What do you think of first when you imagine a light bulb? I can guess that it is most likely the standard A19, tear drop shaped bulb that has made its presence known around the world since the late 1800’s. In addition to the A19 bulb, we are familiar with the A21, A23, and the A15 shapes, as well. But what does the “A” in these types of bulbs mean?
Great question, and I’m glad you asked!
The “A” stands for arbitrary, and in the case of bulbs we take the definition of that to mean the shape was decided upon based on random choice of its designer. The number that follows the “A” in these bulbs indicates the bulb’s size when measured in 1/8” increments. For instance, a standard A19 bulb diameter measures at 19/8”, or when converted is 2.375”. The A21 bulb measures 21/8”, or 2.625” in diameter, but keep in mind that the A21 bulb is generally also slightly taller than the A19 which may prevent it from properly fitting in some fixtures. Be mindful of this when selecting a replacement bulb for your application.