5 Different Fluorescent Tube Sizes and How to Choose One

Fluorescent light bulbs

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Fluorescent light tubes are glass lamps that produce light through a unique action of ionized mercury atom causing a powdered phosphor coating inside the bulb to glow (fluoresce). Although LED (light-emitting diode) light bulbs are increasingly popular and have largely replaced standard incandescent light bulbs in residential applications, tube-like fluorescent bulbs (lamps) and fixtures are still popular in some applications, due to their low energy consumption and the quality of the diffused light they emit. Fluorescent light fixtures using long tube-shaped lamps are often used for workshop and hobby lighting, for example. A 4-foot-long fluorescent tube over a workbench can be much more effective than standard incandescent or LED bulbs in traditional light fixtures. Fluorescent lighting can also work well for providing supplemental light for growing plants, or for needle-crafters and other hobbyists.

And there are also screw-in forms of fluorescent lamps that fit the sockets on standard light fixtures and in floor and table lamps. Known as CFL (compact fluorescent light) bulbs, they were regarded as an excellent energy-saving alternative to incandescent bulbs—at least until the advent of LED technology.


One very legitimate reason why the use of fluorescent bulbs is waning, in favor of increased use of LED bulbs, is that fluorescent bulbs contain a small amount of hazardous mercury. This makes fluorescent bulbs difficult to dispose of safely. Always consult your local waste management agency for advice on what to do with burned-out fluorescent bulbs. Another reason to avoid fluorescent lighting is that LEDs are more cost-effective over the long run. Though more expensive initially, LED bulbs generally last much longer than fluorescent tubes, making them a better long-term bargain.

Fluorescent tubes (officially known as "lamps") come in five basic types, identified as T2, T5, T5HO, T8, and T12, and The "T" designation stands for "tubular," and the following number refers to its diameter in 1/8-inch diameter increments. A T12 bulb, for example, is 12/8 inch in diameter, or 1 1/2 inches. A T2 bulb, on the other hand, is 2/8, or 1/4 inch, in diameter.

Here's what you should know about each basic fluorescent tube size.

What Is a Fluorescent Light Tube?

All fluorescent tubes (lamps) consist of a sealed glass vessel that contains a small amount of mercury and an inert gas, usually argon, at a very low pressure. The inside of the tube or bulb is coated with phosphor powder. When the light fixture is turned on, electrodes inside the tube or bulb ionizes the mercury vapor, which causes light to be emitted when the ionized atoms strike the phosphor coating lining the glass. This is a much different mechanism than that used by incandescent bulbs, which produce light when an inner metal filament gets hot enough to glow from the electricity passing through it. And it is also different than LED bulbs, which produce light from electrical current passing through microchips.

Fluorescent Tubes vs. Fluorescent Bulbs

In a fluorescent tube, there is an electrode at either end of the tube. A device known as a ballast steps up the 120-volt line current in the circuit to a level sufficient to energize the mercury vapor within the tube. The electron flow moves from the electrode at one end of the tube to the electrode at the other tube, and light is produced as ionized mercury atoms strike the powdered phosphor that lines the inside of the tube.

A screw-in fluorescent light bulb is essentially a miniaturized version of the long bulbs used in shop lights and similar fixtures. In the bulb version, a miniaturized ballast is integrated into the base of the screw-in portion of the bulb. Most screw-in fluorescent bulbs consist of small T2 tubes tightly coiled together so that the bulb remains roughly the size of a standard incandescent light bulb.

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    T2 Bulbs

    An EcoSmart 23 Watt Dimmable Soft White CFL Light Bulb

    The Spruce / Bill Lewis

    • Best for: Replacing incandescent bulbs in light fixtures and lamps.

    T2 fluorescent bulbs have tubes that are 2/8, or 1/4 inch, in diameter. This is the size that is often used for CFL (compact fluorescent light) bulbs that are configured to screw into standard 120-volt light fixtures and lamps. There are also straight T2 tubes that are often used for the table lamps that provide task lighting used for crafting and sewing.

    In CFL bulbs that screw into standard light fixture sockets, the ballast that steps up current is integrated into the screw-in base rather than placed in the light fixture itself. This allows the CFL bulb to be used in standard light fixtures that can also accept incandescent or LED bulbs.

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    T5 Bulbs

    Energy saving T5 light tubes with golden connectors

    Jukkrapun Thipsupa / Getty Images

    • Best for: Task lighting in kitchens, low ceiling fixtures.

    T5 tubes are 5/8 inch in diameter—larger than the tiny T2s, but smaller than the larger tubes normally used in shop lights. These bulbs are space savers but still put out the same amount of light as the larger bulbs. These bulbs are good for fixtures mounted on lower ceilings and on the underside of cabinets above countertops. Add these to your kitchen to give you a balanced lighting scheme of soft light that is sure to please. Being small, the space required to accomplish this lighting task is minimal, and the result is great. People use these above and below cabinets to add a unique look to their kitchen, but kitchens are not their only use. Be creative in using these space-saving light bulbs.

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    T5H0 Bulbs

    Stack of T5 bulbs

    Bryngelzon / Getty Images

    • Best for: Exceptionally bright illumination; large, tall rooms.

    T5H0 bulbs are identical to the T5 bulbs with one exception—they are much brighter (HO stands for "high output). These bulbs are said to be twice as bright as T5 bulbs. They are a great addition to rooms with taller ceilings. These bulbs used in fixtures can be great accent lighting in rooms with wall-mouted pictures, and areas that require more light, such as rooms with tall ceilings. These bulbs are so powerful that they are often used in the up-high fixtures used in warehouses and warehouse stores.

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    T8 Bulbs

    Linear fluorescent T8 light tube

    Temdor / Wikimedia Commons / CC BY 4.0

    • Best for: Outdoor light fixtures.

    T8 bulbs are 1 inch (8/8) in diameter. They are more expensive than T12 bulbs but will start much faster. They are excellent at starting at all temperatures, including temperatures below freezing. The starting temperature is set by the type ballast you have installed in the fixture that you buy. The ballast is responsible for starting the light fixture and determines the type of bulb required in each fixture.

    Be sure to do your due diligence before making a selection of light fixture, especially if you need a cold weather-start light fixture. A rapid-start, instant-start, or electronic type ballast may be the right choice for you. Your local dealer can help you choose the right fixture if you give him or her the parameters of your needs for your installation.

    Continue to 5 of 5 below.
  • 05 of 05

    T12 Bulbs

    Linear fluorescent T12 light bulb

    Temdor / Wikimedia Commons / CC BY 4.0

    • Best for: Industrial uses, warehouses.

    T12 tubes are 1 1/2 inch (12/8) in diameter. They are bigger, bulkier, and the least expensive to buy, although, they are not the most efficient tubes. The biggest drawback of these tubes is that they are warm-weather bulbs. They need temperatures to be 60 degrees Fahrenheit or higher to start and operate properly. Below that temperature, the light may appear to flutter and is much dimmer during start-up. The color of T12 bulbs tends to be somewhat greenish, so they are not commonly used in residential applications.

    These are now less popular as users have migrated smaller and more efficient T5 and T8 tubes. Older factories are seeing the benefits of moving away from these older style tubes, and with the new age of LED lighting, it looks as though the days of these old-style fluorescent tubes are numbered.

How to Choose a Fluorescent Light Tube

You really don't have a choice when it comes to choosing fluorescent tube size for a specific light fixture: The tube must match the specifications of the light fixture—A T5 tube will not fit in a fixture designed for T8 fluorescent lamps. The exception is CFL T2-size bulbs, which will fit in any standard screw-in light socket.

The tube diameter, as defined by the T rating, is just one element that goes into choosing a fluorescent tube. There are several other considerations when it comes to matching a fluorescent tube to a specific fixture:

  • Wattage: Light fixtures will be rated for the wattage of fluorescent tubes they accept. This will vary according to the diameter and length of the tube. For example, T12 bulbs are typically 48 inches long and use 40 watts to provide 2500 lumens of light output. T5 bulbs, on the other hand, are typically about 45 inches long and use only 28 watts to produce the same 2500 lumens of light output.
  • Light output (Lumens): This is a measure of the amount of illumination provided by the fixture. The higher the number, the brighter the light.
  • Color rendering index (CRI): This refers to the color of the light produced, on a 100-point scale, where 100 is the relatively warm light produced by standard incandescent light bulbs. CRI ratings under 70 tend to produce a greenish color that's not all that pleasing. For most residential use, ratings of 80 to 89 are considered best. T12 tubes generally have a CRI of about 62, while T5 and T8 tubes have a more realistic CRI of 85.
  • Correlated color temperature ( CCT): is a measurement of the color appearance measured in degrees Kelvin within a range of 2700K to 6500K. 2700K is the warmest (closest to yellow) and 6500 is the coolest (closest to sunlight). Most light fixtures will allow you choose tubes with different CCT ratings. If you are using the fixture as a grow light for plants, for example, you'll want to choose one with a color temperature close to natural sunlight.
  • Shape: Your light fixture may accept traditional straight tube, or it may be designed for curved or round tubes. CFL (compact fluorescents) have narrow tubes that are coiled to create a shape roughly the size and shape of traditional incandescent light bulbs, so that they can be used in standard light fixtures.
  • Base type: Most common fluorescent light fixtures use bases with two pins that fit into slits in the light fixture. But CFL bulbs have screw-in bases that fit standard light fixtures. And there are also single-pin tubes designed for fixtures with that socket design.

For example, a typical fluorescent tube might carry a designation such as this:
F/32/T8/ Warm White/ Linear Fluorescent/ 3000K/ 28000 Lumens/ 48", interpreted as follows:

F= "Fluorescent"
32= 32 watts
T8= diameter (8/8, or 1 inch)
Warm White= Color
Linear fluorescent=Tube shape
3000K= Color temperature
2800 Lumens=Light output
48"= Tube length

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