JD,
I know you got a tape measure so...
Measure the three dimensions (L x W X H).
If you measurement is in inches then divide that total 1728 (inches per cubic foot)
Then multiply by 7.48 or.....
Leave it in inches and divide it by 231.
So there is 7.48 gallons in a cubic foot or
1 gallon in 231 cubic inches.
Same cat, different skinner.
Say your well is 24" long by 18" wide by 12" deep.
24" x 18" x 12" = 5184 cubic inches
divided by 231 = 22.44 gals. or....
5184 cubic inches divided by 1728 (" per cubic ') = 2.98 cubic feet
Multiplied by 7.48 gal per cubic foot = 22.44
Meow.