Two questions, one calculator
"Pool volume" answers how many gallons (or liters) your pool holds, from its shape and dimensions. That's the number you need before ordering chemicals, sizing a heater or pump, or filling a new pool for the first time. "Salt needed" answers a narrower, very common follow-up for saltwater pools specifically: given your gallons and a salt test reading, how many pounds to add to reach a target level. Run "Pool volume" first if you don't already know your gallons, then carry that number into "Salt needed."
Both calculations are deterministic once you have the right inputs. There's no rounding shortcut or lookup table involved, just the geometry of your pool and the definition of parts per million.
Why the oval formula is different from rectangular and round
Rectangular and round pools are unambiguous solids: a rectangular pool is a rectangular prism (length x width x depth), and a round pool is a cylinder (pi x radius squared x depth). Both formulas here use the exact conversion of 1 cubic foot to 7.480519 US gallons (1,728 in³ per ft³ divided by 231 in³ per gallon, both exact), which is why the results land a hair off the rounded "7.5" and "5.9" shortcuts printed in some pool guides.
"Oval" pools are a different case. The shape sold under that name is a stadium or racetrack outline: a rectangle with a semicircle capping each end, not a true mathematical ellipse. That means there's no clean formula to derive from scratch the way there is for a rectangle or circle. This calculator uses the multiplier Hayward publishes in its Aqua Rite salt chlorine generator manual for oval pools: length x width x average depth x 6.7. That figure is noticeably higher than a true ellipse of the same footprint would give (pi/4 x 7.480519 = 5.875), because a stadium shape holds more water than an ellipse inscribed inside the same rectangle. The two rounded ends bulge outward past where an ellipse's curve would fall.
The salt math, from the definition of ppm
A concentration in parts per million is milligrams of solute per liter of solution. That definition alone is enough to derive the dosing formula without relying on any pool-specific figure. One pound of salt is exactly 453,592.37 mg (the international avoirdupois pound), and one US gallon is exactly 3.785411784 liters. Dissolving that one pound into G gallons raises the concentration by 453,592.37 / (G x 3.785411784) ppm. Rearranged for how many pounds raise a G-gallon pool by a given ppm increase: pounds = (target ppm - current ppm) x G / 119,826.4, a unit-conversion identity, not a number specific to any brand of salt or cell.
Pool and salt-cell literature commonly rounds that 119,826 constant to "120,000" for easier mental math (a 0.15% rounding, immaterial for a chemical you're adding gradually and retesting). This calculator keeps the unrounded constant. Worked example: an 8,000-gallon pool testing at 1,000 ppm, targeting the commonly cited optimal of 3,200 ppm, needs (3,200 - 1,000) x 8,000 / 119,826.4 = 146.9 lb of salt.
Why the target range is a range, not one number
Hayward's Aqua Rite manual lists 2,700-3,400 ppm as the ideal salt range for its cells, with 3,200 ppm as the optimal target. The same manual's troubleshooting section notes the "Check Salt" warning triggers below 2,700 ppm (flashing) or 2,400 ppm (steady, cell shuts down), while a noticeably salty taste tends to show up around 3,500-4,000 ppm. Other manufacturers' cells specify similar but not identical windows, so this calculator defaults to 3,200 ppm as a commonly cited target but leaves it fully editable. Check your specific cell's manual for its exact spec before dosing.
There's no formula for lowering salt: the only way to reduce concentration is diluting with fresh water (partial drain and refill). That's also why this calculator returns zero pounds, not a negative number, when your current reading is already at or above target.