Updated September 30, 2026. Quick answer: Box 1g of Form 1099-B is the amount of loss the broker disallowed under the wash sale rule. The disallowed loss is added to the basis of the shares you bought back, so the loss is deferred into the new shares.
More IRS forms and schedules for individual filers, read line by line: Form 6781 instructions.
Schedule D and Schedule E (Form 1040) for 2025, read line by line: Schedule D lines 1b, 2 and 3, Schedule D lines 6 and 7 and Schedule D lines 8b, 9 and 10.
Form 8949 for 2025, read code by code: Form 8949 code L, Form 8949 code O and Form 8949 code W.
Read box by box from the IRS form text: Form 1099-DA boxes 1h and 1i.
The IRS form or worksheet behind this, read line by line: Form 8949 instructions, the wash sale rule and Schedule D instructions.
What the broker must report
The instruction is “Report wash sale loss amount disallowed.” It says: “You must report any loss disallowed under section 1091 if both the sale and purchase transactions occur in the same account with respect to covered securities with the same CUSIP number.” The 1099-B instructions read here are the 2026 edition on irs.gov. Those are instructions to the payer that files the form, so they show what a box is meant to hold, not what tax follows from it.
Beyond the same account
A broker may go further: “You are permitted but are not required to report in box 1g all loss disallowed under section 1091.” and “For example, you may report a disallowed loss even though a security is sold in one account and repurchased in a different account.” So a missing Box 1g does not prove no wash sale happened across accounts. See the wash sale and crypto page and the RSU sell-to-cover page.
What happens to the basis
The instruction is “Increase the adjusted basis of the acquired securities by the amount of the disallowed loss reported in box 1g.”
The IRS example, worked
The IRS example has a customer buy 100 shares for $1,000, sell them for $600, and buy 75 shares with the same CUSIP number in the same account within 30 days. The instruction states: “Since Alex’s loss is $400 ($1,000 minus $600) but the loss on 75 shares is disallowed, Alex’s disallowed loss is $300.” It then says “The $300 in disallowed loss is added to Alex’s basis in the newly purchased 75 shares.” CMG’s arithmetic on that example, computed and not an IRS formula: the loss is $1,000 minus $600, which is $400; 75 of the 100 shares were bought back, so the disallowed part is $400 times 75, divided by 100, which is $300.
Exceptions the broker need not apply
The instructions list cases where the broker does not apply the rule, among them that “The purchased security is transferred to another account before the wash sale”. Those are instructions to the broker, not tax advice for you.
Common mistakes
- Reading a blank Box 1g as proof of no wash sale. The instruction lets brokers go beyond the same account but does not require it.
- Treating the disallowed loss as gone forever. The instruction adds it to the basis of the new shares.
- Assuming the rule is limited to one account. The instruction says a broker may report across accounts.
Related: 1099-B Box 1d Proceeds and Box 1e Cost Basis Explained, 1099-B Box 2: Short-Term or Long-Term Gain or Loss, the wash sale and crypto page, and the capital gains harvest window.
Every box in one place: the 1099-DIV, 1099-INT and 1099-B boxes table. If your withdrawal came from an IRA or 401(k), see the Form 1099-R Box 7 codes.
Sources
- IRS, Instructions for Form 1099-DIV (Rev. January 2024): irs.gov/pub/irs-pdf/i1099div.pdf
- IRS, Instructions for Forms 1099-INT and 1099-OID (Rev. January 2024): irs.gov/pub/irs-pdf/i1099int.pdf
- IRS, Instructions for Form 1099-B (2026): irs.gov/instructions/i1099b
Optional next step
Want help with this decision?
Ask an adviser how this choice fits your broader retirement and financial plan.
- No fee to request a match
- No obligation to hire
Expect questions about your situation, your name, email and phone, and phone verification by text. A match is not guaranteed.
The form is run by Kapitalwise, our advisor-matching partner. Kapitalwise sends your details to advisers who pay for the introduction, so expect calls and texts. Clear Money Guide is paid when you submit the form, whether or not you ever hire anyone. This is free to you and there is no obligation to hire anyone.
Opens here. Nothing loads and nothing reaches Kapitalwise until you press the button.
How our partner relationships workSource: the IRS instructions for the filer of each form, read at irs.gov on September 30, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.