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    There are many light bulb sizes and types, but the most widely used fluorescent lamp is the straight tube. It is measured in diameters in eights of an inch. The same is true for the incandescent lamp. A lamp that is an inch in diameter is called T8. Common sizes are from T2 to T17.


    It should be stressed that the nominal lengths of a T8 and T12 are not gauged on the lamp but the fixture. The measurement is between the interior faces of the holder sockets. The actual lamp length is about an inch shorter. The T5’s nominal length however, is two inches more than the actual end to end bulb measurement.

    Bi-Pin Bases

    While the light bulb sizes differ, a lot of them have bases that hook up to the sockets. Majority of these bases have a couple of pins. These are known as bi-pin bases.

    These bi-pins are available in various sizes, but the most widely used are the medium bi-pins. These are usually set on the T8 and T12 lamps. A smaller bi-pin is used on the T5 lamps.

    There are also single pin bases, but these are usually used on lengthy (i.e., 8 ft) tubular T8 and T12 lamps. These are hardly used in residential homes. Most of the recessed double contact bases are utilized in industries.

    There are also some mogul bi-pin bases. These are used on the 60″ T17, pre-heat light bulb sizes. Most of them are found on the 90 watt versions, but some are on 82 watts. The mogul bi-pin bases are also found on the 60″ T17 and the 48″ T12.

    The T12

    For a time, the T12 (1 ½ in diameter) base lamps were sold in 24″ (30 w), 36″ (25 w), and 48″ (40 w). Due to the 1992 Energy Policy Act, 48″ 40-watt T12 CW, WWX, WW and D lamps can no longer be made. Low wattage 34W was designed in their place. For all intents and purposes, the T12 have been rendered obsolete.

    Most of the T12 made were straight bulbs with medium bi-pins. The specs were 15 inches (14 watts), 18 in (15 watts), 24 inches (20 watts), 36 inches (25 watts), 48 inches (25 watts), 48 inches (34 watts) and 48 inches (40 watts).

    The T8 Light bulbs

    The T8 have become the standard in most homes in the U.S. A T8 will produce less light than a T12 but in a fixture produce light equal to a T12. The 18 inch T8 uses 15 watts. Other common T8 bulbs are the 24 inch (17 watts) and the 36 inch (25 watts).


    The T5 comes in various lengths, including the 6” (4W), 9” (6W), 12” (8W), 21” (13W), 34“(39W), 46” (28W) and 46” (54W). A mini bi-pin is used for the T5. The medium bi-pin is too big for this type of lamp.

    The light bulb sizes are always dependent on the technology. While the T8 is the most common today, it can change when the technology evolves.

    1. John L Crockett

      January 1, 2011 at 8:34 pm

      I have some very old strings (25 bulbs per string) of outdoor Christmas lights. I can no longer find replacement
      bulbs in the stores. I can find bulbs on line but they do not provide the base dimensions. The dimensions of
      my bulbs are about:
      1″ Dia
      3″ Long
      Base is about 5/8″ or 16.4 mm Diameter
      Can you tell me what bulbs to look for on Line?
      Thank you,

    2. Kathleen Hubbard

      February 15, 2011 at 12:44 pm

      It appears from the picture that the bases of the F15 & F20 are the same as a standard light bulb. Have you tried looking at the GE web site? They are the most likely to post the diameter of the base. Or check the GE bulb container, although the store brand of WalMart’s florescent bulb container gave no such dimension. I don’t have a standard incondescent bulb container any more, having gone to all florescents in my home.

      I think the best way to shop for them is to take one of the old bulbs along to the store with you, if possible.

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