The bulb shape and size of a fluorescent lamp are expressed by a code consisting of the letter "T" (which designates that the bulb is "tubular" in shape) followed by a number which expresses the diameter of the bulb in eighths of an inch. They vary in diameter from T-2 (1/4 inch) to T-12 (1 1/2 inches). In nominal overall length, fluorescent lamps range from 6 to 96 inches, which is always measured from back of lamp holder to back of lamp holder.
For, example, the actual over all length of the 40-watt rapid start T-12, 48 inch lamp is 47 3/4 inches. Circline lamps, which are circular, are available in four sizes: 6 1/2 inches, 8 inches, 12 inches and 16 inches outside diameter. There are also U-shaped fluorescent types (Curvalume®) with T-8 and T- 12 bulbs. The width of U shaped types are measured for the distance between the ends. The overall length is measured from the face of the bases to the outside of the glass