To be absolutely certain:
* Check your owner's manual: This is the most reliable source.
* Look at the drain plug itself: If you can see it, the size might be stamped on the plug head.
* Consult a repair manual specific to your engine: This will give you precise details for your vehicle's model and engine code (e.g., 4.7L, 5.7L Hemi). Knowing your engine's specific displacement will help greatly.
Using the wrong size wrench could strip the drain plug, causing a significant repair problem. Always double-check before attempting this task.