How the calculation works
Salt is measured in parts per million, and 1 ppm is 1 milligram per litre. Ten thousand US gallons is 37,854 litres, so raising it by 100 ppm takes 3,785 grams — 8.35 lbs per 100 ppm per 10,000 gallons. Scale that to your pool and the gap you need to close.
Worked example
A 20,000 gallon pool reading 2,600 ppm, aiming for 3,200 ppm. The gap is 600 ppm, so the dose is 8.35 × 6 × 2 = about 100 lbs — two and a half 40 lb bags.
Assumptions
- Pool salt is close to pure sodium chloride; the calculation assumes no significant filler.
- The reading came from a test kit or a strip, not from the chlorinator display — panel readings drift as the cell ages.
Limitations
- Salt is never consumed. It only leaves through splash-out, backwashing, leaks and rain overflow — so if the level keeps dropping, something is losing water.
- Nothing removes excess salt but fresh water. Overshooting means draining and refilling, so add in stages and re-test.
Questions people ask
How much salt does a 20,000 gallon pool need?
Starting from zero, roughly 535 lbs to reach 3,200 ppm — about thirteen 40 lb bags. Topping up an existing pool takes far less; enter both readings above.
What kind of salt should I use?
Pool salt, or food-grade salt at 99.8% purity or better. Avoid rock salt and water-softener pellets — the iron and other minerals in them stain plaster and shorten cell life.
Why does my chlorinator say “low salt” after I added salt?
Usually it has not dissolved yet — give it 24 hours with the pump running. If the reading still disagrees with your test kit, the cell may be scaled or near the end of its life; both make the panel under-read.
Can I swim after adding salt?
Once it has fully dissolved, yes. Undissolved salt sitting on the floor is abrasive, so brush it out first.