Light Bulb Buying Guides, Lighting 101

Shedding light on identifying which fluorescent tube you require

identifying which fluorescent tube you require

The process of identifying which fluorescent tube you require can be a bit of a minefield, but if you follow our step-by-step guide, you will make light work of it.

Diameter and cap

Tubes come in an array of diameters; each has a specific code, which consists of the letter T and a number. The number signifies how many eighths of an inch the diameter is. The different diameters have different cap/bases. Are the different diameters interchangeable? No, apart from the T8 and T12 tubes. Most of the time a T8 Tube can be used to replace a T12 tube (which for the most part have been phased out) but only if the fitting uses a starter.

Code Diameter Cap
T2 7mm W4.3
T4 12mm G5
T5 16mm G5
T5 Circular 16mm GX13
T8 26mm G13
T9 Circular 29mm G10q Base
T12 38mm G13

Length and wattage

The wattage of a fluorescent tube denotes the length of the tube, in most instances if you know the wattage of a fluorescent tube you automatically know the length. T4 tubes however are the exception, they have several lengths for the same wattage, to ensure you select the correct T4 option you must include the pins in the overall measurement.


Tubes come in a variety of shades of white. Here are some examples of codes used to identify which shade you require.

2700, 726, 827, 927 – Very Warm White

3,000, 530, 129, 29, 830, 930 – Warm White

3500, 535, 35, 23, 135, 835 – White

4000, 640, 33, 133, 840, 940 – Cool White

6000, 6500, 765, 54, 154, 860, 865, 960, 965 – Daylight


The codes on the side of a tube provide a clear indication of the information that is required to identify the correct tube, here are some examples.

L 58W / 840: 58W – 58W tells us the wattage and 840 that the colour is white.

FHE 14W / 830: 14W tells us the wattage, FHE (Fluorescent High Efficiency) the diameter and 830 that the colour is warm white.

F18W/T8/835: 18W tells us the wattage, T8 the diameter and 835 that the colour is white.

TL-D 36W/840: 58W tells us the wattage and 840 that the colour is Cool white.

Check out our range of Fluorescent Tubes Fluorescent Tubes: The Lightbulb Co. UK

If you are in any doubt as to which fluorescent tube you require, please contact our helpful team who will be happy to assist to you on 01869 362222.


About Sonja Dean

Sonja is a lighting expert at The Lightbulb Company with a background in hospitality and a keen awareness of the important role lighting plays in our everyday lives. She has worn many hats with us over the years, developing an extensive knowledge of lighting and the lighting industry. She is passionate about working with customers, understanding their needs, and providing them with the best solutions.