Sometimes it is hard to tell when looking at a flat tire to see if it is replaceable. It’s best to never try to repair a tire on your own, and to take the tire to your local tire store for repairs. Never let anyone repair a tire without taking it off the rim. A small nail hole on the outside may have caused extensive damage to the inside of the tire.
Most holes that are ¼” or less and that are on the tread face are repairable; however always check out the whole tire when deciding on whether or not to make the repair. Any damage to the sidewall should not under any circumstances be fixed. The stress applied to tire sidewalls can cause the tire to blow out. If a tire has been driven on while at low pressure for any amount of time, it should not be repaired as there could be extensive damage to the sidewall of the tire.
One thing to keep in mind is the amount of tread left on the tires. Tires with less than 2/32nds left of tread should not be repaired, and should instead be replaced. Always check the age of the tires when determining whether or not to repair or replace.