What is numbers 20 in F20-T12 fluorescent bulb?

In the designation "F20-T12," the "20" refers to the bulb's wattage, indicating that it consumes 20 watts of electricity. The "T" denotes the shape of the bulb, which is tubular, and the "12" specifies the diameter in eighths of an inch—meaning the bulb is 12/8 inches, or 1.5 inches in diameter. Thus, an F20-T12 fluorescent bulb is a 20-watt, 1.5-inch diameter tubular fluorescent light.