Light Bulb Shapes, Sizes and Base Types Explained

When thinking about the image of a typical bulb, most of the people picture that one sort of bulb many of us have been using all our childhood. It’s the traditional A-type bulb. However, bulbs have been evolved significantly and now you have them in numerous categories divided by shapes, sizes and base types.

Of course, such a variety of shapes and sizes can be not only overwhelming but even confusing, yet, the principles of how light bulbs are divided are, in fact, very logical and you will get the whole picture once you learn about all the possible types. The respective parameters (shape, size and base type) appear on the package of the product. This normally is in the form of letters followed by a number followed by an optional letter. The first letter or letters indicate the shape. While the numbers indicate the diameter but the optional letter indicates the length of the bulb.

Modern light bulbs can be used in a variety of fixtures, and each fixture has its own particular requirements of size and shape of the bulb that it is compatible with. This article will briefly cover each category and provide you some useful information so that you actually know which type of bulb you need for different situations and fixtures. Why is it good to know this? Simply because the next time you will go to purchase a new light bulb for your fixture, you will know what you are looking for.

Bulb shapes

The main shape designations of light bulbs are:

  • A – Standard household incandescent light bulb shape (widely used for home lighting, universal);
  • B – Bullet tip;
  • BT – Bulged or Blown Tubular bulb shape (used for table lamps);
  • BR – Short reflector style light bulb;
  • C – Candle or Conical bulb shape (used for chandeliers and decorative lamps);
  • CP – Crystalline Pear bulb shape;
  • E – Ellipsoidal bulb shape;
  • ER – Extended reflector light bulb shape;
  • F – Flame style candelabra bulb shape;
  • G – Globe bulb or circular bulb;
  • HK – Hexagonal Candle;
  • K -Krypton or narrow reflector light bulb shape;
  • MB – Halogen bulb;
  • MR – Quartz reflector lamp (low wattage);
  • P – Pear bulb shape;
  • PAR – Parabolic Aluminized Reflector (PAR) lamp;
  • PS – Elongated standard incandescent bulb;
  • R – Reflector light bulb style;
  • S – Strait Sided bulb style;
  • T – Tube lamp shape.
Light bulb shapes. Source -

Light bulb shapes. Source –

However, only three of all the shapes are widely used for everyday purposes (e.g. household), namely:

A-shaped bulbs. “A” type bulb shapes are standard types for household lighting but you will see different variations and types of them. However, the standard bulb sizes are A19 and A60. The letter indicates the shape of the bulb but the number following it is the diameter of the bulb in eighths of an inch. For instance, A-19 bulb means that it is s “A” type and the diameter would be 19/8″. For non-imperial measured bulbs, the number after the bulb shape is the number of centimeters at the widest point of the bulb.

B and C-shaped bulbs. B-shaped bulbs are also known as Candelabra bulbs. Bulbs with this shape have a bulged base that attenuates to a rounded tip. C-shaped bulbs look very similar to holiday lamps. The bulbs of this shape are very wide-spread in chandeliers, night lights, and lower wattage applications.

PAR-shaped bulbs. PAR light bulbs use a parabolic mirror that focuses all the light. For LEDs, the PAR designation is used for defining the shape since most of the LEDs lack any have a reflective lower surface. The most common of PAR lamps are PAR20, PAR30, PAR38 and PAR42 type of bulbs; nevertheless, those are just a few of the types available. These bulbs come in various beam angles and they are widely used in spotlights and flood lights.

Different LED bulb shapes. Source -

Different LED bulb shapes. Source –

Diameter and length

As for the diameter and length, the categorization is a lot more simple. The main diameter classifications are:

  • 8 = 1 inch;
  • 11 = 1.375 inches;
  • 16 = 2 inches;
  • 20 = 2.5 inches;
  • 30 = 3.75 inches;
  • 38 = 4.75 inches.

As for the length, the bulbs are divided into S (short) and L (long) categories but not always you will see the length indicator mentioned in the product description.

Bulb base types

Altogether you can divide all the bulbs into 6 base types:

  • Screw bases;
  • Twist and lock bases;
  • Specialty bases;
  • Bi-Pin bases;
  • Fluorescent pin bases;
  • Compact fluorescent lamp plug-in bases.

Light bulb base types. Source –

However, just two of these base types can be considered as the main ones, namely, screw bases and pin bases. Each of the base types work differently as their main function is to connect the light bulb to the electrical voltage and the technology of how it is achieved is different in each case.

Screw bases

Screw bases are the most common light bulb base. You might also hear them referred to as Edison base light bulbs. In an incandescent or halogen screw base light bulb, two contact wires link the filament to the base where the electrical voltage is connected to the bulb. Screw bases are made in several sizes, and on the packaging you will see that they are referred to by a descriptive name and size. Unlike in the case of the light bulb shapes, the number in the light bulb base indicates the light bulb base diameter in millimeters, for instance, the candelabra E12 has a diameter of 12 millimeters. The most common screw base sizes are E5, E-5, E10, E-10, E11, E-11, E12, E-12, E14, E-14, E17, E-17, E26, E-26, E27, E-27, E39, E-39, E40 and E-40.

Pin bases

Pin bases are mainly used in linear fluorescents, plug-in compact fluorescent bulbs, MR16s, and a few types of HID light bulbs. The principle of pin base bulbs differs significantly from screw base lamps. Whereas the screw base connects the light bulb to the voltage with two contact wires connected to the metal base, pin base light bulbs contain two pins that stick out of the base and connect the bulb to the voltage. Electrical current then can pass through the pins and flow into the light bulb to excite the filament and produce light.


Blogger, editor, developer who loves green living. Interested in photovoltaics and solar lighting. Reviewing solar products since 2013.

You may also like...

5 Responses

  1. Zane Dunnam says:

    Thank you SO much for your detailed explanation WITH PICTURES! My wife and I have installed new outdoor ceiling fans which require special candelabra bulbs. When searching for replacement bulbs, we were having a difficult time finding the right type and especially size that would fit between the bulb base, and the metal soffit above the base. Our bulbs can be no longer than 3.25 inches in order to cant them sideways and then insert the bulb base into the receptacle to begin screwing down and into the receptacle. The fan/light manual suggested a B10 bulb shape with an E12 base; max-40W. Wow. Most packaging for fan bulbs do not provide all the specifications needed or provide enough detail to make a purchasing decision. So we had to just buy multiple bulbs to see which ones would fit. Your article had the MOST detailed and definitive information in one place for us to understand the many different light bulb shapes, sizes, and bases.

    Thank you again SO much for writing such a great and in-depth article! Keep up the good work.

  2. Kris says:

    But how can I tell if the bulb I am holding in my hand is an E-10, E-27, E-40?

  3. Joseph Karsok says:

    Just checked on several boxes of light bulbs–General Electric, Sylvania, and 2 others . None of them have any base designation on box or bulb. Is an E26 bulb the standard base size? If measuring, at what point am I to measure since these are threads?

    • Arthur says:

      It’s most likely E26 which is the most common base. It refers to 26 millimeters (which is 1 inch). You can determine it by measuring the diameter of the base (the thickest parts).

Leave a Reply

Your email address will not be published. Required fields are marked *