Pool salt calculator: how much salt to add?
A salt calculator that starts from your manual's target. Pounds and bags, free, no sign-up, nothing leaves your browser.
Your dose shows here
Type your gallons, your salt reading and the target from your manual.
pounds = gallons x ppm to add x 0.0000083454 / (purity percent / 100)
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This is arithmetic on the numbers you typed. Retest the way your salt system's manual says before adding more.
How the pool salt calculator works
You bring four things you already have: the pool's gallons, today's salt reading, the target from your salt system's manual, and whatever the bag says. The rest is one line of arithmetic, and all of it stays on screen.
Start with the gap: your target minus your reading, in ppm. Multiply it by your gallons and by 0.0000083454, the pounds of salt that one ppm takes in one US gallon. That number isn't a rule of thumb.
It comes straight from two definitions NIST calls exact: a US gallon is 3.785411784 liters, and a pound is 453,592.37 milligrams. We count 1 ppm as 1 milligram of salt per liter of water. Strictly, USGS counts a ppm by weight, so going by the liter is our shortcut. Fresh water weighs 995 to 1,000 grams a liter between 32 and 90 degrees Fahrenheit, so the temperature effect for fresh water is less than half a percent.
Say your pool holds 18,000 gallons and needs 600 ppm. That's 90.13 lb of pure salt. From a bag that's 99.8% salt you'd pour 90.31 lb to get it, which the dose shows as 90 lb: whole pounds from 10 lb up, tenths below that.
Bags come after the dose. With 40 lb bags you buy three, pour two and 10 lb of the third, and keep 30 lb for later. Pour all three because the bag was open anyway, and the reading climbs about 797 ppm instead of 600. The result lists what each whole bag does to the reading, so you see that before you open the third one.
No gallon figure? We work one out from the surface shape, times the average water depth, times 7.4805 gallons per cubic foot, rounded to whole gallons. Ovals come in two shapes, and the calculator asks which one you have. Even a careful measurement is an estimate, so the result says what a 10% miss would cost in pounds.
Who this is for
You own a saltwater pool, the salt reading came back low, and the manual says where it should be. Maybe the generator flagged low salt. Maybe it rained all week, or it's opening day. Above-ground pools work the same way. Just measure the water, not the wall. Pool techs can use it too. It's the same arithmetic on the same four numbers. If you're still choosing between salt and chlorine, come back once the salt system is in.
Pool owner questions about pool salt
How much salt do I need for my pool?
Gallons times the ppm you need to add, times 0.0000083454. An 18,000-gallon pool that needs 600 ppm takes 90.13 lb of pure salt. Your bag's purity nudges it up a little, and the bag size turns it into a shopping list. Before you start, have three numbers ready: the gallons, a fresh test, and the target in your manual.
What should my salt level be?
Whatever your salt system's manual says. We don't print a range, because the one that counts is the one the maker wrote for your model. If the paper manual is gone, search your system's model name with the word "manual". If the manual gives a low and a high limit, type them in too: they change the warnings, never the dose.
How much salt raises a pool 1,000 ppm?
83.45 lb of pure salt for every 10,000 gallons. Per gallon that's 0.0000083454 lb for each ppm, or 8.35 lb per 1,000 gallons per 1,000 ppm. A bag that's less than pure needs a little more, in proportion: divide by the purity as a decimal (0.998 for 99.8%).
How much does one bag of salt raise the ppm?
It depends on the pool. A 40 lb bag of pure salt raises 10,000 gallons by about 479 ppm and 20,000 gallons by about 240. Multiply the bag's weight by its purity percent divided by 100, then divide by your gallons times 0.0000083454. Type a bag size and we show what each whole bag does to your reading.
How many bags of salt do I need?
Round the dose first, then count bags from it. A 20,000-gallon pool that needs 500 ppm takes 83 lb. With 40 lb bags that's three bags: two full and 3 lb of the third. Pour all three and you add about 719 ppm, not 500. Salt doesn't come back out with another dose, so we'd keep the 37 lb for next time.
How many gallons is my pool?
Surface area times average water depth times 7.4805. Measure the water, not the wall. An 18 ft round above-ground pool with 44 inches of water holds about 6,980 gallons. Work it from a 48-inch wall and you get 7,614, about 9% too many, and that error follows straight into the salt.
How many gallons is an oval pool?
First decide which oval you have. A 16 x 32 ft pool with straight sides and 4 ft of water holds about 13,676 gallons. One that curves all the way round, with the same length and width, holds about 12,032. That's 13.7% apart, and 7 lb of salt apart on a 500 ppm top-up.
How do I lower the salt level in my pool?
Adding salt only ever raises it, so the figure we can work out is dilution: some water out, the same amount of fill water in. If your reading is 4,000 and that's 500 over your target, with fill water at 0 you'd replace 12.5% of the water. If your fill water tests at 250 ppm, it's 13.3%. Whether and how to drain is your manual's call, or a pool pro's.
How much salt do I add when I open the pool or fill it for the first time?
The same sum, starting from a real reading. Test the filled pool and type what the test says. Type 0 only if it reads 0. Then retest the way your manual says before you add more. If the gallons were a guess, the retest can show when that guess needs another look.
How much salt to add to a pool
The chart on a salt bag does the same sum, except it rounds your pool to the nearest row and your reading to the nearest column. That's fine until the rounding lands on the far side of a whole bag.
Test the water and write the number down. Find the target in the manual, not on a forum. Get the gallons from the builder's paperwork, or measure the surface and the water depth. Subtract the reading from the target, multiply by the gallons and by 0.0000083454, and divide by the bag's purity as a decimal (0.998 for 99.8%). Round it, then work out how much of the last bag you actually pour.
Then put the rest of the bag away and test again the way your manual says. A mismatch is a reason to check the gallons and the test, rather than trust the chart.
Sources and last checked
- Unit definitions: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement. 1 US gallon = 231 cubic inches exactly = 3.785411784 liters; 1 pound = 453.59237 grams exactly; 1 cubic foot = 1,728 cubic inches.
- Parts per million by weight, and the density of water by temperature: USGS Water Science School, "Saline Water and Salinity" and "Water Density".
- Pool salt sold in 40 lb bags: Morton Salt, Professional's Choice Pool Salt product page.
Last checked: October 8, 2026 (unit constants version 2026-10-07).
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