A19 or A21?
If you’ve ever searched for a 100 watt or 125 watt equal household LED light bulb, you’ve probably noticed that the standard A19 shape if often replaced with a slightly larger A21 shape. This is because lamp names like A19 and A21 are actually describing their shape through a letter code (A) and their diameters in eighths of an inch. A typical A19 is 2.375 inches in diameter (19/8 = 2.375) and 4.13 inches in height, while an A21 is 2.625 inches in diameter (21/8 = 2.625) and about 5 inches in height.
The “A” shape was one of the first light bulb designs, and it remains popular in homes to this day. Save for the slightly larger bulb and longer neck, the A21 looks just like an A19, and it is intended for many of the same fixtures and purposes. A-shape bulbs usually have a standard medium base that screws into sockets made for all kinds of residential light fixtures, though they are most common in table lamps and ceiling fans. These sorts of fixtures are often paired with 40 to 60 Watt bulbs that produce an average of 450 to 800 Lumens.
To mitigate this high power consumption, LED manufacturers have been trying to create energy-efficient solutions with 100-Watt equal (and above) A-shape LEDs. Higher-lumen LEDs are often made in an A21 shape (and not an A19 shape) because the increased size helps manufacturers get around the heat issues by providing a larger surface area for heat to escape from, thus protecting the sensitive internal electronics. 100-Watt equal A21 lamps are a lot less complicated than A19 lamps to produce and are helping speed along the development of higher-lumen, energy-efficient residential bulbs.
Most of the time, the size of the A21 lamp will not make a difference, but it may in some fixtures. Before purchasing an A21 lamp, make sure your fixture will be able to accommodate the larger measurements. So if you want a certain room brighter the LED A21 is the way to go!