How this calculator works
Salt water pools measure dissolved salt in parts per million, which for water is the same as milligrams per litre. Raising the level by one ppm in one litre therefore needs one milligram, and in a 50,000 litre pool one ppm needs 50 grams. Multiply by the number of ppm you want to add to get the dose. Adding salt also displaces a little water, so the final reading can come out slightly different from the target; add about 80 percent of the calculated dose, circulate it fully, then measure again. Lowering salt works the other way: the only practical method is to drain part of the pool and refill with fresh water.
Formula
salt in kg = change in ppm x volume in litres / 1,000,000. For a reduction, the fraction of the pool to replace = (current - target) / current, and the water to replace = volume x that fraction.
Worked example
A 50,000 litre pool reading 800 ppm, raised to 3,000 ppm, needs 2,200 ppm of salt: 2,200 x 50,000 / 1,000,000 = 110 kg, which is six 20 kg bags. To bring a 4,000 ppm pool down to 3,000 ppm, replace 1,000 / 4,000 = 25 percent, that is 12,500 litres.
Advertisement
Frequently asked questions
What salt level should a pool be at?
Most salt chlorine generators specify a range, commonly about 2,700 to 3,400 ppm, and the exact figure is in the generator manual. Stay inside the manufacturer range rather than a general figure.
Why did my salt level overshoot?
The volume estimate is often the error. Pool volume from drawings or builder figures is more accurate than a calculation from average depth, and a little water is displaced when salt dissolves.
How long before I can swim?
Run the pump until the salt has dissolved and the reading is stable, which usually takes a day. Do not add salt while the generator is running, and never pour it into the skimmer.
Can I use water softener salt?
Pool salt is usually pure sodium chloride with no additives. Some water softener salts contain rust inhibitors or other additives, so check the label for pool suitability before using them.