Adjust RSU cost basic to include the wash sale loss disallowed

kevin2026 · ‎March 14, 2026

I have RSU wash sale loss disallowed sales. Using the TT 2025 desktop version, when adding the loss to the new sale in box 1g, the tax due increases.

For example, I have a sale of 20 RSU shares with wash sale loss of $1000, sold on 05/06/2025. A new sale of 20 RSU shares acquired on 09/02/2026 and sold on 09/03/2026 with small gain of $12. The tax due increases when adding the wash sale loss $1000 to this new sale, using these steps:

1. Check "I have other boxes on my 1099‑B to enter"

2. Enter 1000 to "Box 1g - Wash sale loss disallowed"

I've tried both desktop and online versions, same problem.

How do I adjust the cost basis correctly?

Can I manually adjust the cost basis by using the option "The cost basis is incorrect or missing on my 1099‑B"?

ReneV4

You are right. To manually adjust your cost basis for the wash sales, you will need to check the "I need to adjust my total basis" box found just under the Total cost or other basis box.

For your first entry in May, enter the proceeds and cost basis as reported, then make the adjustment in the Total adjustments to gain (or loss) box. Enter the adjustment of +$1,000, then look for the W box and check it (non-deductible loss for a wash sale).

For your second sale in September, I see 09/02/2026 above, so be sure you are inputting the correct year. Enter the proceeds and cost basis as reported. Then, you will not use Code W. Instead, select Code B (Cost basis incorrect on form). Enter your adjustment as a negative amount -$1,000, resulting in your $12 gain becoming a loss of $988.

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