Hmm-mh. I started to calculate gallons, and then quit. Some of your dimensions don't work out.

For example, you've drawn the dimensions of the angled perimeter line in the lower right corner as the hypotenuse of a right triangle with a rise of 7.5' and a run of 6'. But . . . that the hypotenuse of THAT right triangle would be about 9 1/2', and not the 11' you measured. I never enjoyed it, when I had to measure a pool like that for a custom LoopLoc cover -- too many chances to screw up, and if I did, I got to eat the cover. I'd always end up measuring it 2 or 3 times, and then drawing it on CAD, to make sure the point to point diagonals I'd measured were very close to what the CAD program calculated.

However, your pool appears to be on the same order as a pool 15' x 22' with an avg. depth of 4.5'. That would be about 11,000 gallons. So, 10,000 is a reasonable guess. If you REALLY want to know, read your water meter before and after you fill it, and subtract average daily use over the fill interval. That should give you the correct value, +/- 5%.