What is a good pool LSI?

Anywhere from −0.3 to +0.3 is considered balanced, with 0.0 ideal. Negative is corrosive; positive is scaling.

Why is CYA correction important for LSI?

Cyanuric acid shows up in your alkalinity test but doesn't protect against scale. Counting it makes your LSI look better than reality — which is how "balanced" pools still etch or scale. We remove it.

How do I fix an unbalanced LSI?

pH is the fastest lever — small changes move the LSI a lot. Then adjust total alkalinity and calcium hardness. Warmer water trends more scaling, colder more corrosive.

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Pool LSI Calculator

The LSI tells you if your water will etch plaster or form scale. Most calculators get it wrong by ignoring CYA — we subtract it for a true carbonate-alkalinity reading.

Balance your whole pool at once →Enter all your readings on one screen and get an ordered dosing plan — the full Water Report.

How to use it

Test pH, total alkalinity, calcium hardness, CYA, and water temperature.
Enter all five.
Aim for an LSI between −0.3 and +0.3. If it's off, adjust pH first (fastest lever), then alkalinity or calcium.

What the LSI tells you

The Langelier Saturation Index is a single number for whether your water is balanced. Below −0.3 it's corrosive (it dissolves plaster, etches surfaces, eats metal and heater parts). Above +0.3 it's scaling (calcium drops out as cloudy water and scale, and clogs saltwater cells). Between −0.3 and +0.3 you're balanced.

The CYA correction most tools skip

Cyanuric acid registers on a total-alkalinity test but doesn't actually buffer against scale. Feed raw total alkalinity into the LSI and the number reads falsely high — telling you corrosive water is "fine." We subtract the cyanurate portion (it's pH-dependent) to use true carbonate alkalinity, and we show you both numbers so you can check our work.

Frequently asked questions

What is a good pool LSI?: Anywhere from −0.3 to +0.3 is considered balanced, with 0.0 ideal. Negative is corrosive; positive is scaling.
Why is CYA correction important for LSI?: Cyanuric acid shows up in your alkalinity test but doesn't protect against scale. Counting it makes your LSI look better than reality — which is how "balanced" pools still etch or scale. We remove it.
How do I fix an unbalanced LSI?: pH is the fastest lever — small changes move the LSI a lot. Then adjust total alkalinity and calcium hardness. Warmer water trends more scaling, colder more corrosive.

Related calculators

Built & reviewed by a working pool-service pro. Target ranges follow CDC MAHC, PHTA, and Taylor guidance. Doses are starting estimates — add a little less than the max, re-test, and confirm against your product label. Educational tool, not a substitute for a water test or professional advice.