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LED tube lights are usually categorised by various key designations. The most common of these are tube length (this can be stated in either imperial or metric measurements) and bulb or lamp size. Lamp size is typically given as a ‘T’ measurement, with widespread standard sizes including T5, T8 and T12.

If you’re wondering exactly what is the difference between T5, T8 and T12 lights, the main point to remember is that the higher the T rating, the thicker a lamp will be in diameter. T equals 1/8 of an inch and the number after the T denotes how many eighths of an inch wide the bulb is – hence T8 is exactly one inch or 8/8ths. You can use this to calculate the diameter of different sized LED tube lights. Therefore, T8 tubes at 1-inch (25.4mm) have a larger diameter than T5 tubes (5/8-inch or 15.9mm), but they are not as wide as T12 (1.5-inches or 38.1mm) lamps.

In standard fluorescent tubes, smaller diameters almost always mean better efficiency. A T5 bulb will use less energy to produce the same amount of light as a T8, while a T12 will run at about 45% higher electricity consumption to output the same amount of light as a T5. Being vastly more power-efficient across the board, newer LED equivalents don’t follow quite the same pattern in terms of percentages. However, the basic principle remains similar, even if the ratios between bulb diameter and energy usage stay far closer together as you move up the lamp sizing scale.

It's also worth noting that different tube sizes will tend to be associated with different lamp bases or sockets. T8 and T12 tubes are mounted to bi-pin G13 bases as standard, while T5 tubes are normally attached to a bi-pin G5 socket fitting. In simple terms, this is essentially the tube light equivalent of standard bulb cap styles and sizes.