20 ft wide X 40 ft long at 5 ft deep is 30,000 gallons but like stated before thats alot bigger then most pools good luck with that by hand hope you have 5 years to build it. lol
Do it I can tell u how to keep it to a 5k budget. Just start digging and ill help u threw the rest of the process depending on ground conditions u might get it done in a summer.