T5 Wattage vs. T12 Wattage

Diameter

• The diameter of a bulb is measured in increments of 1/8 inch, according the sizes.com. For example, a T5 bulb is 5/8 inch in diameter, while a T12 is 12/8, or 1 1/2 inches in diameter. T12 bulbs are capable of using a higher wattage and producing more light due to the increased space for gas in the bulb when compared to a T5.

T5 Lengths

• T5 fluorescent bulbs are available in lengths of 6, 9, 12, 21, 34 and 46 inches. Each length has its own wattage and thus brightness. A 6-inch bulb carries 4 watts, while a 9-inch bulb uses 6 watts. A 12-inch bulb is 8 watts, 21-inch bulbs are 13 watts and 34-inch bulbs are 39 watts. The 46-inch bulb comes in two wattages, a dimmer 28 watts and a brighter 54 watts.

T12 Lengths

• T12 bulbs have fewer lengths than the T5, but more wattages due to the introduction of energy-efficient bulbs. The T12 comes in lengths of 15, 18, 24, 36 and 48 inches. The 15-inch bulb carries 14 watts, while the 18-inch carries 15 watts and 24-inch bulbs use 20 watts. A 36-inch bulb comes in two wattages. The original is a 30-watt bulb, and an energy-efficient bulb uses 25 watts to create the same amount of light. A 48-inch bulb comes in 25 watts and 40 watts, with an energy-efficient 34-watt version available, as well. T12 bulbs also come in 8-foot lengths, with wattages of 60 and 75 watts.

Shapes and Uses

• The T in T5 and T12 stands for "Tubular," the round and long shape of the bulb. A T12 bulb is sold in straight, round or U-shaped forms. T5 bulbs are available in straight forms or in specialty shapes including U-shaped, two- or four-pronged push-in bulbs. The T5's thin diameter is used in a variety of specialized shapes for specific fixtures. Place circular bulbs in round ceiling fixtures, for instance, or use U-shaped bulbs in large lamps and designer ceiling fixtures. Push-in T5 bulbs are used in a variety of lamps, fixtures and some work lights.