You can use the fluid displacement method for irregularly shaped solid objects:
Fill the container with water and mark the water level.
Drop your object inside and mark the level again. Make sure your object isn't soluble in water.
For scaled containers, you can just subtract the original volume from the new volume. And that's it, congrats!
But if your original container doesn't have a scale:
Take out the object.
Fill your container with water to the second mark, pour that water into a graduated cylinder/another measuring vessel.
Repeat step 5 for another marked level and subtract the volumes.
Pat yourself on the back — you found the volume of an irregular-shaped object!
Read more