Calculating Internal Volume
There are several reasons why you may need to find the internal volume of your design: the amount of oil needed to fill a gear box, the volume of a tank with a pump, heater, or other items inserted, or the fluid volume of a bottle or jug.
The process is easy and quick.
Preparation
Part (Can be a multibody part): No preparation necessary.
Assembly: Save the assembly as a part including all components.
Create a box around your model
Start a sketch that completely encompasses your model on the left, bottom, and right side. The top will be the “fluid” level. In the case of the gear box, I want to find the entire interior volume so my sketch completely encloses my model. And since I know one of your standard planes goes through the middle of the part (remember to try and be symmetrical to the planes) this sketch will be on that plane.
Now do a midplane extrude to completely enclose your model. Make sure to uncheck Merge result.
Combine
I now have 6 bodies in this file: 5 from the original assembly and the one that I called oil, which will eventually represent the interior volume.
Select the body that is the “fluid”, then hold “Ctrl” and select the other body (bodies). Right click and select “Combine.” You want the Main Body to be the “fluid” and everything else to be the Bodies to Combine.
We are going to Subtract the “fluid” (the rectangular block) from the gear box.
Click the green check mark. A dialog box will appear. To select the Bodies to Keep – click on Selected bodies, then check Body 2. We want to keep just the interior fluid.
What remains is an exact shape of the interior.
Check the mass properties to get the volume which can then be converted to whatever units you need. In this example, the volume is 72.46 in3 which converts to 0.31 gallons. (multiply by .00433)