Pool LSI Calculator
Check whether water is eating plaster or scaling the heater. CYA corrected, temperature aware, built for the phone in your pocket.
Free. No signup. Works on your phone at the pool.
Used to correct alkalinity
1000 if unknown, ~4000 for salt
Saturation index
-0.15
Slightly corrosive. Fine for a week, but trend it up with pH or calcium before it etches.
Log this reading to the customer's record
Pool Service Log calculates LSI on every stop, saves each reading and dose with a GPS timestamp and photos, and texts the homeowner a service report. When a plaster warranty claim shows up next spring, you have the history.
No credit card. Keep your current billing software.
How the LSI is calculated
The Langelier Saturation Index compares the water's actual pH to the pH at which it would be exactly saturated with calcium carbonate (pHs). LSI = pH minus pHs. Positive means the water wants to deposit calcium. Negative means it wants to dissolve it, and your plaster is the nearest source.
pHs depends on four things: temperature, calcium hardness, carbonate alkalinity, and total dissolved solids. This calculator uses the standard Carrier formulation with carbonate alkalinity corrected for cyanuric acid.
Quick targets for plaster pools
- pH 7.4 to 7.6
- Total alkalinity 70 to 90 ppm
- Calcium hardness 250 to 450 ppm
- CYA 30 to 50 ppm (60 to 80 for salt)
Frequently asked questions
What is a good LSI for a pool?▼
Aim for -0.3 to +0.3, and ideally -0.1 to +0.1. Below -0.3 the water is aggressive and dissolves calcium from plaster and grout. Above +0.3 it deposits scale on tile, salt cells, and heater exchangers.
Why does CYA change the LSI?▼
Cyanuric acid shows up on a total alkalinity test but does not buffer like carbonate does. The calculator subtracts roughly one third of your CYA reading from total alkalinity to get carbonate alkalinity, which is what actually drives saturation.
What TDS should I enter for a salt pool?▼
Salt counts toward TDS. A salt pool at 3,200 ppm salt usually sits around 4,000 ppm TDS. For a traditional chlorine pool with no TDS meter, 1,000 is a safe default. TDS has a small effect on the result.
Which reading should I change to fix a low LSI?▼
pH moves LSI the fastest and cheapest. Raising pH by 0.2 raises LSI by 0.2. If pH is already at 7.6 or higher, raise calcium hardness or alkalinity instead so you are not fighting constant pH drift.
Does water temperature really matter?▼
Yes. The same water can be balanced at 85°F and corrosive at 50°F. Winterized pools and cold-season plaster damage usually trace back to LSI drifting negative as the water cools.
More free pool tools
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CYA Drain Calculator
How much water to drain and refill to bring stabilizer, hardness, or salt back down.
Pool Volume Calculator
Gallons for rectangle, round, oval, and kidney pools from a tape measure.