The short answer
To work out how much salt to add, multiply your pool gallons by the increase in ppm you need, then multiply by 0.00000834. Raising a 20,000-gallon pool from 0 to 3,200 ppm takes about 534 lb of salt, or 14 bags of 40 lb. Most chlorinators run best at about 3,200 ppm; check your cell's manual for its range.
How the calculation works
Salt concentration is measured in parts per million (ppm) by weight. One ppm in one litre of water is one milligram. So:
- Metric: kilograms of salt = litres × ppm increase ÷ 1,000,000
- US: pounds of salt = gallons × ppm increase × 0.00000834
The 0.00000834 comes from a US gallon of water weighing about 8.34 lb.
Worked example
A 20,000-gallon pool reading 2,500 ppm, with a target of 3,200 ppm:
- Increase needed: 3,200 − 2,500 = 700 ppm
- Salt: 20,000 × 700 × 0.00000834 = 117 lb
- Bags: 117 ÷ 40 = 2.9, so 3 bags of 40 lb
Salt needed to reach 3,200 ppm from fresh water
| Pool size | Pounds of salt | 40 lb bags | Kilograms |
|---|---|---|---|
| 10,000 gal (37,850 L) | 267 | 7 | 121 |
| 15,000 gal (56,780 L) | 401 | 11 | 182 |
| 20,000 gal (75,710 L) | 534 | 14 | 242 |
| 25,000 gal (94,640 L) | 668 | 17 | 303 |
| 30,000 gal (113,560 L) | 801 | 21 | 363 |
For an existing pool, every 100 ppm increase needs about 8.3 lb of salt per 10,000 gallons.
How to add salt
- Test first. Pools that have been chlorinated for a while already contain some salt.
- Turn the chlorinator off and run the pump.
- Pour the salt around the shallow end and perimeter, not into the skimmer.
- Brush to help it dissolve and stop it settling on the floor.
- Wait 24 hours, retest, then turn the cell back on.
Adding about 80% of the calculated amount first, then topping up after retesting, avoids overshooting.
Frequently asked questions
How much salt do I need for a 10,000-gallon pool?
Going from fresh water to 3,200 ppm takes about 267 lb of salt, or seven 40 lb bags. Raising an existing pool by 500 ppm takes about 42 lb.
What salt level should a saltwater pool be?
Most salt chlorine generators are designed for about 2,700 to 3,400 ppm, with 3,200 ppm a common ideal. Some cells run higher, so follow your manufacturer's range.
What kind of salt should I use?
Use pool salt or plain sodium chloride that is at least 99% pure, without anti-caking agents or iodine. Rock salt and water-softener pellets with additives can stain the pool.
What if my salt level is too high?
Salt does not evaporate or get used up quickly, so the only way to lower it is to drain some water and refill with fresh. The calculator shows what percentage to replace.
How long after adding salt can I swim?
You can usually swim once the salt has dissolved, after about an hour of circulation. Wait about 24 hours before retesting and turning the chlorinator back on.
Sources
Related calculators
About this calculator. Every formula on this page is tested against worked examples from the sources above before publishing. Results are estimates for planning; confirm with your supplier or local code.
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