There is a specific flavor of shock that arrives each spring in tech-heavy zip codes: the high earner who had taxes “taken out” of every RSU vest, yet owes a five-figure check in April. Nothing went wrong, exactly. The system worked precisely as designed — and the design has a gap built into it. This guide shows you how to measure your personal gap and close it before it becomes a penalty.
Where the gap comes from
When RSUs vest, payroll treats the value as supplemental wages — the same category as bonuses. Federal rules allow employers to withhold on supplemental wages at a flat 22%, jumping to 37% only on the portion of a year’s supplemental wages above $1 million. Most employers use exactly these rates for simplicity.
Meanwhile your actual tax on that income is set by your marginal bracket on the full return. A software engineer with $220,000 of salary and $150,000 of vests is paying federal tax in the 32-35% range on those vested dollars — while only 22% was deposited. Every vest mints a 10-13 point IOU. Stack four quarterly vests and the IOU is five figures.
Sizing your personal gap in five minutes
- Project the year’s vest income. Shares scheduled to vest × a reasonable price assumption. Your equity portal lists the schedule.
- Find your marginal federal rate on (salary + bonus + vests). For 2025, the 32% bracket starts at roughly $197k single / $394k joint of taxable income, 35% at about $250k / $501k, and 37% at about $627k.
- Gap per vested dollar = marginal rate − 22% (federal), plus your state’s shortfall if any.
- Multiply. $150,000 of vests at a 13-point federal shortfall = $19,500 you will owe beyond what payroll deposits — before state.
Priya’s year
Priya earns $240,000 salary in California and expects $160,000 of vests. Combined marginal rate on the vests: 35% federal + 9.3-10.3% state ≈ 45%. Withheld on vests: 22% federal + 10.23% CA ≈ 32%. Gap ≈ 13 points × $160,000 = ≈ $20,800. Knowing the number in February, she splits it across two estimated payments and an increased W-4 — and April becomes a non-event.
Penalties, and the safe harbors that switch them off
Owing money in April is annoying; owing it with penalties is avoidable. The underpayment penalty is essentially interest on tax you should have prepaid through the year. You owe no penalty if, through withholding and timely estimated payments, you covered any one of these safe harbors:
- 90% of the current year’s total tax, or
- 100% of last year’s total tax — 110% if your prior-year AGI exceeded $150,000, or
- you owe less than $1,000 after withholding.
For people whose income jumped (new grants, IPO year), the prior-year harbor is gold: pay in 110% of last year’s smaller tax and you can settle the rest in April penalty-free, keeping the cash working meanwhile.
Three ways to close the gap
1. Crank up payroll withholding
File a new W-4 with an extra flat amount per paycheck (Step 4c). Elegant feature: withholding is treated as paid evenly through the year no matter when it actually happens, so a late-year W-4 surge can retroactively cure earlier quarters in a way estimated payments cannot. Some stock-plan systems also let you elect a higher rate on equity events specifically — the cleanest fix when available.
2. Quarterly estimated payments
Pay directly at IRS Direct Pay against deadlines in April, June, September and January. Suits people who prefer to keep paychecks intact and write deliberate checks. Pair each major vest with a payment and you are naturally matched to the income.
3. Sell-and-set-aside
If you sell shares at vest anyway (a defensible default — see concentration risk), immediately park the gap percentage of proceeds in a high-yield savings account earmarked “April.” Behaviorally simple, and the money even earns a little interest on the way.
Don’t forget the state layer
States run their own supplemental withholding rates, and the same arithmetic applies. California withholds 10.23% on equity compensation while its top brackets run well above that, and its SDI payroll tax applies without a wage cap. New York, New Jersey and others have their own flat rates that may or may not match your bracket. Recent movers face the extra wrinkle of allocation: income from a vest is generally sourced to where you worked during the vesting period, so one vest can owe two states. Check both layers, not just federal.
A system you can run in one evening a year
- each January, pull your vesting calendar and price-estimate the year’s vests;
- compute the gap with this guide’s arithmetic;
- choose a safe harbor (110% of last year is usually the easy one) and a closing method;
- set calendar reminders on the four estimated-payment dates;
- in December, sanity-check year-to-date withholding against the target and top up via W-4 if short.
That is the whole defense. The gap never goes away — it is structural — but measured once and funded deliberately, it stops being a surprise and becomes a line item.
Key takeaways
- Federal tax on RSU vests is usually withheld at the flat 22% supplemental rate (37% only after supplemental wages pass $1M in the year) — not at your real marginal rate.
- If your marginal bracket is 32%, 35% or 37%, each vest quietly builds a debt of 10-15 cents per vested dollar that comes due in April.
- Safe-harbor rules protect you from penalties: pay in at least 90% of this year's tax, or 100/110% of last year's, through withholding or estimated payments.
- You can close the gap three ways: extra W-4 withholding, quarterly estimated payments, or setting aside cash from immediate sales.
- State supplemental rates have the same problem — California's 10.23% equity withholding often undershoots top-bracket residents too.
Frequently asked questions
Why doesn't my employer just withhold the right amount?
Federal rules treat equity vests as supplemental wages and let employers use the flat 22% rate, which is simple to administer at scale. Some companies allow you to elect a higher withholding rate on equity — worth asking your stock-plan administrator — but many don't.
Is the 22% a special RSU tax rate?
No. It is only a withholding deposit, not your actual tax. Your real tax is computed on your full return at your normal brackets; the 22% is just the amount prepaid on your behalf.
What is the underpayment penalty actually like?
It works like interest on the shortfall, computed quarterly at IRS rates. It is not catastrophic, but on a five-figure gap it is real money — and entirely avoidable with safe-harbor planning.
Do quarterly estimated payments have deadlines?
Yes — generally mid-April, mid-June, mid-September and mid-January. A vest early in the year creates an obligation early in the year; you cannot always wait and pay everything in Q4 without some penalty exposure, though increased payroll withholding late in the year is treated as if spread evenly.
Educational disclaimer: This guide is general information, not financial, investment, tax or legal advice. Figures refer to the tax years stated and change over time; rules differ by jurisdiction and personal circumstances. Verify current figures with the IRS / HMRC and consult a qualified professional before acting. See our full disclaimer.