Published on August 30th, 2014 | by Peter Young


A Guide To Understanding Modern Light Bulbs: Shapes And Sizes

light bulb shapes

Modern light bulbs will come in a variety of different shapes and sizes and, needless to say, it can get pretty confusing. Typically, there are eight different light bulb shapes that one might find in use around their home. Why so many? Modern light bulbs are used in a variety of different fixtures, and each fixture has its own unique set of requirements when it comes to the size, shape and the way the light bulb projects its light. As a result, selecting the proper light bulb has become more like finding a needle in a haystack. In order to help you understand which shapes and sizes you’ll need, we’ve come up with the following guide:

Modern Light Bulbs: Shapes And Sizes

These are the eight most common light bulb shapes you’ll find in your home:

  • Arbitrary (A)
  • Bulged Reflector (BR)
  • Candle (C)
  • Globe (G)
  • Quartz Reflector Lamp (MR)
  • Parabolic Aluminized Reflector (PAR)
  • Blown Reflector (R)
  • Twist

It’s important to note that while these names are fairly common throughout the industry, there is no universal set of terms to describe the shape of a light bulb. For example, a compact fluorescent light bulb (CFL) will be described as a “twist” or “spiral” shaped bulb. Because of this, we recommend that you bring your old light bulb with you to the store so you can compare it with your new light bulb to ensure it’s the proper shape and size.

The following graphic should help to give you a better idea of what these bulb shapes look like:

Light bulb shapes

And since, for some reason, MR bulbs are not included in the above diagram, here’s a graphic and common use indicator:

MR lightbulb shape

As we mentioned earlier, all of these light bulb shapes will come in different sizes. As you’re browsing the light bulb aisle and comparing different packages you may come across designations like A-19, PAR-20 or G-25. The first part of these designations (A,PAR, G) is referring to the actual shape of the bulb itself. The second part (the number) is the measurement of the bulb’s diameter at its widest point, and this is expressed in 1/8ths of an inch. So for example, a bulb labeled A-19 is an Arbitrary shaped bulb (A) that measures 19 eighths of an inch in diameter at the bulbs widest point.

Check out this graphic to get a visual idea of how this measurement works:

measuring bulbs

Here is a list of the sizes that are available for each of the eight most common light bulbs you’ll find in your home:

  • Arbitrary (A15, A17, A19, A20, A21, A23)
  • Globe (G9, G11, G12, G16, G16.5, G19, G25, G30, G40)
  • Candle (C6, C7, C9, C11, C15)
  • Multifaceted Reflector or Quartz Reflector Lamp (MR8, MR11, MR16, MR20)
  • Blown Reflector (R12, R14, R16, R20, R25, R30, R40)
  • Bulged Reflector (BR25, BR30, BR38, BR40)
  • Parabolic Aluminized Reflector (PAR14, PAR16, PAR20, PAR30 Short or Long Neck)
  • Twist (T2 Coil, T3 Coil, T4 Coil)

Even though there are wide variety of light bulb shapes and sizes available on the market today, there are three shapes and sizes in particular that you are most likely to encounter throughout your home. Check out the following list to see which ones those are and where you’re likely to find them:

  1. A-19. This shape and size of light bulb is by far the most common one that you’ll find in your home.  It’s often used in lamps, hallways lights, overhead sconce fixtures, ceiling fans and more. Crazy to think, but the shape of this bulb hasn’t changed since Edison produced the first commercially practical model in 1879.
  2. R-20/BR-30. These light bulbs are some of the more common ones that you’ll find in your home, and are often used as flood lights or recessed can lighting.
  3. MR16. These sleek, stylish bulbs grace many track lighting and high end kitchen fixtures, and provide spot-lighting (not a lot of diffuse light–much more focused) for things architects want to draw attention to, like kitchen islands and marble countertops.
  4. Candelabras. These little light bulbs come in at number three and are often used as accent lighting and on chandeliers.
  5. PAR38/40. These bulbs, along with the heavier bulge reflector bulb (BR40) are usually your outdoor lighting. These bulb types provide a lot of light (high number of lumens/lux), and are typically higher wattage bulbs. The main difference between BR and PAR is the focus. BR tend to be more spread out, whereas the PARs can be much more focused, like the MR16s.
  6. CFL Twist or Spiral. These bulbs are becoming more and more popular and are designed as a replacement for the A-19 since they are more energy efficient than their incandescent counterpart.
  7. T8/T12. These tube lights are very common in laundry, kitchen, and garages, as well as sometimes being used outside under covered awnings (the ballasts are definitely not waterproof). These have historically been magnetic ballasts with fluorescent, ~32 watt T12 bulbs, but LEDs are starting to exert their influence in this arena, with newer hybrid LED T8s able to be used in both magnetic and electronic ballasts.
  8. G25. These are decorative bulbs that light up a lot of bathroom vanities.

Filament types

In many cases, the exterior of the bulb may not tell you what you need to know about how much energy it’ll use. Incandescents put out light by superheating a filament wire. As you can imagine, that’s not very efficient. 90% of the energy is wasted as heat. CFLs do not have filaments–they are powered by warmly heating a gaseous mixture that, when it gets “excited” glows brightly. This is why CFLs are dim when you first turn them on, and then put out more light as they warm up. LED is an acronym for Light Emitting Diode, meaning that a piece of semiconductor material connects a negatively charged cathode and a positively charged anode. The transfer electrons along this semiconductor material, and this process produces light.

Recently a trend in hip restaurants is to use everything vintage–bowls, decorations, and, yes, lighting. This is an unfortunate development on the lighting front because it means that many progressive restaurants are going back to incandescent bulbs to capture that retro look. However, thankfully, new LED bulbs have started to enter the market that have stretched the diode into filament-looking shape:

Yes, these are LEDs!

These LEDs carry the benefits of other LEDs: supreme efficiency, bright lighting at whatever color spectrum desired, and long life. Cool, huh?

If you want to learn more about how light bulbs work, check out this article on light bulb colors and temperatures. Also, be sure to check out some of our other green home improvement projects: Green Living Ideas, after all, is a top 20 home improvement website!

Photos courtesy of Edison Light Globes, Lamp Tech, Feit,

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About the Author

graduated from Pacific Lutheran University (PLU) with a degree in journalism and has made sustainability and eco-conscious living mainstays of both his professional and personal life. It was during his time at PLU that he began his journey with sustainability and it's what has led him to writing for Green Living Ideas. He currently resides in Honolulu and works for Pono Home, an energy efficiency company focused on reducing carbon emissions and promoting a healthier, greener lifestyle.

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