What size are Christmas light bulbs?

The direct answer: the common outdoor sizes are C6, C7, C9 and the 5 mm mini, and the number after the C is a size convention based on the bulb diameter in eighths of an inch. In published dimensions: C6 is about 3/4 inch across and 1 1/8 inch tall, C7 is 1 inch across and 1 1/2 inch tall, C9 is 1 1/4 inch across and 2 1/2 inch tall, and a mini is 5 mm across. C7 bulbs screw into an E12 candelabra base; C9 bulbs screw into an E17 intermediate base. Those two base sizes are not interchangeable, and that is the single most useful fact on this page.

The size chart

What the C number actually means

C stands for the cone or strawberry shape shared by the whole family, and the number is the diameter convention — the figure divided by eight, in inches. That is why C9 is bigger than C7 and C7 is bigger than C6, and why there is no C8. Published dimensions do not land exactly on the arithmetic for every size, so treat the number as the ordering system it is rather than a precise measurement: use the published diameter and height when you need to know whether a bulb will fit behind a shutter or inside a storage reel.

C6, C7 and C9 all share the same strawberry silhouette. What changes is scale, and with scale comes the base: C7 takes the smaller E12 candelabra screw base, C9 takes the larger E17 intermediate. A C7 bulb will not thread into a C9 socket and the reverse is also true, which is the cause of most mismatched-replacement frustration.

Viewing distance decides the size

This is the only rule you need for choosing. A bulb has to be big enough to register at the distance it is read from, and no bigger — because a bulb that is too large for its viewing distance reads as a glare source and makes the wire between bulbs obvious.

• Thirty to fifty feet away — a roofline, a garage front, a gable. C9. The faceted 1 1/4-inch lens is the only one in the family that keeps its shape from the curb.
• Ten to twenty feet — porch fascia, path markers, a small front elevation. C7, or C9 if the house is set well back from the road.
• Three to six feet — railings, columns, windows, wreaths, wrapped trunks. 5 mm mini. A C9 at hand height is a bulb people knock.
• Close enough to touch. 5 mm mini on green or brown wire, at tight spacing. Industry spacing guidance goes down to 2.5 inches on a wreath for this reason.

Spacing changes with size, too

Size and spacing are a pair, and buying the right bulb at the wrong spacing produces the same disappointment as buying the wrong bulb. Published guidance gives 6 inches as the maximum bulb spacing for a tree, gutter or house trim, 4 inches for poles and banisters, and 2.5 inches for wreaths. C9 socket wire is commonly sold at 12-inch spacing, which is the standard for a roofline because the bulbs are large enough to carry the gap.

The other sizes you will see

• G-series globes (G30, G40, G50) — round rather than strawberry-shaped, and the number works the same way. A party-string look rather than a Christmas-trim look, and popular on patios.
• T5 and M5 — small conical and faceted-conical minis. Same scale as the 5 mm, different lens shape, and usually sold on the same wire-in strings.
• C3 and C5 — smaller than C6 and increasingly rare outdoors. Fine on a wreath or a small tree.
• 5 mm wide angle vs 5 mm conical. Not a size difference, an optics difference: wide-angle throws light sideways, which is what you want on a wrapped surface. Conical points forward, which is what you want on a tree you are looking into.

Buying replacement bulbs without getting it wrong

1. Identify the base, not the bulb. Unscrew one and look at the thread: E12 means C7, E17 means C9. The listing for a replacement box states the base.
2. Match the voltage. Fitting a wrong-voltage replacement into a working run is one of the standard causes of a repeatedly blown plug fuse. Replacing bulbs and fuses covers the diagnosis.
3. Match LED to LED. Dropping an incandescent bulb into an LED run changes the load for that socket and does not look the same next to its neighbours.
4. Buy the box, not the bulb. Replacement C9 LEDs are sold in boxes of 25 for roughly the cost of a few singles, and a roofline you keep for a decade will want them.

If you are deciding between the two large sizes rather than identifying what you already own, the head-to-head is in C9 vs C7 Christmas lights, and the spacing detail for a roofline is in the C9 bulb spacing guide.