How the pool salt calculator works
Salt levels are measured in parts per million by weight. One US gallon of water weighs 8.34 lb, so the pounds of salt to add are the pool volume in gallons, multiplied by the gap between the target and current level in ppm, multiplied by 8.34, then divided by 1,000,000. If the current level is already at or above the target, the gap is negative and the result is held at zero, because this calculator cannot take salt out. The number of bags is the pounds divided by the bag size and rounded up, since part bags are not sold.
Example: a 20,000 gallon pool at 0 ppm with a target of 3,200 ppm. The gap is 3,200 − 0 = 3,200. Then 20,000 × 3,200 = 64,000,000, and 64,000,000 × 8.34 = 533,760,000. Dividing by 1,000,000 gives 533.76 lb. At 40 lb per bag, 533.76 ÷ 40 = 13.344, which rounds up to 14 bags.
Choosing a target and testing the water
The right target is the one in the salt chlorine generator manual, not a fixed number. The manual also states the acceptable range. Test the water with a salt test strip or kit before adding anything. Salt only goes up when salt is added; the practical way to lower it is to drain some water and refill, so overshooting means extra work and extra water. Add the calculated amount in stages, brush it until dissolved, let the pump circulate it through the pool and retest before adding the rest.
What this calculator does not do
This page covers salt only. It does not dose chlorine, pH adjusters, total alkalinity, calcium hardness or stabiliser, and it is not a guide to shocking a pool. It also assumes the volume entered is the water volume, not the pool shell size, and that the salt is plain pool salt rather than a blend. Bag weights differ between products, so enter the weight printed on the bag. To work out the gallons first, use the pool volume calculator.
Frequently asked questions
How much salt raises 20,000 gallons by 3,200 ppm?
The gap is 3,200 ppm. Then 20,000 × 3,200 = 64,000,000 and × 8.34 = 533,760,000. Dividing by 1,000,000 gives 533.76 lb, and 533.76 ÷ 40 = 13.344, so 14 bags of 40 lb.
How much does one 40 lb bag raise salt in 10,000 gallons?
10,000 gallons weighs 10,000 × 8.34 = 83,400 lb. One bag is 40 lb, and 40 ÷ 83,400 = 0.0004796. Multiplying by 1,000,000 gives about 480 ppm per bag, so check the level after each bag rather than adding the whole dose at once.
Why does the calculator show no salt needed?
When the current level is 3,500 ppm and the target is 3,200 ppm, the gap is 3,200 − 3,500 = −300. Multiplying by the volume and 8.34 gives a negative number, which is held at 0 lb and 0 bags. Salt cannot be removed by adding another chemical; draining and refilling lowers it.