The shortcut, first
If you would rather not learn any of this, you do not have to. Snap a clear photo of the bulb you have now (zoom in on the markings), tell us the room it is in, how many you need, and the color of light you like, and send it to a GoodBulb Lumen Master. Decoding bulbs and finding exact replacements is literally our job. Ask a Lumen Master →
Still want to read the code yourself? Here is how.
What the code actually means
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A bulb code looks like a random jumble, but it is a standardized shorthand for a bulb's specs. Read it in three parts: base, shape, size.
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1. The base (how it connects)
Usually one or two letters, followed by the base diameter in millimeters.
- E = Edison screw base. E26 is the standard 26 mm screw base in the U.S.
- B = bayonet (push-and-twist), common in some fixtures and outside the U.S.
- GU = twist-and-lock pins. The number is the gap between the pins in millimeters, so GU10 has pins 10 mm apart.
2. The shape
One or two letters for the bulb's form:
- A = Arbitrary, the classic household bulb shape
- BR = Bulged Reflector, used in recessed cans and floods
- G = Globe, the round bulb in vanity bars
- T = Tubular, long and narrow
- PAR = Parabolic Aluminized Reflector, a focused flood or spot
- HID = High-Intensity Discharge · X = Xenon
3. The size
The number after the shape is the bulb's widest diameter, measured in eighths of an inch. Divide by 8 to get inches.
- A19 = an Arbitrary bulb 19/8 inch wide = 2.375 inches
- BR30 = a Bulged Reflector 30/8 inch = 3.75 inches
- T8 = a Tubular bulb 8/8 inch = 1 inch
Letters at the end
Suffixes vary by brand, but often flag features: W for wattage, LED for the technology, H for halogen, D for dimmable or for daylight (5000–6500K). When in doubt, these are exactly the details worth confirming before you buy.
Put it together
- E26 A19 = a standard U.S. screw base, classic bulb shape, 2.375 inches wide. The everyday household bulb.
- GU10 BR20 = twist-lock pins 10 mm apart, bulged-reflector shape, 2.5 inches wide. A common track or recessed light.
- T8 = a one-inch tubular, the standard shop and office fluorescent-replacement tube.
One honest warning
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There are tens of thousands of bulbs, and the codes get specialized fast. Ordering the wrong one is the most common reason for a return. If the fixture is unusual, or you just want it right the first time, send us a photo and we will match it. And when you do replace it, a GoodBulb LED, made in America and built for 25,000 hours, means you will not be decoding that code again for a very long time.
Shop bulbs by shape → · Ask a Lumen Master →
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