Tax Planning for High Income Earners: Why Your Bonus Is Under-Withheld by Design
A $120,000 equity vest lands in a September paycheck. The company withholds $26,400 on it, which looks about right to anyone who does not stop to check. The actual federal tax on that money is closer to $40,700.
That $14,000 gap is not a payroll error. It is how the withholding system is built, and it is the most common blind spot in tax planning for high income earners whose pay changed partway through the year. The people it catches are not careless. They are busy, they have never had a reason to audit their own withholding, and the number on the pay stub looked plausible.
Your bonus is withheld at a flat rate, not at your rate
Employers can withhold federal income tax on supplemental wages, meaning bonuses, commissions, and equity vests, at a flat 22% when the payment is identified separately from regular wages. That is spelled out in IRS Publication 505 for 2026.
Twenty-two percent is a rule of administrative convenience. It is not a prediction about you.
If your marginal rate is 32% or 35%, every dollar of that supplemental pay is under-withheld by 10 to 13 cents. On a large vest or a signing bonus, that becomes real money, and it becomes real money in a year you were not watching for it.
For 2026, the 32% bracket starts at $403,550 of taxable income for married couples filing jointly and $201,775 for single filers. The 35% bracket starts at $512,450 and $256,225. A household comfortably inside either one is not being withheld anywhere close to its actual rate on a bonus.
Why last year's tax bill stops protecting you
Most people who avoid underpayment penalties do it without ever learning the rule. Pay in 90% of what you owe for the current year, or 100% of what you owed last year, and the penalty does not apply. It is the closest thing the tax code has to a safety net, and plenty of high earners have been standing on it for years without ever looking down.
For the third quarter of 2026 the IRS underpayment rate is 7%, annualized. That is the cost of the net not being there.
The part most people have backwards
Here is what almost nobody knows about that safety net. It is not 100% for you.
If your 2025 adjusted gross income was over $150,000, or over $75,000 filing separately, the prior-year figure becomes 110% rather than 100%. That is on the 2026 Form 1040-ES.
Read that again with a raise in mind. The event that increases your income is the same event that carries you over the threshold and lifts the bar you have to clear. Your tax goes up, and your protection against being penalized for it gets 10% more expensive, in the same year, automatically.
So the household most likely to be leaning on last year's number is the household for whom last year's number has quietly stopped working. Matching it to the dollar is not a defense. It is a shortfall with a penalty attached, and it is the clearest reason tax planning for high income earners has to happen mid-year instead of at filing.
An example, with rounded numbers
A married couple filing jointly, combined salaries around $500,000 for 2026. In September one of them vests $120,000 in company stock. Call it $620,000 for the year.
Withheld on the vest at the flat supplemental rate: $26,400. Actual federal tax on that slice of income, which spans the 32% and 35% brackets: roughly $40,700. Shortfall from a single predictable event: about $14,000.
Their 2025 AGI was over $150,000, so the 110% bar is the one that applies to them, and last year had no vest in it. Matching last year is not close.
This is an illustrative example with rounded figures, not any client's actual result. The shape is the point: one foreseeable event, one flat withholding rate, and a gap nobody sees for six months.
When this does not apply to you
If your income dropped this year, the math runs the other way and you are probably over-withholding. That is a cash flow problem rather than a penalty problem. Worth fixing, not urgent.
If your 2025 AGI was $150,000 or less, the 100% prior-year safe harbor still applies to you and is easier to clear.
If the extra income arrived as evenly spread regular wages instead of a lump, ordinary withholding usually keeps pace. The flat-rate gap is specific to supplemental pay.
And if you are self-employed rather than on a W-2, the withholding mechanics above are not your lever at all. Estimated payments are.
What tax planning for high income earners looks like in August
Pull your most recent pay stub and your 2025 return. Compare year-to-date federal withholding against 110% of the total tax on that return. That is your actual bar, not the number you have been assuming it is.
If you are tracking short, you have two levers and one date. The third quarter estimated payment is due September 15, 2026, and you still have four months of paychecks left to raise withholding against. Those two levers do not behave the same way, and the difference decides which one is worth using this late in the year. [Internal link 1: last week's post, which covers exactly that] walks through it.
Publication 505 also asks for a new W-4 within 10 days of a change that drops your withholding below your liability, which is worth knowing if the raise already happened.
Doing that comparison in August costs you twenty minutes. Doing it in April costs you the penalty and every option you had for fixing it.
Most of tax planning for high income earners is not exotic strategy. It is running the obvious check while the calendar still lets you act on the answer.
Your next step
We're currently offering our Tax Savings Blueprint, a forward-looking planning service for the 2026 tax year. We review your returns and current-year documents, project where your 2026 return is heading, and walk you through it on a call, so you leave with a clear picture of your tax situation, the strategies available to you, and a set of actions to put in motion.
Schedule a free intro call to learn more.
This is for general educational purposes and isn't personalized tax advice. Every situation is different, talk with your CPA before acting on any strategy discussed here.