When it comes to Christmas lights, both C7 and C9 bulbs are common, but they're not the same. A C7 bulb size is significantly smaller than a C9, and they each fit into a different size of light socket. If you're purchasing replacement bulbs for an existing light strand, it's best to take one of the old bulbs with you to compare the size to the new bulbs. If you're shopping for new sets, be sure to read the sizing information to ensure you're getting the bulb size that's best for your holiday decorating dreams.
C7 vs. C9 Bulb Size
A C7 bulb is shorter than a C9 bulb, and also has a narrower base. The base end that screws into a socket fits an E12 socket, with the 12 indicating 12 mm across. Candelabra and nightlight bulbs also fit this size socket, but those bulbs are not necessarily the same height or width a C7. A C7 stands about 1.5 inches tall and is 1 inch in diameter. The C indicates a conical shape, or the classic Christmas light bulb shape.
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By comparison, a C9 bulb size is larger and has a wider base, fitting into an intermediate socket called an E17 that's 17 mm across. A C9 bulb stands an inch higher than a C7, measuring 2.5 inches tall with a 1.5-inch diameter, according to Lit Professional Lighting. As for wattage, the average incandescent C7 bulb is 5 watts, while a similar C9 is up to 10 watts, making it appear brighter when viewed from afar. Newer LED lights in the same C7 and C9 sizes generally have less wattage and may not get as bright, but they'll use less electricity.
Which Size to Use
The best way to decide which bulb size to choose for your strands of lights is to consider where they'll be displayed. Since C7 is smaller, it's a good choice for smaller displays, such as on a medium-sized Christmas tree, or on a small shrub outside. The Christmas Designers site notes that the C9 bulb size is most common for lighting the perimeter of a home, although strands of C7 bulbs are also an option. The size and added brightness of the C9 are why they're still the most common for linear displays outlining the shapes of buildings, or of specialty displays in the shape of a Christmas tree, for instance, as the view looks crisp and bright at night. If you simply prefer the look of one size bulb over another, choose what you like; after all, you'll see them every day during the holiday season.
LED bulbs are a nice option for display on a cut Christmas tree because they don't get as hot to the touch and are less likely to cause a fire. The needles on a Christmas tree dry out over time, making them more of a fire hazard over time. This is why it's a good idea to unplug the indoor Christmas lights when you plan to be away for a while, or at night before bed.