RSU Adjusted Cost Basis Calculator (Form 8949)

An RSU adjusted cost basis calculator tells you the wage-backed number your broker will not compute: vest FMV per share × shares sold—the same fair market value your employer already reported as ordinary income on Form W-2 when your RSUs settled. Without that adjustment on Form 8949 column (e), Schedule D can tax the same vest dollars twice—once as wages through withholding, again as phantom capital gain when you accept a Form 1099-B import that shows $0 cost basis.

Verified against IRS Instructions for Form 8949 (2025) and Publication 525, accessed 25 June 2026. As of the 2026 filing season, major plan administrators still file $0 or incomplete cost basis on Form 1099-B for standard RSU lots—Form 8949 remains where you document the wage layer brokers and tax software omit.

Why RSU cost basis adjustment matters

Adjusted cost basis for RSUs sits at the intersection of two ledgers that do not reconcile automatically:

Under IRC Section 83, the compensation element of RSU settlement is ordinary income equal to FMV × shares on the settlement date. Your employer reports it on Form W-2 and documents per-vest detail in an equity tax supplement. The broker knows your sale proceeds—not the payroll wage layer.

The double-taxation failure mode: W-2 withholding taxes vest FMV as wages. You import Form 1099-B with basis = $0. Schedule D then taxes the full vest FMV again as capital gain. Same dollars, two tax categories.

For deeper filing walkthroughs, see how to report RSU sales on Form 8949 and Schedule D and IRS Pub 525 RSU Tax Basis Adjustment Guide. ESPP readers should use the ESPP Cost Basis Adjustment calculator guide—this article is RSU-only.

Use the RSU Tax Basis Adjuster calculator

The RSU Tax Basis Adjuster accepts four inputs per lot and outputs the numbers you need before opening tax software:

Adjusted basis per share = Vest FMV per share (settlement date)
Capital gain per share   = Sale price per share − Adjusted basis per share

Where I'm less sure—some employers delay W-2 RSU income recognition until the calendar year of settlement when vest dates straddle year-end, so a December vest may not show on last year's W-2. Your mileage will vary depending on plan administrator timing; match the settlement year's equity supplement to your sale lot.

Equity supplement → W-2 → Form 8949: field mapping

Methodology (25 June 2026): We mapped equity tax supplement field labels from published plan-administration guides by Adobe, Salesforce, ServiceNow, and Snowflake to the per-share inputs the RSU Tax Basis Adjuster accepts, then cross-checked against W-2 supplemental layouts from Fidelity Stock Plan Services and Morgan Stanley Shareworks.

Anecdotally, employees who pull sale proceeds from 1099-B and skip the equity supplement are the largest source of RSU overpayment we see in support threads—software imports make the error silent.

Step-by-step: report adjusted basis on Form 8949 and Schedule D

Step 1 — Gather documents before importing 1099-B

Step 2 — Match each sale row to one vest event

FIFO is common but not universal. Specific identification beats averaging when your broker supports lot-level sells. If you sold 150 shares from two vest tranches in one trade, split into two Form 8949 rows—one per vest—with separate basis calculations.

Step 3 — Calculate adjusted basis per row

Column (e) = Vest FMV per share × Shares sold from that vest lot

Run the math in the RSU Tax Basis Adjuster—enter per-share vest FMV, broker-reported basis (usually $0), sale price, and share count to see phantom-gain exposure before filing.

Step 4 — Select checkbox and adjustment code B

When basis was not reported to the IRS, or was incorrect, use Part I Box B (short-term) or Part II Box E (long-term). Enter adjustment code B in column (f)—"basis reported to IRS was incorrect"—when broker basis excludes vest FMV wage income.¹

Steel-man: "TurboTax imported my 1099-B—shouldn't that be enough?"
Best case for import-only filing: Open-market stock with correct broker basis and no RSU history.
Rebuttal: RSU lots fail that test routinely. Import proceeds into column (d); override column (e) yourself. Tax software rarely reconstructs W-2 wage layers without manual input.

Step 5 — Reconcile Schedule D

Net capital gain = Sum of (proceeds − adjusted basis) across all RSU Form 8949 rows

Gain should reflect only price movement after compensation was taxed—not vest FMV dollars again.

Worked example: Priya, staff engineer at Salesforce (single vest, single sale)

Priya vests 120 RSUs on 20 August 2025 at $298.40/share FMV ($35,808 on W-2). She sells 60 shares on 14 April 2026 at $322.75.

At a 35% federal marginal rate, accepting $0 basis costs roughly $6,266 in phantom federal tax on top of the $35,808 already taxed as wages through W-2 withholding.

Priya enters $17,904.00 in Form 8949 column (e), keeps proceeds $19,365.00 in column (d), checks Part I Box B, and uses code B in column (f).

Worked example: Marcus, platform lead at ServiceNow (multiple vests, FIFO)

Marcus vests 50 shares on 1 February 2026 at $892.10 and 75 shares on 1 August 2026 at $945.60. He sells 100 shares on 10 January 2027 at $978.25 (FIFO).

Proceeds: 100 × $978.25 = $97,825. Correct gain: $5,940. A $0 1099-B import shows $97,825 gain—taxing $91,885 of vest wages a second time on Schedule D.

At 37% marginal rate, the phantom overpayment approaches $34,000 in federal tax alone. Marcus splits the sale into two Form 8949 rows matched to vest supplements—not one blended import row.

Original research: tax software RSU 1099-B import behavior

On 25 June 2026, we tested how five consumer tax products handle a synthetic RSU Form 1099-B import ($0 broker basis, $298.40/share vest FMV omitted) against the wage-backed basis the RSU Tax Basis Adjuster produces.

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I haven't tested every state add-on module or amended returns on Form 1040-X—anecdotally, employees who amend after discovering the error often find the software still defaults to broker $0 basis on re-import. Treat the table as a default expectation for fresh 2025 returns.

Pros and cons: filing approaches for RSU sales

Taken position: For employees with fewer than eight RSU sale lots per year, run the RSU Tax Basis Adjuster once per lot, then enter Form 8949 rows manually—one evening prevents thousands in phantom tax. Above that volume, pay for lot-level reconciliation or maintain a spreadsheet tied to vest IDs at sale time.

Working checklist

Verdict: vest FMV is your RSU adjusted cost basis

The rsu adjusted cost basis calculator exists because brokers and tax software will not compute wage-backed basis for you. Form W-2 already taxed the vest layer; Form 1099-B reports $0 because you never purchased shares. Form 8949 is the bridge to Schedule D.

Taken position: Treat every RSU sale as a two-document reconciliation—equity supplement plus 1099-B—before touching tax software. Run the RSU Tax Basis Adjuster first; enter column (e) yourself. That habit prevents the most expensive RSU filing mistake on Schedule D.

Form 8949 reconciles amounts reported on Form 1099-B with amounts you report on your return—the administrative step that turns W-2 RSU compensation into correct cost basis.

Frequently Asked Questions

What is RSU adjusted cost basis?

Answer: Vest FMV per share multiplied by shares sold from that lot—the same amount your employer reported as wages on Form W-2 at settlement. That product is the basis you enter on Form 8949 column (e).

Source: IRS Publication 525

Why does my 1099-B show $0 cost basis for RSUs?

Answer: Brokers record what you paid for shares. RSUs settle as wages—you paid $0—so the broker never sees the vest FMV layer already on Form W-2. You must add it on Form 8949.

Source: IRS Instructions for Form 1099-B

How does the RSU Tax Basis Adjuster calculator help?

Answer: It computes adjusted basis, correct capital gain, and phantom gain if you accept broker $0 basis—giving you the column (e) number before opening tax software. Educational only; confirm with your equity supplement and W-2.

Source: RSU Tax Basis Adjuster calculator

Which Form 8949 adjustment code fixes RSU double taxation?

Answer: Code B—"basis reported to IRS was incorrect"—when broker-reported basis excludes vest FMV wage income already taxed on Form W-2.

Source: Instructions for Form 8949

Does sell-to-cover change my adjusted basis?

Answer: No—sell-to-cover shares withheld for taxes still vest at FMV; that FMV is your basis for any shares you later sell from that vest, including net shares deposited to your brokerage account.

Source: RSU sell-to-cover withholding guide

Can I fix RSU double taxation after filing?

Answer: Consider Form 1040-X with corrected Form 8949, equity supplements, and W-2 within the refund statute window.

Source: IRS About Form 1040-X

Where does RSU vest income appear on my W-2?

Answer: Typically in Box 1 wages with detail in the employer's equity tax supplement—not a separate box. Cross-check the supplement total against per-vest FMV.

Source: IRS Publication 525

Footnotes

Primary Sources

Disclaimer: This guide discusses general U.S. federal tax principles only and is not personalized tax, legal, or investment advice. Employer plans, state taxes, and cross-border assignments can change results. Confirm facts with the sources cited and a qualified tax professional.

Research note: Editorial publish 25 June 2026 for rsu adjusted cost basis calculator and Form 8949 RSU filing queries—step-by-step basis adjustment to prevent double taxation on Form 1099-B imports.