TurboTax Amendment Showing Incorrect Tax Due Before Changes — Why?

[UsedVanilla33]

Dear all, I’m looking for help understanding an issue while amending my 2025 tax return in TurboTax. My original return shows a $3,978 refund. However, I realized I forgot to include a 1099-B for stock sales (long-term), totaling about $3,000 in capital gains. Based on this, I would still expect to receive a reduced refund after amending — not owe taxes. Here are the exact steps I followed: How do I amend my return? → Start an amendment Why are you amending? → Adding or updating a tax form → My form isn’t listed TurboTax states: “We’re going to change the refund monitor to $0 to show the difference between your original and amended return.” I land on “Here’s the info for your amended federal return” I go to: Wages & Income → Stocks, Cryptocurrency, Mutual Funds, Bonds → Edit At this point, before I even add anything: The top-left numbers suddenly change to: Federal due: $1,370 CA due: $540 This is very confusing because: I haven’t added my missing 1099-B yet I expected the refund monitor to show $0 initially (as TurboTax explained) Instead, it immediately shows that I owe money, and I don’t understand how these numbers are calculated I then proceeded to enter my 1099-B (~$3k capital gain), and at the end TurboTax still shows that I owe these amounts, which doesn’t seem correct given my original refund. I’ve been stuck on this for a while and can’t figure out what’s happening. Questions: Why does TurboTax show a tax due immediately when starting an amendment? What do these “Federal due” and “CA due” numbers represent at that stage? Are these real amounts owed, or just interim calculations? Any guidance would be greatly appreciated. Thanks in advance.

[ReneV4]

To ensure that the calculations are correct, don't rely on the refund monitor during the amendment. Instead, do this:

1. Complete your entry for the missing Form 1099-B
2. On the left panel, go to Tax Tools, Tools, then click on View Tax Summary
3. Look for Form 1040-X, Line 21 (Overpayment) or Line 20 (Amount you owe)
4. If you have a $3,000 long-term capital gain, your federal tax increase (by tax bracket) should be roughly: 0 for low income, $450 for 15% and $600 for 20%
5. For California: Approx. $280 (9.3%)

If Line 21 on your Form 1040-X shows a positive number (like $3,400), that is your remaining refund. If it shows a number on Line 20, that is new money you have to send back to the IRS.

Note: If you have not yet received your original $3,978 refund, do not mail or efile your amendment yet. The IRS recommends waiting until your original return is fully processed and the first refund is issued before filing the 1040-X.