How the calculation works
Volume is surface area times average depth, converted to gallons. A rectangle uses length × width × average depth × 7.48, because a cubic foot holds 7.48 gallons. Round and oval pools use 5.9 instead of 7.48, which accounts for the missing corners. Kidney and freeform pools use 5.6 with the widest and narrowest widths averaged.
Measuring depth
Measure from the water line, not the top of the wall. If your pool has a flat bottom, enter the same number for both depths. A gradual slope from 3 to 8 feet averages to 5.5, which is what the calculator does for you.
Why it matters
Every dose of salt, chlorine, shock or acid is worked out per 10,000 gallons. Guessing the volume 20% high means overdosing by 20% every week of the season. Work it out once, write it on the pump housing, and use it in the salt, shock and chlorine calculators.