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Christmas light bulb sizes are named with a letter for the bulb shape and a number for its diameter — usually in eighths of an inch. A C9 bulb is 9/8 inch across, a G40 globe is 40/8 inch, and the small mini and M5 bulbs are measured in millimeters instead. Once you know that the code is a size-and-shape label, not a color or a technology, choosing between mini, M5, C6, C7, C9, and G40 gets a lot simpler.
How the Christmas bulb size code works
Two systems are in play. The C and G codes use eighths of an inch: the letter is the shape (C for the cone/strawberry bulb, G for a round globe), and the number is the maximum diameter divided into eighths. So a C7 is 7/8 inch, a C9 is 9/8 inch, and a G40 globe is 40/8 inch, or about 1.5 inches. The mini and M5 bulbs use millimeters instead — both are 5mm across — and "M5" adds a faceted LED lens to that small footprint. Knowing this, you can decode almost any Christmas light listing at a glance.
Christmas light bulb size chart
Here is every common size side by side, from the smallest tree bulb to the biggest patio globe.
| Bulb | Size | Shape | Base | Typical use |
|---|---|---|---|---|
| Mini (5mm / T1¾) | 5 mm | Small conical | Molded / replaceable | Trees, wraps, indoor strands |
| M5 | 5 mm | Faceted "ice" LED | Molded | Trees, wreaths, sparkle |
| C6 | ~0.75 in (6/8") | Faceted strawberry LED | Molded | Trees, tidy rooflines |
| C7 | ~0.75 in (7/8") | Strawberry | E12 candelabra | Windows, small trees, eaves |
| C9 | ~1.13 in (9/8") | Strawberry | E17 intermediate | Rooflines, ridge caps, fences |
| G40 | ~1.5 in (40/8") | Round globe | E12 / molded | Cafe / patio strings, swags |
| G50 | ~1.9 in (50/8") | Round globe | E12 / E17 | Bold globe strings |
The mini and M5 bulbs (5mm)
The 5mm mini is the classic tree light — a tiny conical bulb about the size of a grain of rice, sold in long strands of 50, 100, or more. The M5 is the LED-era version: the same 5mm footprint but with a faceted "ice" lens that scatters light into a brighter sparkle. Both wrap branches cleanly on thin wire, which is why they dominate Christmas trees. See mini Christmas lights and M5 Christmas lights for the full breakdown.
The C-family strawberry bulbs (C6, C7, C9)
The strawberry or cone bulbs step up in size and presence. C6 is the smallest at about 0.75 inch, almost always a faceted LED molded to the wire — a sparkly, tidy tree and roofline bulb. C7 is a 7/8-inch bulb on a screw-in E12 candelabra base, a mid-size classic good for windows and neat eaves. C9 is the largest at 9/8 inch on a bigger E17 intermediate base, the go-to roofline bulb because it reads clearly from the curb. The screw bases on C7 and C9 let you replace single bulbs. Compare the two big ones in our C7 vs C9 guide, or read up on C6, C7, and the best C9 sets.
The G-family globe bulbs (G40, G50)
Globe bulbs are round rather than pointed, sized in the same eighths-of-an-inch system. G40 is the classic 1.5-inch cafe or patio bulb used for warm, ambient string lighting on decks and pergolas as well as Christmas swags; G50 is the bigger, bolder 1.9-inch version. Their wide spacing spreads a soft glow rather than a dense sparkle. See G40 globe Christmas lights for details.
Which size should you choose?
- Christmas tree: 5mm mini, M5, or C6 — small bulbs that wrap branches and disappear into greenery.
- Roofline and eaves: C9 for maximum curb visibility, C7 for a tidier, closer look.
- Patio, deck, and pergola: G40 or G50 globes for warm, ambient overhead light.
- Windows and mantels: C7 or mini, scaled to the frame.
Size is only half the decision — color and spacing matter just as much. Once you have picked a bulb, see how to choose Christmas light color and our Christmas light spacing guide to plan bulb counts and placement.
Size, base, and replacement bulbs
When you buy spare or replacement bulbs, three things must match: the size code (C7, C9, G40, and so on), the base (E12 candelabra for C7 and many G40s, E17 intermediate for C9), and the technology and voltage (LED or incandescent, and the volts printed on the bulb). Get all three right and a burned-out bulb is a quick swap; get the base wrong and it simply will not thread in. Molded mini, M5, and C6 sets are the exception, since their bulbs generally are not individually screw-out replaceable.
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