How much salt does a salt water pool need?

Most residential salt chlorinators in New Zealand run between 3,000 and 4,000 ppm. Hayward, Pentair and Jandy all nominate 3,200 ppm as the optimum. For a 46,400 litre pool starting from fresh water, reaching 3,200 ppm takes about 148 kg of salt, which is eight 20 kg bags.

Can I put too much salt in a pool?

Yes. High salinity can corrode the chlorinator cell and pool fittings, and salt cannot be filtered or chemically removed. The only way to lower it is to drain part of the pool and refill with fresh water, so it is better to add salt in stages and retest.

How long does pool salt take to dissolve?

Around 24 hours with the pump running. Brush it across the floor rather than leaving it piled up, and do not test the salinity until it has fully dissolved or the reading will be low and you will overdose.

Do I need to add salt every year?

Not usually. Salt is not used up by the chlorinator and does not evaporate with the water. You only replace what is lost through backwashing, splash-out and overflow, which is normally a bag or two a season rather than a full dose.

Pool Salt Calculator NZ

A salt chlorinator does not make chlorine out of nothing. It makes it out of salt already dissolved in the water, and it needs a specific concentration to work. Too little salt and the cell will not generate, so the pool goes green while the unit reports a fault. Too much and the cell can be damaged, and there is no way to take salt back out other than draining water and replacing it. This calculator tells you exactly how much salt to add. Enter the volume of the pool in litres, the salinity your test kit or chlorinator is currently reading, and the target salinity from your unit's manual. It returns the weight of salt to add in kilograms, how many bags that is at the bag size you buy, and how far each bag moves the reading. Because the calculation is a simple concentration, it is only as accurate as your volume figure, so measure the pool properly rather than guessing.

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Pool volume

Current salinity

ppm

Target salinity

ppm

Salt bag size

148.5 kg

of pool salt to add

Bags to buy8

Salinity rise3,200 ppm

Salt already in the pool0 kg

Each bag raises salinity by431 ppm

Check the target salinity in your chlorinator manual before dosing; units differ, though 3,200 ppm is the figure most major manufacturers nominate. Salt cannot be removed once it is in. If your current reading is already above target, the only way down is to drain some water and top up with fresh.

How it works

Salinity in parts per million is the same thing as milligrams of salt per litre of water. That single fact is the whole calculation.

The rise you need is the target reading minus the current reading. Multiply that rise by the number of litres in the pool and you have the mass of salt in milligrams. Divide by a million to get kilograms.

Bags are rounded up, because you cannot buy nine tenths of a bag. The last split shows how far a single bag moves the reading, which is the number to use when you are topping up rather than starting from scratch.

Salt does not evaporate and it is not consumed by the chlorinator, so once it is in the water it stays there. You only lose it through backwashing, splash-out and rain overflow, which is why a pool needs topping up rather than redosing each season.

Worked example

A newly filled 46,400 litre pool reading 0 ppm, with a chlorinator set to the common 3,200 ppm target, buying 20 kg bags.

The rise needed is 3,200 ppm. Multiply by 46,400 litres to get 148,480,000 mg of salt, which is 148.5 kg. At 20 kg a bag that is 7.42 bags, so you buy 8.

Each 20 kg bag lifts this pool by about 431 ppm. Add the salt with the pump running and the chlorinator switched off, brush it across the floor so it does not sit in a pile, and leave it circulating for 24 hours before you retest. Salt takes time to dissolve, and a reading taken too early will send you out for a ninth bag you do not need.

Related calculators

Pool Volume Calculator: work out the litres this calculator needs.
Pool Running Cost Calculator: the annual cost of keeping the pool going.
Dilution Ratio Calculator: concentrate to water ratios for other pool chemicals.
Cost of Owning a Pool Calculator: the full ownership picture, not just chemicals.

Data sources: Parts per million is milligrams per litre by definition. Target salinity ranges follow the operating range published by salt chlorinator manufacturers, typically 3,000 ppm to 4,000 ppm with 3,200 ppm nominated as the optimum by Hayward, Pentair and Jandy. Salt becomes noticeable to taste from about 3,500 ppm and can corrode metal fittings above 4,500 ppm. Confirm against your own unit.