What this calculator does
Salt chlorinators need a salt concentration in the water to work, typically around 3,000 to 3,500 ppm. Getting a 20,000 gallon pool from zero to 3,200 ppm takes about 534 lb of salt, which is a substantial quantity.
Topping up is a much smaller job than starting from scratch, and the difference is worth appreciating before you buy. That same pool going from 3,000 to 3,200 ppm needs only 33 lb. Salt does not evaporate; it only leaves through splash-out, backwashing and rain overflow, so an established pool loses very little.
The formula
One part per million is one milligram of salt per litre of water, so the salt required is the pool volume multiplied by the shortfall in ppm. The pool volume in US gallons is converted to litres at 3.785 litres per gallon. The result is the mass of salt to add, shown in both pounds and kilograms.
| Term | Meaning |
|---|---|
| ppm | Parts per million, equal to one milligram per litre. |
| Salt chlorinator | A cell that converts dissolved salt into chlorine by electrolysis. The salt is not consumed, only cycled. |
| Target range | Usually 2,700 to 3,400 ppm, though the correct figure is whatever your chlorinator specifies. |
| Pool salt | Sodium chloride, typically 99.8% pure, without the anti-caking agents found in table salt. |
The inputs explained
| Field | What to enter |
|---|---|
| Pool volume (US gal) | Pool volume in US gallons. An error here translates directly into an error in the salt, so it is worth calculating rather than estimating. |
| Current salt level (ppm) | Current salt level from a test, in ppm. Enter zero for a new fill. |
| Target salt level (ppm) | Target concentration. Use the figure your chlorinator manufacturer specifies rather than a general number. |
When to use it
Starting a new saltwater pool
A new fill needs the full quantity at once, which for a typical pool is several hundred pounds.
Topping up after rain or backwashing
Water lost and replaced dilutes the salt, and the top-up is far smaller than the initial dose.
Correcting a low chlorinator reading
A chlorinator warning of low salt usually means a modest top-up, and calculating it avoids the common mistake of adding far too much.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much salt does a top-up need?
The same pool at a range of current salt levels.
| Current salt level | Salt to add | In kilograms | Increase needed |
|---|---|---|---|
| 0 ppm | 534.1 lb | 242.27 kg | 3,200 ppm |
| 2,400 ppm | 133.5 lb | 60.57 kg | 800 ppm |
| 2,800 ppm | 66.8 lb | 30.28 kg | 400 ppm |
| 3,000 ppm | 33.4 lb | 15.14 kg | 200 ppm |
Questions
How much salt does a saltwater pool need?
Enough to reach the concentration your chlorinator specifies, usually around 3,000 to 3,500 ppm. From a fresh fill, a 20,000 gallon pool needs roughly 534 lb to reach 3,200 ppm. Check the manual rather than using a general figure, since cells differ.
Can I add too much salt?
Yes, and it is difficult to undo. The only way to lower salt is to drain part of the pool and refill with fresh water. High salt can damage some chlorinator cells and makes the water taste noticeably salty. Add in stages and re-test rather than dosing the full amount at once.
Why does my salt level drop?
Splash-out, backwashing, filter cleaning and rain overflow, all of which remove water. Salt itself is not consumed by the chlorinator, which cycles it between chloride and chlorine indefinitely. Evaporation leaves the salt behind and actually raises the concentration.
What kind of salt should I use?
Pool grade sodium chloride, typically 99.8% pure and free of anti-caking agents, iodine and other additives. Table salt and water softener salt often contain additives that can stain the pool or foul the cell.
For comparing household water use, see the bath vs shower calculator. For the cost of a shower, see the shower cost calculator.