Formula
Pounds of salt equal the gallons times the ppm you still need times 8.34, divided by 1,000,000. The 8.34 is pounds of water in a US gallon, which is how a ppm becomes a weight. Bags are that weight divided by the bag, rounded up.
pounds = gallons × (target − current) × 8.34 / 1,000,000
bags = ceil(pounds / bag weight)
If the water is already at the target, the answer is zero. A 40 lb bag in 10,000 gallons is about 480 ppm. The product label and the chlorinator manual take precedence over this arithmetic. Add salt in stages and test again. This page does not describe mixing, broadcasting, or combining salt with acid, shock, or any other chemical. Gallons come from the pool volume calculator when you do not already know them.
Worked examples
10,000 gallons, from 0 to 3,200 ppm, 40 lb bags. 10,000 × 3,200 × 8.34 / 1,000,000 = 266.88 lb. That is 7 bags to have on hand. One bag is about 480 ppm in that pool, so the seventh bag is not automatically a full pour.
The same pool, already at 3,200 ppm. Nothing to add.
10,000 gallons from 0 to 3,600 ppm. 300.24 lb at the same 8.34. A chart that says 300 lb is this formula, rounded.
Reference
What one 40 lb bag adds, using 8.34 lb per gallon.
| Gallons | ppm from one 40 lb bag |
|---|---|
| 5,000 | 959 |
| 10,000 | 480 |
| 15,000 | 320 |
| 20,000 | 240 |
Questions
Does this tell me how to add the salt?
No. It tells you the pounds. The bag label and the chlorinator manual say how the salt goes in, how long to wait, and what the cell wants. Do not mix salt with other pool chemicals.
Why 8.34 pounds per gallon?
That is the figure salt charts use for a gallon of water. Some tables use 8.345. The difference on 10,000 gallons and 3,200 ppm is about 0.16 lb. Your manual’s chart wins either way.
Should I dump every bag the page rounds up?
The bag count is what you buy so you are not short. Pour most of it, circulate, and test before the last of the bag. The page does not invent a ppm cap.
Sources
- Pounds = gallons × Δppm × 8.34 / 1,000,000. The 8.34 is the salt-chart figure for a gallon of water. Some manuals print 8.345. Use the chart in the chlorinator manual.
- No mixing procedure is given. The bag label and the chlorinator manual take precedence.