Shoppers of LED Light bulbs often confuse the specifications for base type (E26 or E27) and bulb shape (A19).
Screw in bulbs use a base called an Edison Screw or ES base. This base was developed by Thomas Edison for the first light bulbs and is still in use today.
There are four commonly used thread size groups for lamps:
- Candelabra: E12 North America, E11 in Europe
- Intermediate: E17 North America, E14 (Small ES, SES) in Europe
- Medium or standard: E26 (MES) in North America, E27 (ES) in Europe
- Mogul: E39 North America, E40 (Goliath ES) in Europe.
The number following the E indicates the size in mm of the external thread screw. Thus a E26 has a 26 mm base diameter.
You may see low cost LED bulbs using an E27 base on eBay and Amazon from foreign suppliers. While these technically will work in E26 bases, this is usually a sure indication that the bulb was not strictly designed for the US market and this bulb may lack a proper North American, UL or ETL safety certification and therefore should be avoided.
Now that we have learned that E26 and E27 are terms used to describe the type of base used what does A19 or bulbs with an A designation mean?
A type describes a bulb that has a pear-like shape. The number that follows the "A" within the A series indicates the width of the bulb in one-eighth inch units or in millimeters.
The most commonly used A-series light bulb type is the A19 bulb which is 2 3⁄8 inches (60 mm) wide at its widest point and approximately 4 3⁄8 inches (110 mm) in length. This is the classic shape that most people are used to when shopping for a "Light Bulb". You can use this handy image below to identify the size of your A Shaped bulb:
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