RSU Tax Withholding: Why Your Employer Withholds Too Little (and the Quarterly Fix)

Here's a rite of passage for tech workers: your first big vest, a five-figure tax bill the following April, and the dawning realization that the taxes "withheld" at vesting weren't the taxes you actually owed. Nobody warns you. Payroll doesn't explain it. Your manager definitely doesn't know.

This is the guide I wish someone had handed me. The mechanics take five minutes to understand, and the fix takes about an hour a year.

What happens when RSUs vest

When RSUs vest, the fair market value of the shares on the vest date counts as ordinary income — just like salary. It shows up on your W-2. Your employer withholds taxes, usually by selling a portion of the newly vested shares ("sell-to-cover") and sending the proceeds to the IRS and your state.

So far, so reasonable. Here's the catch: the amount withheld is not based on your tax bracket.

The 22% trap

The IRS classifies RSU vest income as supplemental wages — the same bucket as bonuses. For supplemental wages, employers are allowed (and in practice, required) to withhold federal income tax at a flat 22% on amounts up to $1 million per year, and 37% on amounts above that. This comes straight from IRS Publication 15, and your payroll department applies it automatically.

But 22% is a withholding convention, not your tax rate. Your actual tax rate is your marginal bracket, determined by your total income. If your salary plus vests plus bonus puts you in the 32% or 35% federal bracket, the IRS still only got 22% at vest time — and you owe the difference in April.

An illustrative example

The numbers below are a simplified illustration, not anyone's real situation. State taxes, Social Security, and Medicare also apply at vest.

Say $100,000 of RSUs vests in a year, and your marginal federal bracket is 32%:

That's $10,000 of surprise tax bill per $100,000 vested — before state taxes, which have their own version of this problem. (California, for example, withholds 10.23% on supplemental wages while its top marginal rate is 13.3%.) Multiply by a few vest dates a year and you see why April hurts.

Why doesn't payroll just withhold the right amount?

Because it can't. Your employer doesn't know your spouse's income, your other investments, your deductions, or your total picture. The flat 22% is a procedural simplification so payroll can process thousands of vests without doing a personal tax return for each employee. It's not a bug in your company's system; it's how the system is designed.

You also generally cannot ask payroll to withhold a higher flat rate on RSU vests specifically — the supplemental-wage rate is set by the IRS. The fix happens on your side, not theirs.

The quarterly fix

The IRS doesn't care how the right amount gets paid during the year — withholding, estimated payments, or a mix. It only cares that enough is paid on time, each quarter. So the standard fix for RSU earners is quarterly estimated tax payments that cover the gap between the 22% withheld and your actual marginal rate.

Step 1: Estimate the annual gap

Once a year (January is a good time), estimate: total vest value expected this year × (your marginal federal rate − 22%). Add the state equivalent: vest value × (your marginal state rate − your state's supplemental withholding rate). That total is roughly what you'll owe beyond withholding.

Step 2: Divide into four payments

The IRS due dates are April 15, June 15, September 15, and January 15 of the following year. Pay a quarter of your estimated gap by each date. Federal payments go through IRS Direct Pay (free) or a card processor if the rewards math works; state payments go through your state's portal.

Step 3: Use the safe harbor

You don't need the estimate to be perfect. The IRS waives underpayment penalties if your total withholding + estimated payments reach a safe harbor: generally 100% of last year's total tax (110% if your adjusted gross income was above $150,000), or 90% of this year's tax, whichever is easier to hit. For most RSU earners with rising comp, the 110%-of-last-year target is the practical one — it's a known number you can pull straight off last year's return, and hitting it means no penalties even if you still owe a balance in April.

Two mistakes to avoid

• Paying the whole gap only in Q4. Estimated taxes are a pay-as-you-go system. A single big January payment can still trigger underpayment penalties for the earlier quarters. Spread it out.
• Forgetting state. The federal gap gets all the attention, but in high-tax states the state shortfall can be nearly as large. Run the same math for your state.

Make tax season boring

When your equity comp is simple — RSUs vested, taxes handled quarterly, documents in one place — tax software handles the return without drama. The complexity that sends people to a CPA usually comes from ISOs, early exercises, or multi-state situations, not from plain RSUs with a quarterly payment habit.

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The bottom line

Your employer withholds 22% because the IRS tells it to, not because 22% is what you owe. The gap between withholding and your marginal rate is predictable, measurable, and fixable with four payments a year. Set up the quarterly habit once, and April goes back to being just another month.