How do I adjust cost basis for stock sale?

pokebowl

I had a bunch of stock in 1 ETF held in a personal brokerage account that I bought at various times and various prices. I sold a large volume of it in 2025 all for 1 price. My brokerage (interactive brokers) allows me to select which lots will be used to compute the cost basis when I made the sale, but it seems that they have lost my selections and used a different allocation for "which lots" I ultimately sold, which greatly lowered my cost basis for the sale.

I have tried to contact them but haven't been able to get an updated 1099-B reflecting the correct cost basis. How do I go about correcting the cost basis on my own?

Taylor

Hello pokebowl,

If your broker won't correct your Form 1099-B, you can still enter the correct cost basis in the software. You'll enter the 1099-B exactly as you received it, entering the cost basis from the 1099-B into the Cost Basis Box.

On the next screen when it asks if you have Adjustments to the Stock Sales, select Yes. The software will then provide a list of adjustments to choose from, the first line will say, "The basis shown in Box 1e is incorrect." By marking this line, the software will provide a box for you to enter the correct basis amount. Generally, your cost or other basis for investment sales should include the price you actually paid for the stocks and any cost of purchase, such as commissions.

To get there in the software: Income > Common Income > Your Investment and Savings Accounts (1099-INT/DIV/B/DA).

Keep detailed records of your original lot selections and calculations showing which shares you sold and their actual purchase dates and prices. You'll want to retain this documentation in case of an audit, as the IRS will receive the original 1099-B information from your broker so you may need to provide your documents as well.