When looking for light bulbs in the 100w or 125w range (or equivalent), you'll notice that the standard base of the product is a different size.
Two standard light bulb base sizes are available today: A19 and A21. Although dozens of other designs and options are in the marketplace, most homes and businesses use these options for their everyday lighting needs.
The designations describe the shape of the base with the diameter measured in eighths of an inch. That means A19 and A21 bulbs have the same form, but the first is 2.375 inches in diameter. The larger model is 2.625 inches in diameter.
You'll also notice a base height difference in the A19 vs. A21 comparison. The A19 is 4.13 inches high, while the A21 is nearly 5 inches.
When you purchase any light bulb with these designations, including LED bulbs, you'll receive a similar user experience.
Even though the A-style base was one of the first introduced with the invention of the light bulb, it continues to be one of the most popular designs. It uses a standard threaded style that works well for convenient connections to fixtures.
Consumers can quickly remove or install light bulbs with minimal risk of injury. Even if the base gets stuck in the fixture, it is reasonably easy to take out – even if the bulb breaks.
Except for the longer neck and larger bulb, the A21 light bulbs look precisely like the A19 version. The size is identical enough that you can install them in the same fixtures or use them for similar lighting needs.
Most fixtures get paired with 40W or 60W bulbs that produce up to 800 lumens, but you can go with something brighter when needed because of this design element. The only trouble in the past has been the heat generated by 100W incandescent bulbs. Ceiling fans, floor lamps, and even some sockets weren't designed to withstand those temperatures.
LED manufacturers create an equivalent wattage experience, measured in lumens, that fits in fixtures while eliminating the heat-generation issue. Higher lumens still use the A21 shape, much like higher wattage designs do, to ensure the technology works as intended.
For most consumers, the size of the light bulb base isn't the consequential part of the buying decision. A19 and A21 bulbs work in virtually all standard fixtures.
What you'd want to review is if the fixture is compatible with the wattage equivalent you need for your home, business, or outdoor application.
Wattage, or watts, is a measurement of electrical power. When you have one watt, you experience one joule transferred per second. If you're looking at electrical applications and circuits, it's equally easy to translate current to voltage.
If you have a traditional 40W incandescent bulb with an A19 base, it will convert 40 joules of energy into heat and light output each second.
The distinction between heat and light output when comparing incandescent and LED technologies is what generates the energy-saving difference.
If you have a 40W incandescent bulb and a 10W LED bulb that produce equal lumens, you're saving 30 joules per second.
You could purchase a 40W LED bulb, but the power and brightness generated by that technology could light up an entire backyard. That's why wattage is no longer considered an essential measurement of energy. You need to know the lumens rating for a direct product comparison.
With an incandescent bulb, some designs lose up to 90% of the joule expenditures as heat. That means you only get 10% of the energy converted to light with A19 or A21 designs.
LEDs generate the same light with much less wattage because the heat generation process doesn't occur.
When you're ready to upgrade to LED lighting, you'll likely buy bulbs with A19 or A21 bases. Track and linear or tube lights come with pins or receptacles to use, which means a different base is necessary for those fixtures.
If you're considering the A19 or A21 format only, you'll have the following choices to consider.
When comparing A19 vs. A21 light bulbs, the primary difference is the size. You can find higher lumens options with a more extensive base, but the rest of the technology remains the same. The minor differences are not enough to prevent you from using either option in modern fixtures.
Choose the lumens you need, select the LED color temperature you want, and you'll have a long-term solution for your lighting needs.