How many gallons is my pond?
The quick formula is length times width times average depth, times 7.48 gallons per cubic foot. For a rectangle with straight walls, it works.
Where it goes wrong
Real ponds have plant shelves, sloped sides and curved edges, and the average depth is where most estimates slip. A pond 24 inches deep at the bottom can average well under that once the shelves are counted. Overestimate the volume and you overdose treatments and buy a bigger pump than you need. Underestimate it and the filter cannot keep up.
How PondTools does it
PondTools calculates the volume from the outline and the depth you set for each shelf. A pond 22 feet 6 inches by 20 feet 6 inches, 24 inches at its deepest, comes out at about 1,940 gallons. Every calculator on the site, from salt to water changes, starts from that number.