RSU withholding gap calculator
See how much your company under-withheld on a vest and what to set aside before April.
RSUs · Forms & reporting
How much did your company actually leave you short when your RSUs vested? More than you think, and you can size it in one subtraction. Your employer withholds at a flat rate that the IRS sets for supplemental wages, and for most people with real equity that rate sits below their true tax rate. The difference is yours to cover, and the IRS often wants it before next April.
Interactive calculator coming soon
The live version is being built. Until it ships, here is the exact math it runs, so you can do it by hand in a few minutes with a vesting statement and last year’s return.
What you need before you start
Pull these four numbers. Everything else is arithmetic.
The vested value
The market value of every batch that vested this year. Shares times the price on each vest date, added up. That total is your RSU ordinary income, the same as if it were salary.
Your real marginal rate
Your true federal bracket plus your state rate, plus the 0.9% additional Medicare tax 2026 if a big year pushes your wages over the threshold. This is the rate that actually applies to the next dollar you earn, not the average rate on your whole return.
What was already withheld
The federal tax your employer held back at vesting, usually shown on your pay stub or vesting confirmation. For 2026 the default federal supplemental rate is a flat 22% 2026, rising to 37% on cumulative supplemental wages over $1,000,000 in the year.
Your state withholding on the vest
Many states also withhold at a flat supplemental rate that can trail your real state rate. California holds back 10.23% 2026 on stock supplemental wages, and New York holds 11.70% 2026 for the state plus another 4.25% 2026 for New York City, and even those high rates can sit under a top earner’s real rate. Add what was withheld for state so the gap is the full picture, not just the federal slice.
The math it runs
The whole calculator is one line:
Real tax owed on the vest, minus everything already withheld, equals the gap.
Real tax owed is the vested value times your real marginal rate (federal plus state plus the Medicare surtax where it applies). Everything withheld is the federal plus state amounts from steps three and four. Subtract, and the result is the cash you still owe on those shares.
If the number is positive, that is your shortfall. Set it aside the day it shows up, because spending it first is how a known bill becomes a scramble.
Watch out
The flat 22% withholding looks like a lot until you compare it to a top bracket. A high earner whose real combined rate is well above 22% is short on every single vest, and the gap compounds across a year of them. The withholding covered part of the tax, not the tax.
Turn the gap into an action
A gap you have measured is harmless. A gap you ignore earns a penalty. Once you have the number:
Do I owe this before April?
Often, yes. Income tax is pay-as-you-go, so a large shortfall can trigger an underpayment penalty if you wait until filing. You usually cover it through a quarterly estimated payment or by bumping your W-4 withholding. The full sizing logic is in the withholding gap, and the under-withholding mechanics are in the under-withholding surprise.
What if I have several vests across the year?
Run the subtraction for each vest and total the gaps. Lumpy vesting is exactly where people get caught, because no single vest looks alarming but the sum does. Then schedule the payments with the quarterly estimated tax checklist.
The number that matters is the gap, not the headline tax. Measure it the day your shares vest, park the cash, and pay it on the IRS schedule. Do that and a tax surprise turns into a line item you already handled. If your vesting is large or arrives all at once, a short fit check can confirm you are not tripping a penalty without knowing it.
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