Measure straight across the widest part of the glass and count the width in eighths of an inch. That count is the number in the shape code. A bulb 2-3/8" across is 19 eighths, so a standard-shape one is an A19.
How to measure
- Switch the light off, let the bulb cool, then take it out.
- Hold a ruler marked in eighths of an inch across the widest part of the glass, with one edge of the bulb at zero, and read where the other edge falls.
- Turn the inches into eighths: 2-3/8 inches is 16 eighths plus 3, so 19.
- Add the letters for the shape: A for the standard shape, B or C for candles, G for globes, BR, R or PAR for reflectors, MR for small reflectors and T for tubes.
Inches to shape number
| Across | Eighths | Codes |
|---|---|---|
| 1-1/4" | 10 | B10, CA10, T10 |
| 1-3/8" | 11 | B11, CA11 |
| 1-7/8" | 15 | A15, ST15 |
| 2" | 16 | MR16, PAR16 |
| 2-3/8" | 19 | A19, ST19 |
| 2-1/2" | 20 | PAR20, R20, BR20, ST20 |
| 2-5/8" | 21 | A21 |
| 3-1/8" | 25 | G25 |
| 3-3/4" | 30 | BR30, R30, PAR30 |
| 4-3/4" | 38 | PAR38 |
| 5" | 40 | BR40 |
The shape decides which letters go with the number.
A measurement can land between two numbers. Feit says dividing the code’s number by 8 usually gives the width, and listed widths vary a little from bulb to bulb, so confirm the code on the old bulb or its box.
What a measurement doesn’t tell you
- The length. It isn’t in the code. Makers list it separately, Westinghouse as the maximum overall length, so measure it too if the bulb sits under a shade or inside a globe.
- The base. Its code, such as E26, E12 or E17, is printed on the bulb or its box. To tell it by its look, see light bulb base types.
- Brightness and ratings. Lumens, watts, dimming and enclosed or wet ratings are on the box. For lumens, see watts to lumens.
Know the code? Read it in full.
Bulb codes