Free RSU tax calculator and equity comp guides for U.S. employees.

Mid-year underreporter notices spike after brokers finish correcting 1099-B files. The IRS matches proceeds it received from the broker. If basis was blank or zero on that file, the notice may propose tax on nearly the full sale amount — even when those dollars already hit your W-2 as vest wages.

Do not treat the proposed balance as a final bill. Pull the notice, your filed Form 8949 / Schedule D, the 1099-B, and the vest confirmation for that lot. The response package is a corrected gain computation: proceeds minus vest FMV basis, with a short explanation that the W-2 already included the vest.

Paying the notice without fixing basis can lock in double tax on the same economic dollars. Ignoring it until the next notice is worse. Calendar the response deadline on the first page and keep a PDF of everything you mail or upload.

If you filed with zero basis on purpose because software imported it that way, this is also your cue to amend before the next cycle — Form 1040-X with the same vest documentation.

IRS Direct Pay’s help pages were updated again in July 2026. For people who just saw a vest stub with supplemental withholding, the useful path is Estimated Tax (Form 1040-ES) for the current calendar year — not “balance due on a return you have not filed yet.”

Sell-to-cover funds the employer’s withholding, not your full marginal tax. If salary plus the vest put you above the 22% bracket, the shortfall is still your problem. Paying within a few weeks of the vest — while the cash is still liquid — is easier than reconstructing the gap in March.

Use the confirmation number from Direct Pay and file it next to the vest PDF. Safe-harbor rules (roughly 90% of this year’s tax or 100%/110% of last year’s, depending on income) still decide whether you owe an underpayment penalty; the payment is how you catch up, not a free pass to ignore the math.

State gaps are separate. California and New York residents often need a state estimated payment even when the federal Direct Pay looks adequate.

Summer start dates are common after spring recruiting. Your old employer’s final vest and your new employer’s first vest can land in the same calendar year on two different Form W-2s. Tax software adds Box 1 wages together; neither payroll system saw the full picture when it withheld.

Match each vest confirmation to the employer EIN on the W-2. Shares may sit in one brokerage account after a transfer, but wage reporting does not move with the shares. Mixing lots is how people adjust the wrong basis later.

Run a combined estimate now — salary from both jobs plus both vest FMVs — against federal and state withholding already taken. If the gap is real, raise W-4 withholding at the new job or make an estimated payment before Q4 vests stack on top.

Social Security tax can also look “double counted” across two W-2s when both employers withheld up to the wage base. Excess Social Security is credited on Form 1040; Medicare usually is not refunded the same way.

Broker workplace pages still get traffic every vest season from people hoping that “not selling” means “not taxed.” For standard RSUs, the fair market value on the vest date is wage income whether the shares sit in your account or go straight to sell-to-cover.

What holding changes is concentration. If a large share of your net worth is already employer stock, another unvested tranche vesting into the same ticker is a risk decision, not a tax-avoidance strategy. The W-2 event is done.

When you eventually sell, 1099-B proceeds should be measured against vest FMV basis. A drop below vest price can create a capital loss even though you already paid ordinary tax on a higher FMV — painful, and a reason to separate “tax paid” from “paper profit.”

If you hold, save the vest confirmation the day shares land. Brokers are slow to learn basis; your PDF is not.

The Taxpayer Advocate Service’s 2026 estimated-tax calendar still lists September 15 as the third quarterly due date for calendar-year filers. W-2 employees skip it until a vest creates a gap between supplemental withholding and actual marginal tax.

If you vested in June or July, open the vest confirmation and add federal withheld on that event to salary withholding YTD. Compare that total to a rough full-year tax estimate that includes remaining scheduled vests. The difference is what September 15 is for.

You can also close the gap by increasing W-4 withholding for the rest of the year. Either path feeds the same annual reconciliation; estimated payments are just front-loaded cash.

Put a reminder two weeks before September 15 with a link to your vest PDF folder. The people who scramble in mid-September are usually the ones who felt “done” when sell-to-cover settled.