VestingTax calculators are planning tools for U.S. employees with RSUs and other equity compensation. They help you compare estimated tax to employer withholding before a vest, sale, or move—not prepare a tax return or take a filing position.
Last reviewed: 2025-05-28. Each calculator also shows the tax year and assumption sources in its results panel.
What the calculators estimate
- RSU vest tax: ordinary income on vest FMV, federal income tax (bracket-based or marginal override), optional FICA (Social Security up to the wage base, Medicare, Additional Medicare above threshold), and state income tax using a flat effective rate when loaded.
- Withholding vs. tax gap: employer supplemental withholding (federal flat rate and state rate you enter or we prefill) compared to estimated total tax on the vest.
- Sell-to-cover and net proceeds: shares sold for withholding, cash left after estimated taxes, using the same rate assumptions.
- 1099-B / cost basis helpers: arithmetic on proceeds, reported basis, and vest FMV already taxed on your W-2—not tax software filing logic.
- Multi-state and comparison tools: simplified day-count or flat-rate allocation between states for rough planning, not residency or sourcing determinations.
- Options / ESPP / vesting schedule tools: scenario math on spreads, discounts, or future vest values with clearly labeled user inputs where official rates are not loaded.
What the calculators do not estimate
- Your actual tax liability after deductions, credits, AMT (except simplified ISO inputs), NIIT, local city taxes, or prior-year carryforwards.
- Employer-specific payroll quirks, 83(b) elections, non-U.S. tax, or grant-document edge cases.
- Full state bracket tables, county taxes, or official multi-state sourcing when you lived or worked in more than one state.
- Investment advice, optimal sell timing, or whether you should exercise, hold, or sell.
- Positions suitable for filing—always reconcile to your W-2, 1099-B, pay stubs, and a qualified tax professional.
Federal withholding defaults
For RSU vests treated as supplemental wages under $1 million, we default to the IRS flat supplemental withholding rate from IRS Publication 15 (Circular E). That rate is what many employers use for payroll withholding; it is not the same as your marginal tax rate for the full year.
Federal income tax in vest calculators uses ordinary-income brackets from IRS inflation-adjusted guidance (for example, Rev. Proc. 2024-40 for tax year 2025) when loaded, stacked on your entered salary and bonus. You can override the marginal rate when bracket math is unavailable or you prefer a single rate.
How payroll taxes are modeled
When enabled, calculators apply employee-side FICA using rates and thresholds from centralized tax-year files:
- Social Security: employee rate and annual wage base (SSA wage base, IRS Pub. 15).
- Medicare: base employee rate on all wages (IRS Publication 15 — Social Security and Medicare Tax Rates).
- Additional Medicare Tax: 0.9% above filing-status thresholds (IRS Topic 560).
We model FICA on the vest as additional wages in the vest year. We do not model employer FICA, prior-year wage base already hit on other employers, or every payroll-system rounding rule.
State rates: official withholding vs. flat estimates
States handle supplemental / bonus withholding differently. When we have a verified, state-published supplemental or bonus withholding rate for a tax year, we load it in the tax-year config and mark it official in the assumptions box, with a link to the state source.
When no verified state supplemental rate is loaded, we use simplified flat effective-rate estimates for both state tax and default withholding prefill. These are labeled estimated in the assumptions box and sourced to VestingTax flat effective rate methodology (2025). They are planning shortcuts—not bracket tables and not citations to individual state statutes.
As of the last review date above, 2025 state defaults in this site use flat estimated rates for all states with income tax. No-state-tax jurisdictions (for example FL, TX, WA) use 0%. You can override any state rate in the calculator; your entry is always shown in results.
Multi-state calculators apply day-count or user-entered allocation factors on top of these flat rates. They do not determine legal residency or which state has taxing authority over your income.
Planning estimates, not filing positions
Outputs are labeled as estimates throughout the site. Withholding is payroll mechanics; estimated tax is simplified math on the inputs you provide. The gap between them is a planning signal—possible balance due or refund at filing—not a promise of what you will owe.
Tax law changes, employer reporting errors, and facts we do not ask about (dependents, other income, ISO AMT, RSUs vesting across state lines) can all change your real outcome. Use these tools to ask better questions, then confirm with your documents and a tax professional.
Source hierarchy
- IRS and state tax agencies first—Publications, withholding tables, revenue procedures, and state DOR guidance when we can link to a stable official URL.
- Brokerage and payroll education pages second— Schwab, Fidelity, and similar plan-administrator explainers for how RSUs show up on W-2 and 1099-B. These describe common employee experience; they are not law.
- VestingTax assumptions last—Flat state effective rates, unavailable tax years, and calculator defaults where official data is missing or not yet loaded. These are clearly marked estimated or user enteredin each calculator's assumptions section.
Every loaded assumption shows its source ID, confidence level, and review date in the calculator assumptions box. If a tax year is not loaded (for example 2026 at launch), calculators ask you to enter rates manually rather than guessing.
Questions or corrections? Contact us. See also the site disclaimer.