I have a fluorescent tube that I need to replace. The label on it says:

enter image description here

The bulb measures 47" end-to-end, including the metal endcaps but not including the pins.

The diameter appears to be 1-1/2".

The pins look like this:

enter image description here

I'm extremely confused by fluorescent light bulb sizes. What is the size designation of this bulb? USA if it matters.

  • 1
    I would think it is a standard 4 foot. Quite a few products are not their actual size. I think the F40 is the important label.
    – crip659
    May 2, 2023 at 13:45

3 Answers 3


In almost all cases you can replace an F40T12 (what you have) with an F32T8 bulb. Which will be a lot easier to find (in 2023), in general, and save 20% on wattage, often with no decrease in light out. T12 bulbs are nearly as obsolete as incandescents.

The number after the F is the nominal wattage, and goes with length in a mysterious manner. That manner varies with diameter. Beware of circular or U tubes with the same wattage when shopping blindly, though.

The number after the T is diameter of the tube in eighths of an inch.

T12 and T8 use the same lengths and pin spacing. T5 is different on both and incompatible.

A fixture with a T12 is likely to be old. If a new tube fails to fix it, the ballast may have expired, and need to be replaced as well, which might call into question the value of repair .vs. replace. Or it may be so ancient it has a separate "starter" that needs to be replaced (look for a small round "can-like" thing sticking up from the fixture behind the tubes - it twists and pulls out.) Ballasts typically last longer than bulbs, but not forever. A new ballast is likely to be more efficient than the old ballast, and won't need a starter.

  • 3
    And once you get to ballast replacement then there is the other option of rewiring to use LEDs. I've been replacing fluorescent fixtures with LEDs in my house over the last several years, but a few of them just refuse to die, and the energy savings just aren't big enough to justify replacement before then (unlike incandescents). May 2, 2023 at 14:25
  • 1
    Ok; armed with this knowledge I went to the hardware store and picked up some LED replacement bulbs that were T8/T12 replacements, 48", and marketed as drop-in replacements (which I guess means they work with the existing ballast; there was another type marketed as "ballast bypass direct wiring", which I didn't get). And.... success! They fit and they work great. Thanks!
    – Jason C
    May 2, 2023 at 15:00
  • 2
    Yup, those replacements do exactly that - they work with the ballast voltages. Easy to install. Only downside is you still need the ballast, where the ones that work on line voltage let you remove the ballast (but that's more work).
    – Ecnerwal
    May 2, 2023 at 15:03
  • 2
    You can only replace F40T12 with F32T8 if you change the ballast to match. May 2, 2023 at 20:34

You want F40T12 tube size

The T-number is the diameter in 1/8'ths of an inch.

  • T5 = 5/8" diam.
  • T8 = 1" diam. (8/8")
  • T12 = 1-1/2" diameter

The 48" long tubes are the most common, and are called F40T12. The "40" is the nominal watts.

Note that T8 tubes will physically fit, but will not work properly with a T12 ballast. If in doubt, pop the ballast cover off and read the labeling on the ballast as to whether it supports F40T12 or F32T8. A bulb/ballast mismatch will perform badly.

And 5000K (kelvin) color temperature

The tubes come in a variety of color temperatures. These decide how "yellow-ish" or "blue-ish" your light looks. You generally want to match all your tubes in the room or premises to the same color temperature so it doesn't look bad.

  • 2700K is "hearth and home" traditional incandescent
  • 3500K is a warm white by fluorescent standards
  • 4100K is normal fluorescent standard
  • 5000K is "daylight" fluorescent, which is what you have.

CRI is how nice the stuff looks.

That is Color Rendering Index, a recent phenomenon. Old tubes have pretty bad CRI. All modern tubes are 80-90 CRI and there are even 98 CRI tubes. 100 is perfect light. You can't tell CRI by looking at the tube, but you can sure compare fixtures of 2 different CRIs by looking at the stuff they're lighting up. Good CRI makes things look natural.

90 CRI is readily available in real fluorescent, which is why I'm staying with it rather than going LED (at least for 48" tubes). LED conversions are also an option; some keep the ballast in place, others require you rewire to bypass the ballast. (and others work either way).


That is a 48" or T-5 bulb. It is noted on the far right ( E7T5)

  • 1
    Ok; your T-? gave me something to start looking for. The diameter is 1-1/2" (I just added that to the post) and I found this size chart so I think it's actually a T12-48". Does that make sense? It says T-5 only has a 5/8" diameter.
    – Jason C
    May 2, 2023 at 13:50
  • 1
    That makes sense. wonder why the markings are T5. Most of those fixtures use a T-8. T-12 is unusually big, but that's what you have.
    – RMDman
    May 2, 2023 at 13:52
  • 1
    T5 is 5/8" diameter, so you are answering incorrectly due to some text on the bulb that isn't what you think it is. This a a T12 (12/8" diameter) tube, which can be replaced with a T8 or T12 tube of the same length, generally.
    – Ecnerwal
    May 2, 2023 at 14:00
  • 1
    @RMDMan T12 was the standard for decades in residential and commercial use. T8 is relatively new - i.e., it probably existed decades ago but it wasn't the standard thing. I remember T12 back around 1980 (high school, long story...) and those were in some used fixtures that had probably been sitting in the garage for 10+ years at the time I first saw them up close. And my current house has some (still haven't replaced them all with LEDs) that could be from the 80s but probably go back to the 60s. May 2, 2023 at 14:02
  • 1
    That's E7t5 - the lower case t should have been a clue...
    – Ecnerwal
    May 2, 2023 at 14:22

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