Why Your RSU Tax Withholding Is Probably Wrong

Henry Supinski, ChFC® · 4 min read · July 2026

If you've ever been surprised by a tax bill the April after a big vest, the withholding on your RSUs is very likely the reason. It's not a mistake by your employer. It's just how the default rate works.

The Default Rate Is a Flat Percentage

Most employers withhold RSU income at a flat federal supplemental rate, commonly 22% up to a threshold and 37% above it, plus state withholding. That flat rate has nothing to do with your actual marginal tax bracket, which for many tech employees with a base salary plus vesting equity is well above 22%.

Where the Gap Shows Up

If your real marginal rate is 32% or higher and your RSUs were withheld at 22%, the difference doesn't disappear. It shows up as a balance due when you file, sometimes alongside an underpayment penalty if the gap was large enough across the year.

How to Close the Gap Before It's a Surprise

• Adjust your W-4 to withhold additional flat-dollar amounts from your regular paychecks.
• Make quarterly estimated tax payments if withholding alone won't cover the gap.
• Sell a portion of shares at vest specifically to cover the true tax liability, not just the withheld amount.
• Model out a large vesting year in advance, especially if it coincides with a bonus or other income spike.

Do This Before Your Next Big Vest

The fix is simple once you know it's needed: figure out your actual marginal rate, compare it to the flat withholding rate, and adjust before the vest, not after you've already spent the difference.

A Hypothetical Vest, Start to Finish

Numbers make this concrete, so here is a hypothetical. An engineer earns a $220,000 base salary and has 1,000 RSUs vesting this year at an average price of $150 per share. The vest adds $150,000 of ordinary income on top of her salary. Her plan runs a sell-to-cover at the flat supplemental rate and remits roughly $33,000 in federal withholding on the vest.

Her actual federal liability on that same $150,000, given everything else on her return, comes out closer to $50,000. Nothing on her paystub flags the difference. The brokerage confirmation shows shares sold for taxes, which feels like the taxes are handled. The gap of roughly $17,000 surfaces the following April as a balance due. Spread across several vests in a year, it can bring an underpayment penalty along with it. The numbers here are invented. The pattern is not.

Sell-to-Cover Is Not the Same as Fully Covered

Most plans default to sell-to-cover. The broker sells just enough shares at vest to fund the statutory withholding. The word "cover" does a lot of misleading work in that sentence. It covers the withholding requirement, not your tax bill. Those are only the same number if the flat supplemental rate happens to match your real marginal rate. For most people with a strong salary plus vesting equity, it does not.

One more mechanic worth knowing. Your cost basis in the remaining shares steps up to the vest-date value. If you sell those shares soon after vest, the additional gain or loss is usually small. Holding them is a separate decision about concentration in one stock. It is not a way to undo the tax already owed at vest.

How We Approach It

I sat through these vest windows myself at SAP, with my own money and my own April surprises. So this is one of the first things we check for a new client with equity. We map the vesting calendar for the year, estimate the real marginal rate on top of salary and bonus, and compare that to what the plan will actually withhold. Then we close the gap on purpose: extra W-4 withholding on regular pay, quarterly estimates, or cash set aside from each vest. If SAP equity is part of your picture, the specifics live on our SAP RSU withholding page. The broader playbook is on our tech employees page.

Questions We Hear

Can I just ask my employer to withhold more at vest?

Some plans let you elect a higher withholding rate on equity income, but many lock the flat rate into payroll and give you no option. That is why the practical fix usually happens on your side: a W-4 adjustment, quarterly estimated payments, or a cash reserve funded from each vest.

Will I owe a penalty if my withholding falls short?

Not automatically. The IRS has safe harbor rules based on paying in a set portion of your current or prior year liability through withholding and timely estimates. Meet a safe harbor and the shortfall is just a balance due. Miss it and a penalty can apply. That math is worth running before a large vest year, not after.

Should I sell my shares at vest or hold them?

Tax is only one input. Because your basis resets to the vest-date value, selling soon after vest usually adds little additional gain. Holding is really a question of how much of your net worth you want riding on one company. We treat that as a concentration decision to make deliberately, not a default.

Does this problem go away at higher incomes?

Partly. Once supplemental wages pass a statutory threshold for the year, employers must withhold at the top rate, which narrows the gap. But most people with meaningful RSU income sit between the flat rate and the top bracket, which is exactly where the mismatch is largest. State withholding can have its own version of the same problem.