My previous article got into wattage, lumen output, etc.  This is about the actual size and shape of light bulbs.

What you may or may not know is that every light bulb type has one or more letters and some numbers that are used to categorize it.  Thing of this as the taxonomy of light bulbs, and if you are ordering one online, it is especially important in order to be able to search for the correct bulb.

The “common” light bulb is an A19.  The letter indicates the shape.  A is used for, well, what I would call a light bulb shape!  Some common types:

A – standard light bulb shape

G – Globe (the rounded style often used in bathroom and vanity lights)

R – Reflector (think spotlight)

PAR – Parabolic Aluminized Reflector. 

B – candelabra shape

MR – metalized reflector

There are many others, but the above are the most common.


The numbers indicate the widest part of the bulb in multiples of 1/8 inch.   So, for example:

A19 – the “common” light bulb is 2 3/8” at its widest part (19 x 1/8th inch)

MR 16 – 2 inches wide (16 x 1/8th)



While this is the bulb’s overall shape and size, the connector also has to be considered, i.e. how the bulb plugs into the light socket.

The standard American “medium screw base” is officially designated E26.  “E” is for Edison screw, a nod to the light bulb inventor and 26 is the diameter in millimeters.  Because they are essentially the same size and interchangeable, you may see E27 bulbs in places like IKEA, since the European screw base is 27mm.  Because the size difference is so small there is no problem using an E27 bulb in a US fixture (as long as it is 120 Volts of course!)   So the good old American light bulb shaped light bulb might be listed as A19 / E26 or A19 / E27.

By contrast, the smaller, “candelabra” base in the US is an E12.

There are other bases, which are typically only found on specialty bulbs such as bi pin halogens, e.g. MR 16 bulbs.  Since those get much more complicated, but sure to take your old specialty bulb with you to the store as there are many bulbs that look very similar but often have subtlety difference base sizes. 


So next time you see an A19 / E26, 800 lumen, 5,500, 85 CRI, 10W LED bulb, you will know that it will be:

– Standard light bulb shaped (because of the A)

– Standard light bulb sized (19 x 1/8”, or 2 3/8” at the widest point)

– Standard medium screw base (E26)

– Putting out 800 lumens of light (about the same as a typical 60W incandescent bulb)

– Cool white (5,500 degrees Kelvin)

– Able to show all colors well (since it has a Color Rendering Index of 85)

– Using only 10W of power (to produce the same amount of light as an old 60W bulb).


Now you just need to choose clear or frosted….!

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