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Filing Guides / Form 1099-B

Form 1099-B equity compensation guide

Broker statement for proceeds, basis reporting, and withholding details for equity sales. This public guide pulls forward the form lines, common errors, evidence requirements, and IRS citations that matter most for equity compensation workflows.

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Tax year 2025

Published 2026-03-03

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Key lines to validate

Box 1a

Use this description to tie the sale to the exact equity lot (ISO exercise shares, ESPP lot, RSU sell-to-cover lot, or open-market lot). Match share quantity and CUSIP/ticker before filing.

Equity use cases

  • • Identify whether a disqualifying disposition belongs to an ISO or ESPP lot.
  • • Separate sell-to-cover transactions from elective sales.

Common errors

  • • Lot description does not match the shares on Form 3921/3922 records.
  • • Multiple lots are summarized without allocating basis by lot.

Required evidence

  • • Broker confirms
  • • Trade confirmations
  • • Form 3921/3922 copies when applicable

IRS citations

Box 1b

Date acquired is used for short-term vs long-term holding period analysis. For equity awards, acquisition date may differ from grant date (for example ISO exercise date, ESPP purchase date, or RSU vest date).

Equity use cases

  • • Determine long-term treatment for post-exercise option shares.
  • • Validate ESPP qualifying disposition holding periods.

Common errors

  • • Using grant date instead of acquisition date.
  • • Missing acquisition date on covered security lots.

Required evidence

  • • Broker lot details
  • • Equity system transaction history
  • • Award agreement dates

IRS citations

Box 1d

Box 1d is gross proceeds. You will report proceeds on Form 8949/Schedule D and then apply basis and adjustments to arrive at gain or loss.

Equity use cases

  • • RSU sell-to-cover often appears as proceeds with separate wage reporting elsewhere.
  • • ISO/ESPP sales flow from proceeds to Form 8949 adjustments.

Common errors

  • • Reducing proceeds for fees before reporting when fee treatment already handled by broker reporting conventions.
  • • Copying proceeds into wage lines.

Required evidence

  • • 1099-B statement detail page
  • • Year-end broker gain/loss supplemental report

IRS citations

Box 1e

Box 1e is broker-reported cost basis. Equity awards regularly need manual basis adjustment on Form 8949 (for example when compensation income is not reflected in broker basis).

Equity use cases

  • • Disqualifying ISO/ESPP sales where compensation component changes tax basis.
  • • RSU lots where wage inclusion was already taxed through payroll.

Common errors

  • • Accepting zero or understated basis without Form 8949 adjustment.
  • • Double counting wage component in both basis and wage income.

Required evidence

  • • Employer supplemental stock plan statement
  • • W-2 wages and equity compensation breakout
  • • Form 8949 draft with adjustment code and amount

IRS citations

Box 4

Box 4 reports backup withholding under IRC §3406 — not routine payroll or broker withholding. Backup withholding is triggered only when a payee has failed to provide a valid TIN or has been notified by the IRS as subject to backup withholding. Most equity holders will see Box 4 blank. When it is populated, the amount is a federal tax credit claimed on Form 1040 Line 25b. It must not be added to proceeds or deducted from basis.

Equity use cases

  • • Backup withholding on stock sales where a valid TIN has not been provided to the broker.
  • • Corrective withholding cases where the IRS has issued a backup withholding notice (B-notice) to the broker.

Common errors

  • • Treating Box 4 as routine broker withholding — it is exclusively backup withholding under IRC §3406.
  • • Ignoring Box 4 when present, causing a missed federal payment credit on the return.
  • • Adding withheld tax into basis or reducing proceeds.

Required evidence

  • • 1099-B copy with withholding
  • • Tax return payment section worksheet

IRS citations

Common error playbooks

1099-B Basis Mismatch Recovery

Broker basis does not include equity compensation amount already taxed through payroll.

  1. 1. Reconcile lot-level grant/vest/exercise records to each 1099-B sale lot.
  2. 2. Compute corrected basis including compensation already recognized as wages where applicable.
  3. 3. Prepare Form 8949 adjustment code and amount for each impacted lot.
  4. 4. Roll corrected totals into Schedule D and preserve audit support packet.

Equity Lot Classification Recovery

Sale lots are present, but ISO/ESPP/RSU classification is unclear.

  1. 1. Map each sold lot back to plan records and acquisition event dates.
  2. 2. Identify qualifying vs disqualifying treatment where relevant.
  3. 3. Rebuild lot worksheet before finalizing Form 8949 columns and adjustments.

Mini check

If broker basis is understated for an equity sale, where do you normally correct it?

Correct answer

Form 8949 adjustment

Why it matters

Equity basis mismatches are generally corrected on Form 8949 with an adjustment code/amount, then carried to Schedule D.

What does Box 12 on Form 1099-B communicate?

Correct answer

Whether basis was reported to the IRS

Why it matters

Box 12 is an indicator flag and does not eliminate the need for basis adjustments when broker basis is wrong.