Pool Salt Calculator

A pound of salt in a thousand gallons of water is 119.8 ppm, which is why every chart rounds it to 120 and why the arithmetic here is simpler than most pool chemistry. Salt dissolves, it does not react, and nothing in the pool consumes it.

So the dose is just target minus current, scaled by volume. What is more useful day to day is knowing what a single bag does, because salt is sold in 40 lb bags and pools are not sold in round numbers:

One 40 lb bag raises a 10,000 gallon pool by about 479 ppm, a 15,000 by 320, a 20,000 by 240, a 25,000 by 192 and a 30,000 by 160.

Metric is simpler still: a kilogram in 1,000 liters is 1,000 ppm. Metric pool salt comes in 20 kg and 25 kg bags, and a 20 kg bag raises 10,000 liters by 2,000 ppm, 50,000 liters by 400 and 75,000 liters by about 267.

Which means bags rarely divide evenly into what you need. That is fine, and it points one way: round down, not up. A pool 40 ppm short of target is a pool you top up next month. A pool 400 ppm over target is water you have to drain.

Worth saying plainly, because people arrive expecting salt to be a consumable like chlorine. It is not.

Salt does not evaporate and nothing uses it up. The cell splits it into chlorine, the chlorine sanitizes, and it reverts to chloride and goes round again. It is a loop, not a reaction that consumes anything.

The only way salt leaves is with water leaving: splash-out, backwashing, rain overflow, a leak, or draining. So the salt level tracks your water losses and nothing else, and a pool that holds its water needs topping up once or twice a season at most.

That has a useful diagnostic side. Salt dropping faster than splash-out explains is water going somewhere, and it is often the first sign of a leak — a reason to run the bucket test before you buy more bags.

Test before dosing, and do not dose against the control panel. The reading on a salt system is not a salt measurement.

It is inferred from conductivity. The cell measures how well the water conducts and works backward to a salt figure. That is a reasonable proxy and it drifts for two reasons.

Temperature. Conductivity rises with temperature, so the same water reads lower when it is cold. A panel showing salt down 300 ppm in April, on a pool that has not lost water, is usually reading spring temperatures rather than a real drop. Units compensate for this, imperfectly.

Cell age. Plates scale and wear, conductivity across them changes, and the calculated figure drifts with it. A cell near the end of its life commonly reads low and asks for salt the pool does not need.

Test strips or a sample at the pool store cost almost nothing and measure the water rather than the cell. Do that first, especially if the panel is asking for a second dose in a season.

There is no chemical that removes salt. Not one. The only method is replacing water, which is why overshooting is worth avoiding rather than correcting.

The math is proportional: replace a quarter of the water and you remove a quarter of the salt. Going from 4,200 ppm back to a 3,200 target means replacing about 24% of the volume — roughly 4,800 gallons on a 20,000 gallon pool.

Do it in stages, and never by emptying the pool. Lower by a third, refill, let it circulate, retest, repeat if needed. An empty shell can float out of the ground on groundwater pressure, and that is a replacement rather than a repair.

Most cells will run happily a few hundred ppm above target and simply post a fault above their ceiling. If you are only mildly over, the cheapest fix is often to leave it and let the next backwash and a winter of rain bring it down.