How we estimate the gap.

The core idea

RSU vests are ordinary wage income, taxed at your marginal rates. Many employers withhold at the optional flat 22% supplemental rate (mandatory 37% on supplemental wages above $1M in a year, per IRS Publication 15); an employer may instead aggregate the vest with regular wages, which changes what's withheld. Two precision notes: the $1M threshold counts all your supplemental wages for the year, bonuses included, while we count only the vest you enter; and if your employer aggregates instead of using the flat rate, your actual withholding differs from this estimate. The estimate compares what the vest actually adds to your federal bill against what gets withheld for it. The difference is your gap.

Marginal stacking, not a flat rate

We compute federal income tax on (salary + vest) and subtract the tax on salary alone, using the 2026 brackets and standard deduction for your filing status. That difference is the tax attributable to the vest. It handles vests that straddle bracket boundaries correctly, which a single "your bracket times the vest" multiplication does not: the first dollars of a vest may be taxed at 32% while the last dollars hit 35% or 37%.

The result is a single estimate under stated assumptions, not a decorated range. The math is exact given the inputs; what's uncertain are the assumptions, so we state them instead of widening the number.

The safe harbor

The headline instruction is the estimated-tax safe harbor (IRS Publication 505, IRC §6654): pay in at least 110% of last year's total tax during the year, through withholding or estimated payments, and no underpayment penalty applies regardless of the final bill. The rate is 100% when prior-year AGI was $150,000 or less ($75,000 for married filing separately); the calculator's checkbox handles that. We surface this first because it converts an unpredictable April surprise into one fixed number you can plan against in July.

What the calculator assumes

• Salary plus vest is your household's ordinary income; the standard deduction applies.
• No itemizing, no other income, no credits, no pre-tax 401(k)/HSA adjustments.
• The whole vest lands in the current calendar year at the value you entered.

What it deliberately doesn't model

• State income tax: shown as a per-state note, never computed. State treatment of RSU income varies enough that a generic percentage would be false precision.
• Social Security and Medicare: withheld separately by your employer. The 0.9% Additional Medicare Tax above $200k in wages is also excluded; employers must withhold it above that threshold, so it's roughly neutral to the gap (the Social Security portion caps at the $184,500 wage base in 2026 regardless).
• Alternative Minimum Tax and the 3.8% Net Investment Income Tax.
• Multiple vests at different prices; enter the year's total instead.
• ISOs, NSOs, and ESPP: different rules entirely.

Data sources

2026 brackets, standard deductions, and thresholds come from IRS Rev. Proc. 2025-32; the Social Security wage base from SSA's Federal Register notice for 2026 (doc 2025-19763). Supplemental withholding rules from IRS Publication 15; safe-harbor rules from Publication 505. Parameters are pinned by automated tests against those sources and refreshed each year when the IRS publishes new figures.

Privacy, because it's a design input

Every computation runs in your browser. Financial inputs are never placed in URLs and never sent to a server; there is no server to send them to. See privacy for the whole (short) story.