlement income is compensation reported on your W-2. Rate estimates that include NIIT on settlement income are overstated. On an $800,000 settlement, that overstatement is roughly $30,000 — enough to distort planning decisions in a meaningful way.

What is my cost basis in the Anduril shares I received?

Your basis is the 409A value used at your Anduril RSU 409A settlement, and your holding period starts on the settlement date. Appreciation above that figure is a capital gain rather than compensation, and gains on shares held more than twelve months qualify for long-term rates. Take the per-share number from your own settlement statement rather than from any published estimate — Anduril has not disclosed it, third-party figures disagree with one another, and your statement is the document that governs.

What happens if Anduril’s valuation falls below my settlement price?

You would hold a capital loss on shares you have already paid ordinary income tax on. Capital losses offset capital gains first, and only $3,000 per year against ordinary income under current law. The tax paid at settlement and any future loss on the shares sit in permanently separate buckets — the loss cannot recover the earlier tax. That asymmetry is the clearest argument for holding your tax reserve in cash rather than in additional Anduril stock.