What is T8 LED tube light? The “T” stands for “tubular” (the shape of the bulb) and the number indicates the diameter in eighths of an inch. A T8 has a 1-inch diameter (or 8/8 inch), T5 has a 5/8-inch diameter, and T12 has a 12/8-inch diameter (or 1-1/2 inch) diameter.