TAX GUIDES 5 GUIDES // 2026 TAX CODE

RSU Tax & Filing Guides

Practical, citation-backed guides explaining federal supplemental withholding, IRS Form 2210 safe harbor rules, Form 1099-B cost basis adjustments, and multi-state RSU allocation formulas.

Author: Sean Beck • Updated for 2026 IRS & State Tax Rules
CITATIONS: IRC § 83 • IRC § 3402(g) • IRC § 6654 • 26 C.F.R. § 1.6045-1
GUIDE 01 // IRC § 3402(g) FEDERAL WITHHOLDING 9 MIN READ • UPDATED 2026-10-15

The IRC § 3402(g) Supplemental Withholding Gap: Why 22% Sell-to-Cover Leaves a 15% Tax Deficit

How Treasury Regulation § 31.3402(g)-1 works, why brokerages default to 22% flat withholding, and why standard sell-to-cover leaves a large April tax bill for tech employees.

Primary Statutory & Regulatory Authorities Covered:
  • 26 U.S.C. § 3402(g) — Overlapping Pay Periods & Supplemental Wages
  • 26 C.F.R. § 31.3402(g)-1 — Supplemental Wage Payments & Mandatory Flat Rate
  • IRS Publication 15 (Circular E), Section 7 — Supplemental Wages
  • 26 U.S.C. § 83(a) — Property Transferred in Connection with Performance of Services
4 Sections • Worked Examples Included
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GUIDE 02 // IRC § 6654 SAFE HARBOR & ESTIMATED TAXES 11 MIN READ • UPDATED 2026-10-18

IRS Form 1040-ES Safe Harbor & Form 2210 Annualized Income Installment Method for Variable RSU Vests

How to prevent IRS underpayment interest penalties and navigate California’s asymmetric 30/40/0/30 estimated tax schedule when your equity compensation fluctuates across quarters.

Primary Statutory & Regulatory Authorities Covered:
  • 26 U.S.C. § 6654 — Failure by Individual to Pay Estimated Income Tax
  • 26 U.S.C. § 6654(d)(1)(C) — 110% Limitation for High-Income Taxpayers (AGI > $150,000)
  • 26 U.S.C. § 6654(d)(2) — Annualized Income Installment Method (Form 2210 Schedule AI)
  • Cal. Rev. & Tax. Code § 19136.3 — California $1,000,000 AGI Safe Harbor Elimination
3 Sections • Worked Examples Included
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GUIDE 03 // FORM 8949 & 1099-B COST BASIS & TAX RETURN FILING 10 MIN READ • UPDATED 2026-10-20

How to Prevent RSU Double Taxation: Reconciling W-2 Box 14 with Form 1099-B on Form 8949

Why brokerage Form 1099-B statements frequently report $0 or unadjusted cost basis on vested RSUs, and how to enter the exact Form 8949 Code B adjustment so you never pay tax twice on the same equity grant.

Primary Statutory & Regulatory Authorities Covered:
  • 26 U.S.C. § 1012 — Cost Basis of Property
  • 26 C.F.R. § 1.6045-1(n) — Broker Reporting of Cost Basis for Equity Compensation
  • IRS Instructions for Form 8949 — Adjustment Code B (Incorrect Basis Reported on 1099-B)
3 Sections • Worked Examples Included
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GUIDE 04 // MULTI-STATE SOURCING INTERSTATE TAX ALLOCATION 10 MIN READ • UPDATED 2026-10-22

Multi-State RSU Tax Allocation & Trailing Liability When Relocating Before Vest (CA, NY, WA, TX)

Why moving from California, New York, or Massachusetts to Washington, Texas, or Florida does NOT eliminate state income tax on your unvested RSU grants—and how state workday allocation ratios work.

Primary Statutory & Regulatory Authorities Covered:
  • California FTB Publication 1004 — Equity-Based Compensation Guidelines
  • New York 20 NYCRR § 132.24 & TSB-M-07(7)I — Nonresident Allocation of Stock Options and RSUs
  • Massachusetts 830 CMR 62.5A.1 — Non-Resident Income Tax Sourcing of Equity Awards
3 Sections • Worked Examples Included
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GUIDE 05 // PRE-IPO & TENDERS PRIVATE EQUITY & LIQUIDITY EVENTS 9 MIN READ • UPDATED 2026-10-25

Pre-IPO Double-Trigger RSUs, Liquidity Waivers & Secondary Tender Offer Taxation

Understanding IRC § 83 and § 409A rules for private technology RSUs at companies like Stripe and Databricks, how double-trigger settlement works, and why tender offers create severe safe-harbor cash crunches.

Primary Statutory & Regulatory Authorities Covered:
  • 26 U.S.C. § 83 — Property Transferred in Connection with Performance of Services
  • 26 U.S.C. § 409A — Inclusion in Gross Income of Deferred Compensation (Short-Term Deferral Rule)
  • 26 C.F.R. § 1.409A-1(b)(4) — March 15 Short-Term Deferral Settlement Deadline
3 Sections • Worked Examples Included
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