U.S. · Tax forms
1099-B box 1g wash sale loss disallowed.
The loss you cannot claim yet, and where it goes instead.
Box 1g is the loss from a wash sale that is not allowed this year. Brokers must report it when the sale and the purchase are in the same account for covered securities with the same CUSIP number. The disallowed amount is added to the basis of the replacement shares. On Form 8949 you enter it in column (g) as a positive number with code W.
Split a loss into allowed and disallowed
Result
What the IRS tells brokers about box 1g
| Rule | What the instructions say | Practical effect |
|---|---|---|
| Required | Report the loss disallowed under section 1091 if the sale and purchase are in the same account, for covered securities with the same CUSIP number. | The usual case: one brokerage account, the same stock bought again. |
| Permitted | A broker may report more, for example a loss where the security was repurchased in a different account. | Box 1g can include wash sales across accounts, but it is not required to. |
| Not required: transfer | No wash sale reporting if the purchased security was moved to another account before the wash sale, or bought elsewhere and moved in later. | The broker may miss wash sales that cross accounts. You are still responsible for them. |
| Not required: separate accounts | No need to apply the rules if securities are treated as held in separate accounts. | Check your own records if you trade the same stock in several accounts. |
| Section 475 election | No need to apply them if the customer gave written notice of a valid, timely mark-to-market election for the account. | Applies to traders who made that election. |
| Replacement basis | Increase the adjusted basis of the acquired securities by the disallowed loss. | The loss is deferred, not lost. |
| Box 5 checked | The broker need not complete boxes 1b, 1e, 1f, 1g and 2 for a noncovered security. | Box 1g may be blank even if you had a wash sale. |
| Wrong amount | On Form 8949 enter the correct nondeductible loss in column (g) as a positive number. | If it is less than the form shows, attach a statement explaining the difference. |
The IRS example
The 1099-B instructions use Alex, who bought 100 shares for $1,000 in September 2025 and sold them for $600 in October 2025. Within 30 days Alex bought 75 shares with the same CUSIP number in the same account. The loss is $400, but the loss on 75 of the 100 shares is disallowed, so the disallowed loss is $300. The broker reports $600 in box 1d, $1,000 in box 1e and $300 in box 1g, and the $300 is added to the basis of the new 75 shares. The calculator above reproduces that split by share count; your broker's form controls when lots, prices or accounts differ.
Entering it on Form 8949
For a wash sale, the Form 8949 instructions say to report the sale, enter code W in column (f) and put the nondeductible loss in column (g) as a positive number. Column (h) is proceeds minus basis combined with column (g). In Alex's case that is $600 minus $1,000 plus $300, a $100 loss. If no part of the loss is a wash sale loss, enter -0- in column (g).
Things the broker may not see
Brokers apply the rule account by account. The IRS says they do not have to treat purchases in other accounts as wash sales, and the Form 8949 instructions send you to Publication 550 for substantially similar securities. If you bought the same stock in an IRA or at another firm, the form can understate the disallowed loss.
Where next
For the basis side see box 1e. To test your own purchase dates use the wash sale rule calculator.
Sources
- Instructions for Form 1099-B: Box 1g, the Alex wash sale example, and the Box 5 rule.. Read November 2, 2026. (IRS page last reviewed April 30, 2026.)
- Instructions for Form 8949 (2025): Code W and column (g) instructions.. Read November 2, 2026. (IRS page last reviewed April 30, 2026.)
Line and box numbers are those printed in the IRS instructions named above and can change from year to year; match each box by its label.
Frequently asked questions
What is box 1g on a 1099-B?
Box 1g is the wash sale loss disallowed: the part of a loss from a sale that cannot be deducted this year because replacement securities were bought.
Does the disallowed loss disappear?
No. The broker adds the disallowed loss to the basis of the replacement shares, so it reduces a later gain or increases a later loss.
How do I report box 1g on Form 8949?
Enter code W in column (f) and the box 1g amount in column (g) as a positive number. Column (h) combines proceeds minus basis with that adjustment.
My broker did not report a wash sale. Does the rule still apply?
Yes. The instructions let brokers skip cross-account wash sales, but the tax rule is not limited to one account. Use your own records and Publication 550.
Is box 1g always filled in?
No. For a noncovered security with box 5 checked, the broker does not have to complete box 1g.
Not tax advice
This page summarizes IRS instructions, mostly written for the people who prepare the form, and is not tax, legal or financial advice. The payer, a CPA or an enrolled agent can confirm what a box or code means for your return. Everything runs in your browser; nothing you enter is sent anywhere.