Pool salt is a simple mass balance: to raise the whole pool from its current salinity to the target, you add a mass of salt proportional to the gallons and the ppm gap. One US gallon of water weighs about 8.34 lb, so the pounds of salt to add are:

delta    = target ppm − current ppm
saltLb   = volumeGal × 8.34 × delta / 1,000,000
bags     = ceil(saltLb / bag size)        (rounded UP)
leftover = bags × bag size − saltLb
cost     = bags × price per bag           (only when a price is entered)

over-salted (current > target):
fraction = (current − target) / current
gallons  = volumeGal × fraction           (drain & replace this much)

dimensions mode: volumeGal = pool-volume engine (shape × avg depth × 7.48 gal/ft³)

We ship 8.34 lb/gal (the mass of one US gallon of water); some calculators round to 8.35, a ~0.1% difference well below display precision. We round the bag count up on purpose — coming up short leaves the salt generator unable to make enough chlorine — and show the exact pounds plus the leftover so you can split a partial bag.

Salt does not evaporate; it only leaves through splash-out, backwash, and leaks. So when your reading is above target there is no chemical to remove it — the only lever is to drain and replace part of the water. We compute that fraction directly. In From dimensions mode the gallons come from the same engine the Pool Volume Calculator uses (7.48 gal/ft³, primary-source-validated shape constants), not a rule-of-thumb multiplier. This is an advisory planning figure, not a compliance record — your SWG cell's manual is the authority on the target salinity range.

Sources: Swim University — Pool Salt Calculator; Omni Calculator — Pool Salt Calculator; PoolChemicalCalculator — Pool Salt PPM Calculator.