Light bulbs are made in a wide range of sizes and shapes. A standardized system of letters and numbers is used to indicate the size and shape of the bulb as well as the size and type of the base. Bulbs also come in a variety of wattages.

Row of light bulbs
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Row of light bulbs.

Bulb Shapes

Closeup Macro Christmas Lights
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Christmas lights are an example of C bulbs.

Letters are used to indicate the shape or "type" of a light bulb. The three most common shapes are Type A, B, and G. Type A is the common pear-shaped light bulb, Type B has a decorative torpedo or flame-like shape, and the globe shape found in many bathrooms is known as Type G. Other types/shapes include Type C (for night lights and Christmas trees), Type R (reflectors) and Type T (tubular).

Bulb Size

Man holding light bulb
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Man with light bulb.

The type designation is often followed by a number. This gives the diameter of the bulb at its widest point in units of eighths of an inch. For example, an A19 bulb indicates a pear-shaped bulb with a diameter of 19 times 1/8 inch, or 2 3/8 inches.

Base Shape

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Lightbulbs have a screw base.

Most household light bulbs have a screw base, also known as an "Edison" base. This type of base is indicated by the letter E. Other types of bases include bayonet, pin and blade.

Base Size

Broken compact fluorescent light bulb
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Light bulbs have different base sizes.

Light bulbs also come with different size bases. The shape and size of the bulb aren't sure signs of the bulb's base size. The size of the base may be specified in several ways: common name (e.g., "candelabra"), diameter of the base in inches (e.g., 1 1/16) or the diameter of the base in millimeters (e.g., E26 is a screw-type base 26 mm in diameter).. There are five standard sizes of screw-in sockets: miniature (E11), candelabra (E12), intermediate (E17), medium or standard (E26), and mogul (E39).

Wattage

White bulb
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Light bulbs all have different wattage power.

Wattage indicates the amount of power consumed by a light bulb. It also provides a general indication of a bulb's brightness. However, a 100-watt bulb doesn't necessarily put out twice as much light as a 50-watt bulb. Higher wattage bulbs also generate more heat, so never exceed the recommended wattage for a lamp or fixture as you might cause a fire.