The letters and numbers on light bulb packages are coded shorthand for the shape and size of each bulb. In other words, they precisely describe the shape or type of bulb as well as its diameter.
It does matter what shape a light bulb is. The shape of the light bulb is important because it affects how the light is cast. Some bulb shapes provide more focused light while others give off a diffused glow in all directions. Plus, light bulb shape can impact the light fixture's design value. For instance, flame-shaped candle bulbs can work well in a chandelier.
Here's what you need to know about light bulb shape codes.
|BT||Bulged tube/chimney/blunt tip/bent tip|
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An A designates the teardrop—or flower bulb—shape of a standard light bulb. This is where the name light bulb comes from. However, over time the name has become broadly used to describe any bulb shape.
Most standard screw-base light bulbs—such as the GE 60 watt incandescent light bulb—are A bulbs. A-19 and A-21 light bulbs, which are most 60 watt and 75 watt light bulbs respectively, are two common examples of A bulbs.
02 of 08
B can indicate one of three different shapes. When it is used by itself, it stands for bullet. A bullet light bulb is a cylinder with a smaller rounded end and a base that tapers in.
BR denotes a bulged reflector bulb. BR bulbs typically have two funnel-shaped reflector areas, one behind the other, to help shape and focus the light. They're narrow at the bottom and then flair out for a wider top.
A BT code can indicate a bulged tube or chimney light bulb—essentially just wide tubes sometimes with an even wider middle. It can also stand for a blunt tip light bulb—the ones that are flame-shaped with a rounded end. In addition, BT light bulbs can stand for bent tip, which is the exact opposite of a blunt tip. To minimize the confusion, many companies just spell out blunt tip or bent tip on the package rather than relying on the code.
03 of 08
A C stands for candle. These are the flame-shaped bulbs with the pointed, bent tips. Some companies have started using CA to designate their candle bulbs. That might have started as an attempt to distinguish candle flame-shaped bulbs from circline lamps.
CIR tells you it's a circular or circline lamp. In other words, it's a fluorescent tube made in a circle.
04 of 08
G is the code for a globular light bulb. These bulbs are often mounted in the open, so they can be seen. One popular location for globular light bulbs is in a multi-socket light fixture above a vanity mirror.Continue to 5 of 8 below.
05 of 08
An S bulb is made for use in a sign or appliance. These are not used in residential lighting.
06 of 08
A T stands for tube or tubular. This is the shape of, and the designation for, a standard straight fluorescent light bulb.
T-12 and T-8 fluorescent tubes are two fairly common examples of bulbs in this shape. Another is the T-10 tubular incandescent light bulb that is commonly used in the gallery light fixtures that mount to a picture's frame.
07 of 08
An R lamp is a reflector lamp. If you have any recessed lights or track lights in your home, you probably have some of these lamps installed.
Variations on the R shape include BR—bulged reflector bulbs, PAR bulbs—which have a parabolic shape, and MR—mirrored reflector bulbs.
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The numbers that follow the letter codes denote the size of the light bulbs. They show the diameter of a bulb in eights of an inch.
Two familiar examples are T-12 and T-8 fluorescent tubes. The older T-12 tubes are 1 1/2 inches in diameter while the newer T-8 tubes are 1 inch in diameter. A common A-19 light bulb is 2 3/8 inches in diameter, and an R-30 flood light is 3 3/4 inches across its widest part.