There are a few variations, but simply: If the cost of repairing the vehicle, plus its salvage value, plus rental reimbursement expense during repair equals or exceeds its actual cash value, it is not economically feasible to repair. This situation is commonly referred to as a "total loss."
A total loss is usually not a good situation even if you hate your car. If you don't agree with the "total loss" determination you typically can get an independent appraisal of the damage (at your expense).
Some companies consider a damaged vehicle a total loss when the total cost to repair it exceeds just 51 percent of the vehicle's actual cash value (because of added storage costs, rental car, etc.), while others don't consider it a total loss until it is at 80% of the ACV.
Add comments of your situation, as insurance companies and adjusters vary in their determination. For example, a new car may be closer to 50% of ACV while an older car may be 90% of ACV before distinguishing it as a total loss. We have also seen some situations that are the opposite.
CarInsurance.com recommends you purchase a separate GAP insurance policy when you purchase insurance on a new vehicle. Alternatively, with many CarInsurance.com companies you can add Loan/Lease (GAP) coverage to your CarInsurance.com policy.