Gallonage help

[Gnarbrah]

Hi all - just had my start up today and I asked the tech what he thought my volume was. He guessed 10-12k.
My paperwork estimates 12,800.
Now I have no clue by looking (kidney shaped) but I found a couple online calculators and they came back in the 20k + range. That seems like a pretty wide swing!
I attached the drawing showing the depths and dimensions. Any help is appreciated! I’m guessing this comes in handy when adding chemicals!

 

[Toxophilite]

Do you remember how many hours it took to fill? If so, check the gpm of your fill hose and calculate off that.

 

[pjt]

If your water utility installed a smart water meter (connected to the internet) at your house, you can login and check water usage to get an accurate volume.