How much salt does your pool need?

How much salt do I add to my pool?

Pounds of salt = gallons × 8.33 × (target ppm − current ppm) ÷ 1,000,000, because a gallon of water weighs about 8.33 pounds (USGS). Example: a 20,000-gallon pool at 2,700 ppm raised to 3,200 ppm needs 20,000 × 8.33 × 500 ÷ 1,000,000 = 83.3 pounds — 2.1 bags of 40 lb, so buy 3. In metric it is simpler: liters × ppm difference ÷ 1,000,000 = kilograms.

What salt level should my pool be?

The level in your salt chlorinator's manual — it differs by brand and model. Hayward's AquaRite manual gives 2,700–3,400 ppm with 3,200 ppm optimal; Pentair's IntelliChlor IC15–IC60 guide recommends 3,600 ppm. Test the water first: Pentair notes most pools already contain some salt from the water source and chemicals.

How do I lower the salt level?

Hayward's manual says the only way is to partially drain the pool and refill it with fresh water. The share to drain is (current − target) ÷ (current − fill water ppm). A 20,000-gallon pool at 4,000 ppm brought to 3,200 ppm with salt-free water needs 20% replaced — 4,000 gallons. Do it in steps and test after the water mixes.

What kind of salt should I use?

Hayward calls for sodium chloride that is more than 99% pure — food-quality or water-softener salt, often sold in 40–80 lb bags as coarse solar salt — and warns against rock salt, iodized salt and salt with anti-caking additives or yellow prussiate of soda. Follow your own chlorinator's manual.

How many gallons is my pool?

Length × width × average depth × 7.48 for a rectangular pool, in feet. The pool volume calculator does it for rectangular, round and oval pools.

How long should I wait before testing the salt level?

Run the pump so the salt dissolves and circulates before you test, and then add any further salt in small steps rather than all at once. Adding too much is easier than fixing it, since the only way to lower salt is to replace water.

Why the formula works

Salt level is measured in parts per million (ppm) by weight, so 3,200 ppm means 3,200 pounds of salt in every million pounds of water. A gallon of water weighs about 8.33 pounds, so a pool of V gallons weighs V × 8.33 pounds, and raising the level by Δ ppm takes V × 8.33 × Δ ÷ 1,000,000 pounds of salt. In liters, one liter of water weighs a kilogram, so the salt in kilograms is liters × Δ ÷ 1,000,000. The formula ignores the small increase in volume from the dissolved salt.

Two worked examples

Adding salt: a 15,000-gallon pool reading 1,200 ppm that should reach 3,200 ppm needs 15,000 × 8.33 × 2,000 ÷ 1,000,000 = 249.9 pounds, which is 6.2 bags of 40 lb, so buy 7. In metric, 60,000 liters from 1,000 to 3,200 ppm takes 132.0 kg. Lowering salt: the same 15,000-gallon pool at 4,200 ppm with a target of 3,200 ppm and fresh water that contains no salt needs 23.8% of the water replaced, or 3,571 gallons drained and refilled. If the fill water already holds salt, the share to replace is larger, which is why you can enter the fill-water ppm.

Common mistakes

Using the wrong volume is the biggest source of error, since the salt is proportional to the gallons; measure the pool, or estimate it with the pool volume calculator, rather than relying on a catalog number. Another is aiming at a generic level instead of the one in your chlorinator manual. Third, adding all the salt in one pass and testing at once: undissolved salt on the floor gives a low reading. Finally, remember that evaporation removes water but not salt, so topping up with fresh water dilutes the pool only as you lose water through splashing, backwashing or draining.

Sources: USGS Water Science School, “Facts About Water” (weight of water); Hayward Aqua Rite Operation and Installation Manual; Pentair IntelliChlor IC15/IC20/IC40/IC60 Installation and User’s Guide. The ppm arithmetic follows from the definition of parts per million by mass. Need the gallons? Use the pool volume calculator; see also the pool chlorine calculator, ppm calculator and brine calculator. How we check our calculators.