T8 Bulbs are more energy-efficient and have a smaller diameter (1 inch) than T12 Bulbs, which are less efficient with a larger diameter (1.5 inches).
if you have a T12, it is larger by a half inch to the newerT8 tubes.However, the diameter is not as important as the bi-pin base. There are two main types of bases for bulbs: The screw base and the pin base
CE ETL Approved Jesled T8 LED Tube Light for Replacing T5 T8 T10 T12 Fluorescent Bulbs, Find Details and Price about T8 LED Tube T8 LED from CE ETL Approved Jesled T8 LED Tube Light for Replacing T5 T8 T10 T12 Fluorescent Bulbs - Jesled ...