The volume of a standard ASME head with dish radius equal to 100% of vessel diameter and knuckle radius equal to 6% of vessel diameter is V = 0.0809 * D^3.
For a 2:1 elliptical head, the volume is V = Pi / 24 * D^3.
For a hemispherical head, the volume is V = Pi / 12 * D^3.
In all of these formulas, use consistent units. In other words, if diameter is in inches, then the volume is in inches cubed.