Why Your RSU Withholding Didn't Cover the Tax Bill (And What to Do About It)

The email from your accountant lands in April. Not a refund -- a balance due for $47,000. Your RSUs vested throughout the year, your employer withheld shares at the standard rate, and you assumed that covered it. It did not. This is the most common surprise our clients describe, and it is almost always preventable.

The reason is straightforward: the default withholding rate on RSUs is lower than your actual tax rate, and the gap widens the more you earn. Understanding exactly why that happens is the first step to making sure it does not happen again.

The 22% Problem

When your RSUs vest, your employer treats the value as supplemental wages. The IRS allows a flat 22% federal withholding rate on supplemental income up to $1 million in a year. Above that, it jumps to 37%. But here is the issue: if you are in the 32% or 35% federal bracket (which you almost certainly are on a California tech salary north of $300,000), the 22% rate leaves a gap. California then applies its own supplemental rate of 6.6% on the first $1 million, but your actual California marginal rate could be 9.3%, 10.3%, or even 13.3%.

A senior director at a large public tech company whose RSUs vest quarterly could easily see a combined gap of 15 to 20 percentage points between what is withheld and what is actually owed. On $300,000 in vesting income, that is $45,000 to $60,000 in unwithheld tax. That number arrives in April as a balance due. As we covered in a previous post, your RSUs do not just add income -- they reshape your entire tax picture, and withholding is only one layer of that.

How California Makes It Worse

California taxes all income earned while you are a resident. For RSUs, the state sources the income based on your workdays between the grant date and each vest date. If you worked in California for half of that period, California taxes half the gain. The calculation gets complicated fast, especially if you joined the company after the grant or if you are considering a move out of state.

The state's flat supplemental withholding rate of 6.6% rarely covers what you actually owe, because your California marginal rate is almost certainly higher. If you are in the 9.3% bracket, that is a 2.7 point gap just on the state side. On top of that, California does not allow the same deductions and adjustments that reduce your federal liability, so your effective state rate on RSU income can be higher than you expect. For a deeper look at how California taxes equity compensation specifically, read our guide to RSU tax planning for California.

The NIIT Surprise

The Net Investment Income Tax adds a 3.8% surtax on the lesser of your net investment income or the amount by which your modified adjusted gross income exceeds $200,000 (single) or $250,000 (married filing jointly).

Here is the part that catches people: when you sell vested RSU shares, the gain counts as investment income. And your RSU income at vest counts as wages that push you past the NIIT threshold. So you are paying the 3.8% on top of your regular income tax, and your employer did not withhold for it at all. The NIIT is not subject to withholding -- it lands entirely on your tax return, and you are expected to cover it through estimated payments or increased W-4 withholding on your salary.

Sell-to-Cover Is Not a Plan

Many employers offer sell-to-cover: they sell enough shares at vest to cover the withholding and send the rest to your brokerage account. It is convenient, and it feels like settling your tax bill automatically. But the shares get sold at whatever the market price is that day, not at a price you chose. If the stock is down from its high, you are selling more shares to cover the same tax bill, which accelerates the dilution of your position.

Sell-to-cover also does not address the gap we just described. It only covers the 22% federal plus 6.6% California. The rest of what you owe -- the bracket gap, the NIIT -- is still unpaid. The right approach is to calculate the full gap and cover it through estimated payments or a higher salary withholding election on your W-4, rather than letting more shares go at an unfavorable price.

What to Do This Quarter

You can fix a withholding gap before next April. Here is the order of operations:

Step 1: Calculate the gap. Add up the RSU income you expect to vest this year. Multiply by your combined federal and state marginal rate (include the 3.8% NIIT if you will exceed the threshold). Subtract what your employer will withhold. The difference is your shortfall.

Step 2: Increase salary withholding. The simplest fix is a W-4 adjustment. Ask your payroll department to withhold an extra fixed dollar amount from each paycheck. The IRS and FTB treat this as timely withholding even if the RSU income has not landed yet, which avoids underpayment penalties.

Step 3: Make quarterly estimated payments. If your salary withholding cannot absorb the gap, file Form 1040-ES (federal) and Form 540-ES (California) and pay the difference in quarterly installments. The safe harbor rules protect you if you pay at least 100% of last year's tax (110% if your AGI was over $150,000). Our year-round tax planning playbook walks through the quarterly schedule and how to coordinate estimated payments with your other income.

Step 4: Review at every vest. Each vest event changes your cumulative income picture. Set a calendar reminder to check your withholding after every vest cycle, especially if the stock price moves significantly.

The Bigger Picture

An RSU withholding gap is not just a surprise bill. It is a signal that your equity compensation has grown beyond the level where default settings work. Once the gap reaches five figures, you are in territory where a once-a-year filing approach leaves money on the table and creates tax bills you did not plan for.

A tax strategist does what a preparer does not: looks at the full picture of your equity compensation, your salary, your other income sources, and your goals, then designs a withholding and payment plan before the income arrives. It is the difference between filing what happened and designing what could happen, which we covered in our customized tax strategy post.

And if the gap is large enough to make you think seriously about your concentrated position, the conversation moves from "how do I cover the tax" to "how do I reduce the position without creating a second tax catastrophe." That is where a multi-year diversification plan starts.

The fix starts with a conversation, not a filing. Call us at (619) 280-2700 for a free 15-minute discovery call. The paid strategy session is where the real plan gets built, and the first call is how you find out whether this kind of planning fits what you need.

FAQ

Why is RSU withholding lower than my actual tax rate?

RSUs are classified as supplemental wages, and the IRS allows employers to withhold at a flat 22% on the first $1 million. If your marginal federal rate is 32% or higher, that creates a gap. California adds another layer with its own flat supplemental rate of 6.6%, which is often below your actual California marginal rate.

Does sell-to-cover at vest cover all the taxes I owe?

No. Sell-to-cover only covers the default withholding amounts (22% federal plus state supplemental). It does not account for your higher marginal bracket, the Net Investment Income Tax, or state rate differences. You are still responsible for the gap.

Can I use a W-4 to fix an RSU withholding shortfall?

Yes. Increasing the extra withholding amount on your W-4 is the simplest fix. Your employer treats it as timely tax withholding, which avoids underpayment penalties, and you can adjust it at any time during the year.

What is the NIIT and how does it affect RSUs?

The Net Investment Income Tax is a 3.8% surtax on investment income above $200,000 (single) or $250,000 (married filing jointly). Your RSU income at vest counts toward that threshold, and when you sell your RSU shares, the gain may be subject to the NIIT. Your employer does not withhold for it.

How do estimated tax payments work for RSU income?

File Form 1040-ES for federal and Form 540-ES for California. Pay the difference between what is being withheld and what you expect to owe in four quarterly installments. The safe harbor rules protect you from penalties if you pay at least 100% of last year's tax (110% if your AGI was over $150,000).

What happens if I do not fix the withholding gap?

You will owe the difference when you file your return, plus potential underpayment penalties from both the IRS and the California Franchise Tax Board. The penalties are calculated on the unpaid amount and the time it was outstanding.

This article is for educational purposes only and does not constitute tax, legal, or investment advice.