The formula
One pound of salt in 1,000 gallons raises salinity by about 120 ppm. So pounds needed = (target − current) × gallons ÷ 120,000. Salt does not get used up by the chlorinator, it only leaves through splash-out, backwashing and dilution from rain, so you usually top up once or twice a season.
Which salt to buy
Use pool-grade or food-grade sodium chloride at 99% purity or better. Water softener salt with rust inhibitors or iodized salt will stain and foul the cell. Bags are almost always 40 lb, which is why the result is rounded up to whole bags.
Adding it
Turn the salt cell off, pour bags into the shallow end with the pump on, and brush any piles so they dissolve rather than sit on the plaster. Wait 24 hours before retesting and switching the cell back on. Don't know your volume? Use the pool volume calculator first.
Dose tables by pool size
Written-out amounts for every product: 5,000 gallons, 10,000 gallons, 12,000 gallons, 15,000 gallons, 18,000 gallons, 20,000 gallons, 25,000 gallons, 30,000 gallons.