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Capital losses are claimed by completing schedule 3, and allowable losses may be used to offset gains within the current year, up to three years prior, or carried forward to future years, depending on the situation. You'll enter a capital loss in the investment section. 

To enter a capital loss  in TurboTax Online:

• Select the image of the magnifying glass (or Find) in the upper-right area of TurboTax
• Enter “capital” in the Find window
• Select "capital gains and capital gains deduction profile" from the list of results, and then select Go
• Follow the onscreen instructions.