The number printed on a fluorescent tube is not the length, not the wattage, and not the brightness. It is the diameter in eighths of an inch, and misreading it is how half the shop-light replacements go sideways at the checkout counter. Understanding fluorescent light bulbs sizes before you unload a fixture from the ladder saves you an irritating trip back to the hardware store and protects the ballast that is usually a bigger deal than the tube itself. This guide breaks down every standard size, pin configuration, and retrofit option you are likely to encounter in a US home.
The T-Number System
Every fluorescent tube carries a T-designation: T5, T8, T12, and so on. The T stands for tubular, and the number tells you the diameter in eighths of an inch.
- T5: 5/8 inch diameter (0.625 inches)
- T8: 1 inch diameter
- T12: 1.5 inch diameter
T4 and T2 exist for specialty applications like undercabinet lighting, but they are rare in residential fixtures. A quick caliper measurement or laying an old tube on a ruler settles any confusion in under a minute.
Common Tube Lengths
Length is the other half of the matching equation. Standard US fluorescent tube lengths are surprisingly consistent across the T-sizes:
- 24 inches (2 ft): 17 to 20 watts, common in undercabinet and closet fixtures
- 36 inches (3 ft): 25 to 30 watts, found in older kitchen boxes
- 48 inches (4 ft): 32 to 40 watts, the workhorse size for garages and shop lights
- 72 inches (6 ft): 55 to 75 watts, rare residential, common commercial
- 96 inches (8 ft): 60 to 95 watts, warehouse and shop fixtures
A 4-foot T8 running 32 watts is by far the most common tube in US residential use. If you are not sure what you have, the label on the fixture end cap will spell it out.
Pin Configurations
Pin type determines whether a tube physically fits into a socket. T8 and T12 tubes use G13 medium bi-pin sockets, which are interchangeable physically (though not electrically). T5 tubes use a smaller G5 miniature bi-pin.
Circular tubes, often called circline, use a 4-pin single-ended connector. U-bend tubes, which are essentially a 4-foot tube folded in half, still use G13 pins on both ends of the U.
T5 vs T8 vs T12
The three main sizes are not plug-and-play substitutes, even when the length matches. Ballast compatibility varies, and pin spacing differs between T5 and T8/T12.
T12 tubes are the oldest and least efficient. Most general-purpose manufacturing ended around 2012, though stock still turns up at closeout retailers. T8 replaced T12 as the residential and light commercial standard and remains the most widely available tube today. T5 is used primarily in high-output commercial fixtures and modern growing setups.
Wattage, Lumens, and Color Temperature
Wattage varies by length and tube diameter. A standard 4-foot T8 pulls 32 watts and produces around 2,900 lumens. A 4-foot T12 pulls 40 watts for similar output, which is why T8 displaced it.
Color temperature comes in four common variants: 2700K (warm), 3500K (neutral), 4100K (cool white), and 5000K (daylight). Shop lights and garages usually ship in 4100K or 5000K. Kitchens under older fixtures often run 3000K for a warmer feel.
Compact Fluorescent (CFL) Pin Bases
CFL tubes for dedicated fixtures use a different base system entirely. Common pin bases include:
- GX24q: 4-pin base for 26W, 32W, 42W triple-tube CFLs
- G24d: 2-pin base, usually for magnetic ballast fixtures
- 2G11: Long twin-tube CFL used in commercial lighting
Screw-base CFLs with a standard E26 medium base are the exception. Those work in any regular lamp socket and do not require a separate ballast.
LED Retrofit by Tube Size
LED replacements are now sized to match every common fluorescent tube. A 4-foot LED T8 tube from Hyperikon, Barrina, or Philips runs $6 to $15 in multi-packs and produces 1,800 to 2,400 lumens while pulling 15 to 18 watts.
Type B direct-wire LEDs eliminate the ballast and usually outperform ballast-compatible Type A versions. A 10-pack of T8 LED tubes running 18 watts each will light a 20×20 garage and cut energy use by roughly 50% compared to the old T12 setup.
How to Identify What You Have
Pull the tube and read the end cap. The manufacturer prints the size code (F32T8, F40T12, F17T8) right on the metal cap. F is for fluorescent, the number after it is the wattage, and the letter-number combination identifies the diameter.
A 4-foot T8 will be marked F32T8. A 4-foot T12 will be marked F40T12. A 2-foot T8 will be marked F17T8. Matching that exact code at the store prevents the single most common mismatched-tube failure. Once you can read the cap and confirm the ballast type, navigating fluorescent light bulbs sizes at Home Depot or Lowe’s takes under 30 seconds. Keep a cellphone photo of the end cap and the ballast label inside the fixture so you never have to pull a tube twice for the same purchase.
