* Ounces are a unit of weight or mass.

* Mils are a unit of length, specifically one-thousandth of an inch.

To help you further, please tell me:

* What are you trying to measure? Are you trying to find the volume of something that weighs 2.5 ounces? Or the thickness of something?

* What is the substance you are measuring? Different substances have different densities, so the volume of 2.5 ounces will vary.

Once you provide more context, I can help you make the correct conversion!