Automotive bulbs are identified by trade-number sizes. The same vehicle often uses several. This chart maps common functions to the bulb sizes you are most likely to need.
| Function | Common bulb sizes |
|---|---|
| Headlight | H11, H7, H4, 9005, 9006, H13 |
| Fog light | H11, H8, H10, 9145 |
| Brake / tail | 7443, 3157, 1157 |
| Turn signal | 7440, 3157, 1156 |
| Reverse | 921, 912 |
| License plate | 168, 194 |
| Interior / dome | 194, 211-2, 578 |
Confirm before buying. Exact sizes vary by year and trim — check the manual, the existing bulb, or a fitment lookup. Many vehicles use different bulbs front and rear.
LED replacements must be the same base size and, for some functions, may need a load resistor to avoid hyper-flashing.