When you browse light bulbs, cords, and sockets, you run into a wall of letters and numbers — E12, E17, E26, G40, S14, A19. It looks like a code you were supposed to have been taught. It isn't. Once you know what the letters and the numbers each stand for, you can identify any bulb base in about five seconds.

The short answer

The three screw-in bases you will actually meet are E12, E17, and E26. The "E" stands for Edison, after Thomas Edison, who patented the screw base. The number is the width of the base in millimeters. An E17 base measures 17 mm across. That is the whole system.

• E12 — Candelabra. 12 mm. The small one. Chandeliers, night lights, decorative string lights, most globe bulbs.
• E17 — Intermediate. 17 mm. The in-between one. Older Christmas light strings, appliance bulbs, some specialty fixtures.
• E26 — Medium or Standard. 26 mm. The one in your table lamp. Standard household lighting and commercial patio string lights.

If you are holding a bulb and want to know which you have: E12 is roughly the width of a pencil, E26 is roughly the width of a quarter, and E17 is the one that is obviously neither.

Light bulb base size chart

What the other letters and numbers mean

The base code tells you what fits the socket. A second code tells you the shape and size of the glass. The letter is the shape; the number is the diameter of the bulb itself.

• G is a globe — round. A G40 is a 40 mm round globe, the classic patio-string look. A G50 is the same shape, 50 mm across.
• S is a sign bulb — straight-sided with a rounded top. An S14 is the standard commercial string light bulb.
• A is the ordinary pear-shaped household bulb. An A19 is the one in most table lamps.
• C is a candle shape, tapered like a flame.

Put the two together and the whole label reads plainly: a G40 E12 bulb is a 40 mm round globe on a 12 mm candelabra base. Shape first, then the base that has to match your socket.

The one thing that matters: shape is a preference, base is a requirement. A G40 and a G50 will both look right on a patio string. A bulb with the wrong base will not screw in at all.

Which bulb do I need?

E12 Candelabra

The most common decorative base. If you bought a string light set with small round globes, or you are relighting a chandelier, this is almost certainly what you need.

• White .5-Watt LED G40 Globe, Shatterproof — $1.25 each, in stock
• Same bulb, 10-pack — $12.50, in stock
• LED Filament G50 Globe, dimmable — $1.62 each, if you want the visible-filament look

Browse all E12 candelabra bulbs →

E17 Intermediate

The least common of the three, and worth checking twice before you order — an E17 will not thread into an E12 or E26 socket. If you are replacing bulbs on an older Christmas string, this is usually the one.

• LED Filament G40 Globe, E17 — $1.62 each, in stock
• 10-pack — $16.20
• 25-pack — $40.50, for a full string

Browse all E17 intermediate bulbs →

E26 Medium

The standard household base, and the one on commercial-grade outdoor patio string lights. If you are lighting a backyard, a patio, or a paper lantern with a pendant cord, this is your size.

• Cool White 0.8-Watt LED S14, Shatterproof — $1.90 each, in stock
• Warm White S14, 10-pack — $19.00, in stock
• Warm White LED Filament S14, dimmable — $2.68 each

Browse all E26 standard bulbs →

Prices and availability shown are current as of September 2026. Check the product page for today's price and stock.

Still not sure?

Measure the base across its widest point in millimeters. That number is your answer — 12, 17, or 26. If the bulb is broken or missing, measure the inside of the socket instead.

Or just ask us. We have been selling lanterns and lights since 2002, and we would rather spend two minutes getting you the right bulb than have you order the wrong one. Reach our team by phone, email, or chat.

Browse by base size: E12 · E17 · E26. Or see all light bulbs.