About RSU Tax Calculator

Why we built this

Every year, tens of thousands of software engineers and tech employees get a surprise tax bill in April. Not because they did anything wrong — because their employer withheld only the IRS-mandated flat rate of 22% on supplemental wages (RSUs, bonuses), while their actual marginal federal rate is 32%, 35%, or 37%. The gap between what was withheld and what is owed is real, it compounds quarterly, and it can trigger an IRS underpayment penalty on top of the tax itself.

Most RSU explainer articles stop at "you may owe more." We built a calculator that quantifies exactly how much more — split by federal, state, Social Security, and Medicare — and tells you the safe-harbor quarterly amount to send to the IRS before the penalty clock runs. The tool runs entirely in your browser: we never see your numbers.

Start with the calculator. For the full story on the withholding gap, read our guide: Why Your Company's 22% RSU Withholding Will Cost You Thousands .

Methodology

The calculator uses 2026 tax-year parameters from the following primary sources:

• IRS Publication 15 (Circular E) — employer tax guide governing supplemental wage withholding rates and the $1,000,000 threshold that triggers the 37% mandatory rate.
• Rev. Proc. 2025-32 — the IRS revenue procedure that set the 2026 inflation-adjusted tax brackets, standard deductions, and phase-out thresholds announced in late 2025.
• Social Security Administration (SSA) — 2026 OASDI Wage Base — $184,500, the ceiling above which Social Security (6.2%) is no longer withheld.
• IRC § 3101(b)(2) / ACA Additional Medicare Tax — 0.9% surtax on wages above $200,000 (Single / Head of Household), $250,000 (Married Filing Jointly), or $125,000 (Married Filing Separately).
• State tax agencies — each state's supplemental withholding rate and progressive bracket tables are sourced from the respective state's employer withholding guide and verified annually. California sourcing rules follow FTB Publication 1005.
• IRS Form 2210 Instructions — safe-harbor rules: 90% of current-year tax, or 100% of prior-year tax (110% if prior-year AGI exceeded $150,000).

The 2026 key figures used in the engine: supplemental flat rate 22% (first $1M), 37% (above $1M); standard deduction $16,100 (Single) / $32,200 (MFJ); Social Security wage base $184,500.

Estimate only — not tax advice

This tool produces estimates for planning purposes only. It does not constitute tax, legal, or financial advice. Individual tax situations involve factors this calculator cannot capture — including itemized deductions, depreciation recapture, AMT exposure, carryover losses, non-qualified deferred compensation, qualified opportunity zone elections, and prior-year credits, among others.

Always verify your specific liability with a CPA, enrolled agent, or tax attorney before making payments or filing. The calculator is a starting point for informed conversations with your tax professional — not a substitute for one.

Who we are

RSU Tax Calculator is an independent publication run by a small team of engineers and financial writers who have personally navigated the RSU withholding gap. The site is editorially independent; it is funded by display advertising (Google AdSense) and affiliate disclosures where applicable.

Author / Editor: Alex Morgan, Finance & Tax Writer
Alex has covered personal tax strategy, equity compensation, and retirement planning for independent publications since 2019. All published figures are cross-checked against IRS source documents before each tax year update.

Found an error or want to suggest a feature? Contact us.