The formulas
Rectangle: length × width × average depth × 7.48 = gallons. Round or oval: length × width × average depth × 5.9. Freeform: use the average length and width with the rectangle formula, then take about 10 percent off. Average depth is the shallow end plus the deep end, divided by two. Measure in feet.
Typical sizes
A 14 × 28 foot pool with a 3.5 to 6 foot depth holds about 14,000 gallons, or around 7 loads at 2,000 gallons each. A 16 × 32 foot pool runs near 19,000 gallons. Spools and plunge pools often hold 3,000 to 6,000 gallons, which is one to three loads.
Why not just use the hose?
A garden hose puts out roughly 500 to 700 gallons an hour. A 15,000 gallon pool would take a full day or more of nonstop running. On a well, that can pull the water level down and stress the pump. On a rainwater system, it can drain your cistern.
Before your fill
Clear a path for the hose from the driveway to the pool, check with your pool builder on timing, and have your start-up chemicals ready. Order a little extra water so you aren't short of the tile line.