For most car batteries, the most common wrench size is 10mm , with 13mm and sometimes 1/2-inch also used depending on the vehicle. Battery terminal sizes vary by make and whether it’s a top-post or side-post battery, so it’s smart to have a small wrench set ready.

Quick answer

  • 10mm : very common on many modern cars, especially imports.
  • 13mm : also common, especially on some domestic and older models.
  • 5/16-inch or 8mm : can show up on some side-post or European-style setups.

Best practical move

If you’re just trying to disconnect a battery, start with a 10mm wrench or socket and keep 8mm, 13mm, and 1/2-inch nearby in case your car uses a different size.

Safety note

Use the correct size so you don’t strip the terminal nut or damage the battery clamp.

If you want, I can also help you figure out the likely wrench size for your specific car make and model.