For partial losses (damage), the loss is the smaller of basis or decline in fair market value. For thefts or total losses, the loss is the adjusted basis of the property, regardless of FMV. A reduction must be made for insurance or other recovery, but there is no $100-per-event or 10%-of-AGI reduction. Casualty losses for business and income-producing property that are not related to a federally declared disaster continue to be deductible. Business casualty losses are claimed on Form 4797, while losses from income producing property not used in a business are deductible on Schedule A, line 16.

Because individuals can’t claim miscellaneous deductions for employee expenses for 2018 - 2025, business casualty and theft losses of property used in performing services as an employee cannot be deducted nor applied in the netting process to offset gains. (2021 Form 4684 instructions, page 6)