Pool Salt Calculator
How much pool salt to add in kilograms or pounds, from litres or gallons: 25 kg Australian bags or 40 lb US bags, with the math shown.
How this is calculated
Pool salt is a simple mass balance: to raise the whole pool from its current salinity to the target, you add a mass of salt proportional to the gallons and the ppm gap. One US gallon of water weighs about 8.34 lb, so the pounds of salt to add are:
delta = target ppm − current ppm
saltLb = volumeGal × 8.34 × delta / 1,000,000
bags = ceil(saltLb / bag size) (rounded UP)
leftover = bags × bag size − saltLb
cost = bags × price per bag (only when a price is entered)
over-salted (current > target):
fraction = (current − target) / current
gallons = volumeGal × fraction (drain & replace this much)
dimensions mode: volumeGal = pool-volume engine (shape × avg depth × 7.48 gal/ft³)We ship 8.34 lb/gal (the mass of one US gallon of water); some calculators round to 8.35, a ~0.1% difference well below display precision. We round the bag count up on purpose — coming up short leaves the salt generator unable to make enough chlorine — and show the exact pounds plus the leftover so you can split a partial bag.
Salt does not evaporate; it only leaves through splash-out, backwash, and leaks. So when your reading is above target there is no chemical to remove it — the only lever is to drain and replace part of the water. We compute that fraction directly. In From dimensions mode the gallons come from the same engine the Pool Volume Calculator uses (7.48 gal/ft³, primary-source-validated shape constants), not a rule-of-thumb multiplier. This is an advisory planning figure, not a compliance record — your SWG cell's manual is the authority on the target salinity range.
Sources: Swim University — Pool Salt Calculator; Omni Calculator — Pool Salt Calculator; PoolChemicalCalculator — Pool Salt PPM Calculator.
How much salt does a saltwater pool need?
A pool starting from zero needs about 3.2 kg of salt per 1,000 litres to reach the 3,200 ppm most chlorine generators want, so a 50,000-litre pool takes roughly 160 kg, or seven 25 kg bags. In US units that is about 267 lb per 10,000 gallons, which Swim University gives as six to seven 40 lb bags. If your tester already reads some salt, only the gap counts, and that is what the calculator above computes: exact kilograms or pounds, the bag count, and the leftover.
Salt by pool size: litres, kilograms and 25 kg bags
Every row below is the full initial charge (a new fill, or a pool opened with no salt in it) taken to a 3,200 ppm target, computed with the same engine the calculator uses.
| Pool volume | US gallons | Salt to reach 3,200 ppm | 25 kg bags |
|---|---|---|---|
| 15,000 L | 3,963 gal | 48.0 kg | 2 |
| 20,000 L | 5,283 gal | 64.0 kg | 3 |
| 25,000 L | 6,604 gal | 79.9 kg | 4 |
| 30,000 L | 7,925 gal | 95.9 kg | 4 |
| 40,000 L | 10,567 gal | 127.9 kg | 6 |
| 50,000 L | 13,209 gal | 159.9 kg | 7 |
| 60,000 L | 15,850 gal | 191.9 kg | 8 |
| 75,000 L | 19,813 gal | 239.8 kg | 10 |
| 100,000 L | 26,417 gal | 319.8 kg | 13 |
Openings run September–October in Australia and New Zealand and April–May in the northern hemisphere, so “season open” means the same full charge in both, six months apart. See salt quantities for a pool measured in litres, southern-hemisphere season for the rest of that opening-season checklist.
Salt by pool size: gallons, pounds and 40 lb bags
| Pool volume | Litres | Salt to reach 3,200 ppm | 40 lb bags |
|---|---|---|---|
| 4,500 gal | 17,034 L | 120.1 lb | 4 |
| 8,000 gal | 30,283 L | 213.5 lb | 6 |
| 10,000 gal | 37,854 L | 266.9 lb | 7 |
| 15,000 gal | 56,781 L | 400.3 lb | 11 |
| 20,000 gal | 75,708 L | 533.8 lb | 14 |
| 25,000 gal | 94,635 L | 667.2 lb | 17 |
| 30,000 gal | 113,562 L | 800.6 lb | 21 |
| 40,000 gal | 151,416 L | 1,067.5 lb | 27 |
Three of those bag counts look like they round the wrong way. A 4,500-gallon pool needs 120.1 lb, a tenth of a pound past three bags; a 15,000-gallon pool needs 400.3 lb, a third of a pound past ten. The count still goes up, because under-salting is the failure mode that leaves the cell unable to make chlorine. In practice you buy the extra bag, it stays nearly full, and the calculator prints the leftover pounds so you know that before you load the truck.
If the pool already has salt in it
Salt is a straight mass balance, so only the gap between your reading and your target is chargeable. Halve the gap and you halve the bags. A 50,000-litre pool sitting at 1,500 ppm needs 85 kg to reach 3,200: four bags rather than seven. That is also why a mid-season top-up after heavy rain or a partial drain is usually one or two bags, not a full charge.
Don’t know the volume? Measure it once with the pool volume calculator. Its shape-by-shape gallonage is the same engine this tool’s From dimensions mode calls, so the two agree to the litre. The target itself comes from the generator: sizing that cell to the pool is a separate job, handled by the salt cell sizing calculator. If the pool is not on salt yet and you are quoting the changeover rather than servicing it, the salt system conversion pricing calculator prices the cell and board, the install hours, the start-up visit and the initial salt charge off this same volume as one job.
The math we run
gallons × 8.34 × (target − current) ÷ 1,000,000 = pounds, then ÷ bag size,
rounded up. One US gallon of water weighs about 8.34 lb; some calculators
round to 8.35, a difference well under a tenth of a percent. In metric the same
balance collapses to litres × ppm ÷ 1,000,000 = kilograms, because a litre of
water weighs essentially a kilogram — the two routes land within 0.1% of each
other (159.9 kg against 160.0 kg on a 50,000-litre pool).
Cross-checking against published worked examples is the honest test of a formula. Omni Calculator’s metric example puts a 40,000-litre pool at about 130 kg (290 lb) of salt to reach the ideal level; the mass balance above gives 127.9 kg for the same pool at 3,200 ppm — the same answer within about 2%, with the difference coming from where each tool rounds. Both agree on the band: 2,700–3,400 ppm with 3,200 as the ideal.
When the reading is above target
Most calculators skip the over-salted case. Salt doesn’t evaporate; it only leaves through splash-out, backwash, and leaks. So when your reading is above target there is no chemical to remove it. The only fix is to drain and replace part of the water, and this tool computes exactly how many gallons or litres. A small overage inside the 2,700–3,400 band usually corrects itself through normal splash-out. If your salt keeps dropping fast instead, that points to a leak rather than normal loss — rule it out with the bucket test before you keep adding bags.
What salt doesn’t do
A salt cell generates chlorine day to day; it does not replace a shock. Sizing that separately is the pool shock calculator, and where salt sits against liquid chlorine or tablets on cost is a different question again: the chlorine type cost comparator runs that comparison. If you’re not sure which chemistry tool a job needs, start from the pool water chemistry hub.
Use pool-grade salt only — never table, rock, or iodized. Switch units freely (gallons or litres, lb or 25 kg bags), add a price per bag for a quick cost line, and print or copy the bag count straight into a quote. There’s no account and no ad-wall. This is an honest planning figure, not a compliance record — your salt cell’s manual is the final word on the target.
Frequently asked questions
How many bags of salt for a 50,000 litre pool?
What salt level should a saltwater pool sit at?
How much salt for a 15,000 gallon pool?
How do I do the salt math in litres and kilograms?
Can I lower pool salt that is too high?
Sources
- Pool Salt Calculator. Swim University. Retrieved .
- Pool Salt Calculator. Omni Calculator. Retrieved .
- Salt Pool Calculator — PPM and salt dose for an SWG. PoolChemicalCalculator. Retrieved .