The rules, procedures and laws regarding vehicle losses vary from state to state and from insurance company to insurance company. In most states, insurance companies have the authority to declare a vehicle a "total loss."
How an insurance adjuster comes to the determination that a vehicle is a total loss depends upon both state laws and then the insurance company's own internal guidelines. If the state has a certain percentage of the car's worth that the damages would come to or exceed to repair the car then the adjuster will find the car to be totaled out. If the state does not have an amount stated in law that the insurance companies must follow then it is left up to the discretion of the insurance company to decide what percent of the car's actual cash value (ACV) makes a car totaled.
Many states only define a total loss of a motor vehicle as when a car is not repairable, or when it costs more to repair than what it is worth. When a state has not set forth specific guidelines then the insurance company normally has the right to determine if a car is a total loss or not using their internal guidelines as we mentioned earlier.
Without a specific state law as a guideline most insurance companies will determine a vehicle to be a total loss when it is between 51 to 80 percent of its actual worth or basically when the repairs costs are more than the value of the vehicle. They may find that the cost of repairs, storage fees, rental car costs, etc make it uneconomical to repair a car and thus declare a car to be a totaled.
A vehicle's value is determined by comparing your vehicle's condition to similar vehicles. This may include input from local auto dealers, private parties or recent sales which the adjusters use in their valuation. Condition, equipment and mileage differences are also taken into consideration.
Sometimes as insurance adjuster can tell by viewing the car and tell that it will be a total loss due to the extensive damage the car received while other times the car will be looked over by mechanics and the cost of the repairs is used by the claims adjuster to determine that the car should be totaled out and not repaired.
To find out if your state has any guidelines regarding when a car should be determined to be a total loss contact your state's insurance regulator.
If they do not have regulations regarding this and it is left up to the individual insurance carrier then ask your insurance agent what your company's guidelines are. If you are in the middle of a third party claim then you can discuss with the insurance adjuster for the insurance company working your claim what their guidelines are for totaling out a vehicle.