Maybe a rather unscientific way , but when I finish my tanks I emerge them in a bucket of water and measure the water displacement...... This way you also (dis)counting the radius and other strange shapes...
You can also fill them completely and then empty them in a kitchen measuring bucket......
([8D].....make sure you clean them properly beforev returning to you wife....)
The math is a nice thing but when difficult when having strange conformal cells

Regards,
Gerald