There are a variety of shapes, lengths and diameters in tube lights in order to suit a wide range of applications. Generally speaking, with LED tubes the most common size and shape are the 4 foot length fixtures with a T8 diameter. Other lengths available in the marketplace include 2 foot, 3 foot and 8 foot. In the past, T12 diameter bulbs were the standard, eclipsed first by T8 and then T5. For older fluorescent fixtures, the most common setup is going to be either a T12 or T8 type due to the costs associated with running T5 fluorescent light fixtures.
When upgrading older existing fixtures, sometimes the issue arises where it is unclear what diameter bulb is currently installed. Luckily, this is an easy fix as the labeling system is standardized across all manufacturers. The “T” stands for tubular, referring to the bulb’s shape, and the number following it represents the bulb diameter in eighths of an inch. This means a T8 bulb will be 1” in diameter, while T12 and T5 bulbs will be 1.5” and ⅝” in diameter respectively. It should be noted that as long as a T8 and T12 are using the same pin base (G13 most likely), they can be used interchangeably, as long as they are within the same milliamp requirements for the ballast if it is using one.
