A bulb’s name carries two codes. A19 is the glass: the letter is the shape and the number is its widest point in eighths of an inch, so an A19 is 2-3/8" across. E26 is the base, the standard screw base in US homes. Match the base to the socket first, then check the shape fits the fixture.
How to read a shape code
The number divided by 8 gives the width in inches at the bulb’s widest point: A19 is 19/8, or 2-3/8". A21 is 2-5/8", a little bigger, and may not fit under a lampshade or inside a globe made for an A19. The height isn’t in the code, so it varies by product.
Bulb shape and base chart
Light bulb shape and base codes
| Code | Shape | Across | Used in |
|---|---|---|---|
| A15 | Standard, smaller | 1-7/8" | Appliances, ceiling fans |
| A19 | Standard | 2-3/8" | Lamps and many fixtures |
| A21 | Standard, larger | 2-5/8" | Larger fixtures |
| B, C, CA | Candle | Varies | Chandeliers, sconces |
| G25 | Globe | 3-1/8" | Bathroom vanities, pendants |
| BR30 | Reflector, wide | 3-3/4" | Recessed cans |
| BR40 | Reflector, wide | 5" | Larger recessed cans |
| PAR38 | Reflector, focused | 4-3/4" | Outdoor floods, track |
| MR16 | Small reflector | 2" | Track and accent lights |
| Base | Name | Looks like | Used in |
|---|---|---|---|
| E26 | Medium | Screw base | Most US household fixtures |
| E12 | Candelabra | Small screw base | Chandeliers, nightlights, sconces |
| E17 | Intermediate | Screw base between the two | Microwaves and appliances |
| GU10 | Twist and lock | Two pins, push and twist | Spotlights on 120 V |
| GU24 | Twist and lock | Two pins, twist to lock | Some fixtures; some A19 LEDs |
| GU5.3 | Bi-pin | Two pins | 12 V MR16 spots, with a transformer |
| E27, E14 | European | Screw bases | Europe’s versions of E26 and E12 |
Width is the widest point of the glass. Heights vary by product.
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A19 and E26 are two different things
Many household bulbs are an A19 with an E26 base, which is why the two codes get mixed up. A19 says nothing about the base: Feit sells an A19 LED with a GU24 pin base, and A15 bulbs, smaller than A19, can use the same E26 base. Check each code against your old bulb or its box.
Same shape, different base
- MR16 spotlights come in two kinds that don’t swap: a GU5.3 base with two pins, run at 12 volts through a transformer, and a GU10 base with two pins you push in and twist, run on household 120 volts.
- E26 and E12 have European look-alikes, E27 and E14: see how to tell a base.
BR30 or BR40 for a recessed light?
Both are reflector bulbs for recessed cans: BR30 is 3-3/4" across and BR40 is 5". Bulb makers disagree on which one a 5" or 6" can takes, so measure the can’s opening before you choose: recessed light bulbs sets out each maker’s sizes. Turn the power off and let the old bulb cool before you change it.
Know the codes? Now the brightness.
Watts to lumens