Pool LSI Calculator: Check Your Water Balance
The Langelier Saturation Index (LSI) tells you whether pool water will eat away at plaster, grout and metal, or leave scale on them. Enter five readings below to get your pool’s LSI. Everything is calculated in your browser and nothing you type is sent anywhere.
What the LSI number means
Water is always trying to reach balance with calcium carbonate. If it holds less than it could, it dissolves calcium from whatever it touches: plaster, grout, tile and, eventually, heater parts. If it holds more, it deposits the extra as scale. The LSI measures which way it is leaning and by how much.
The calculation compares the pool’s actual pH with its saturation pH (pHs), the pH at which this water would be exactly balanced given its temperature, calcium, alkalinity and dissolved solids. LSI is simply pH − pHs.
- Negative: the water is corrosive. It wants calcium and will take it from your surfaces.
- Zero: the water is exactly balanced.
- Positive: the water is scale-forming. Expect cloudy water and crusty deposits, worst where the water is hottest.
The target: −0.3 to +0.3
Aim for an LSI between −0.3 and +0.3. Inside that band the water is close enough to balance that it neither etches surfaces nor scales them in any way you will notice. Many service pros aim for slightly positive, around 0 to +0.3, because a little scale-forming tendency protects plaster better than a little corrosive one. The PHTA CPO Handbook treats up to +0.5 as balanced; many pros aim tighter.
| LSI | What it means | What to do |
|---|---|---|
| Below −0.5 | Corrosive | Raise pH, alkalinity or calcium soon. Plaster and metal are being attacked. |
| −0.5 to −0.3 | Slightly corrosive | Nudge pH or alkalinity up at the next visit. |
| −0.3 to +0.3 | Balanced | Nothing. Keep testing. |
| +0.3 to +0.5 | Slightly scale-forming | Nudge pH or alkalinity down. |
| Above +0.5 | Scale-forming | Lower pH and alkalinity before scale builds up, especially on heated pools and spas. |
How each reading moves the LSI
- pH has the most direct effect. Raising pH by 0.1 raises the LSI by 0.1. It is usually the quickest thing to adjust.
- Temperature: warmer water is more scale-forming. The same water reads about 0.3 higher at 104 °F than at 78 °F, and a balanced summer pool can turn corrosive in cold winter water. That is why this calculator will not run without a temperature.
- Calcium hardness: doubling calcium raises the LSI by about 0.3.
- Alkalinity: doubling the alkalinity that counts raises the LSI by about 0.3. See the CYA section below for which alkalinity counts.
- TDS: higher dissolved solids lower the LSI slightly. It matters most for salt pools.
Why CYA changes the answer
A total alkalinity test counts everything in the water that buffers pH. In a stabilized pool that includes cyanurate from the cyanuric acid (CYA), and cyanurate does nothing to hold calcium in balance. Only the carbonate alkalinity does.
So before the LSI is calculated, total alkalinity is corrected by subtracting about a third of the CYA reading, the standard correction at normal pool pH. With 100 ppm total alkalinity and 60 ppm CYA, the calculation uses 80 ppm. On an outdoor pool carrying 80–100 ppm of stabilizer, skipping this correction makes the water look more balanced than it is. That is why CYA is required here, and why the calculator shows the carbonate alkalinity it used.
If the pool has no stabilizer, as with many indoor pools and spas, enter 0.
What about TDS?
Total dissolved solids shift the calculation a little. The standard pool table steps its constant up from 12.1 (under 800 ppm) through 12.2, 12.3 and 12.4 to 12.5 (over 5,500 ppm). If you leave TDS blank, the calculator assumes under 800 ppm and says so under the result. Salt pools usually read 3,000 ppm or more, which lowers the LSI by about 0.3, so enter TDS for a salt pool.
How often to check the LSI
You do not need a full LSI every visit. pH, chlorine and alkalinity move week to week; calcium, CYA and TDS change slowly. Most companies test the core readings at every stop and run a full LSI on a regular schedule, such as monthly and whenever a pool is filled, drained or showing trouble. Re-check when the seasons change, because temperature alone can move a pool out of balance.
For how readings fit into the rest of the job, from visit logs to service reports, see what pool service software should do and how to choose one. If you are checking water balance at every stop, a tight route gives you the time to do it: see how to plan pool routes that don’t waste your day.
How AquaFlow Pro handles water balance
In AquaFlow Pro, technicians log readings on their phone at every stop, and the app calculates LSI with the same CYA correction used here. It keeps every reading in the pool’s history. Each company chooses how often its technicians run a full LSI, and the app reminds them when one is due without ever holding up a routine visit.
Every plan starts with a 30-day free trial, no credit card required.