I have never really known our pool size in gallons - the pool was there when we bought the house 14 years ago.
It is an 30 x 15 ft kidney shape - with round 8 ft deep bottom - small shallow end which quickly gets deep.
Recently I have been guessing it is about 15000 gallons using various pool size calculators.
Anyway I thought I might try using the 'Pool Calculator' to check if my guess was close. So tonight the FC was 2.5 - I added 60 oz of 12% chlorine and after mixing/circulating the water for an hour the FC was 6. Using the Pool Calculator this gives a pool size of 16500 gallons. Understanding there is some +/- 0.5 error in my FC testing (TF100) - this still gives me confidence that I am in the 15000 - 17500 gallon range.
Edit - it is kidney shaped not oval....