How to measure a light bulb base
Measure the widest diameter of the metal base in millimeters. For pin bases, measure center-to-center pin spacing. Match to the table below.
For screw bases (E-type)
- Take a ruler or calipers and measure across the widest point of the threaded metal section.
- Round to the nearest whole number — this is the E-code diameter in millimeters.
| Diameter measured | Base code | Common name |
|---|---|---|
| ~12 mm | E12 | Candelabra (US) |
| ~14 mm | E14 | Small Edison screw (EU) |
| ~26 mm | E26 | Medium / Standard (US) |
| ~27 mm | E27 | Medium / Standard (EU) |
For pin bases (G-type)
- Measure the distance between the center of the two pins.
- Look at pin shape — are they straight wires, loop/folded wires, or mushroom-capped?
| Pin spacing | Pin shape | Voltage | Base code |
|---|---|---|---|
| 4 mm | Straight wire | 12V | G4 |
| 5.3 mm | Straight thick | 12V | GU5.3 (MR16) |
| 9 mm | Looped wire | Mains | G9 |
| 10 mm | Mushroom-tip, twist-lock | Mains | GU10 |
For bayonet bases (B-type)
Bayonet caps have no thread — they push in and turn. Measure the metal cap diameter: approximately 22mm = B22. The two side pins visible on the base also confirm it is bayonet.
If the label is on the fixture
Most light fixture sockets have the base type and maximum wattage stamped on a label inside the socket housing. Look for it before measuring.