Volume is length × width × average depth. A rectangle holds 7.48 gallons per cubic foot; a round or oval pool is π/4 of the rectangle around it.
For a pool with a shallow and a deep end, the average of the two depths is close enough for dosing.
The product label always wins — if it says something different, follow the label.
How do I measure my pool?
Measure the inside of the water, not the deck. For depth, measure from the water line to the floor at the shallow and the deep end.