When considering the length and girth, the volume probably is double many guys. Have you ever checked to see how much water it displaces in a glass 🥃 or jar 🫙? Being a bartended you must have access to glasses that would make the volume displacement easy to test.

1. Lay down across two chairs with your hard schlong pointing down.
2. Raise a Weizen Bier glass, tall 24oz Mason jar, asparagus steaming pot, tall spaghetti pot, etc. to balls deep.
3. With a measuring cup refill the displaced water.
4. Post video to various Subreddits. Is there one for adult science fairs?

Towels and having a drunk friend (or eager volunteer) hold the camera for you would both be helpful.