Slide in your test readings to get your Langelier Saturation Index — the one number that tells you if your water is balanced, eating your plaster, or laying down scale.
Typical target: 7.4–7.6
Typical target: 60–120 ppm
Typical target: 200–400 ppm
Optional — set 0 if you don’t use stabilizer. Above 0 we subtract the cyanurate share from alkalinity for a truer LSI.
Ideal range: −0.3 to +0.3
LSI = pH + TF + CF + AF − TDS
= 7.50 + 0.65 + 2.10 + 1.96 − 12.1 = +0.11
TF, CF and AF are the NSPF temperature, calcium-hardness and alkalinity factors (looked up from the industry tables and interpolated). TDS = 12.1 for a chlorine pool.
Balanced water — it won’t corrode surfaces or lay down scale. Re-test weekly; temperature swings alone can push you out of range.
Using stabilizer? Add your CYA above for a truer reading — it inflates the alkalinity test without being part of the carbonate balance.
The Langelier Saturation Index measures how saturated your water is with calcium carbonate. Water always wants to reach equilibrium (LSI = 0). When it’s under-saturated (negative LSI) it pulls calcium out of whatever it can — plaster, grout, tile, metal — to balance itself. When it’s over-saturated (positive LSI) it dumps the excess back out as scale on surfaces, heaters, and salt cells.
Say a chlorine pool tests at pH 7.5, total alkalinity 90 ppm, calcium hardness 300 ppm, and 80 °F. The NSPF factors are TF 0.65, CF 2.10, and AF 1.96, with a TDS constant of 12.1:
LSI = 7.5 + 0.65 + 2.10 + 1.96 − 12.1 = +0.11 — comfortably balanced. Heat that same water to a 100 °F spa and the temperature factor jumps, pushing the LSI toward scaling — which is exactly why heaters and spas scale first.
The LSI is a single number that tells you whether your pool water is balanced, corrosive, or scale-forming. It combines pH, total alkalinity, calcium hardness, and water temperature (plus total dissolved solids) into one index. A value near zero means the water is in equilibrium with calcium carbonate — it won’t dissolve your plaster and grout, and it won’t deposit scale on surfaces and equipment.
Aim for an LSI between −0.3 and +0.3, with 0.0 being perfect. Inside that band the water is considered balanced. Below −0.3 the water turns corrosive (it etches plaster, dissolves grout, and attacks metal); above +0.3 it becomes scale-forming (cloudy water, calcium buildup, clogged heaters and filters). Anything beyond ±0.5 needs prompt correction.
LSI = pH + temperature factor + calcium factor + alkalinity factor − a TDS constant. The three factors come from the NSPF/industry standard lookup tables (each reading maps to a factor), and the TDS constant is 12.1 for traditional pools or 12.2 for saltwater pools. If you enter cyanuric acid, the calculator first subtracts its share from your alkalinity (see below) so the alkalinity factor uses true carbonate alkalinity. Just slide in your readings and read the result.
Yes, indirectly. A total-alkalinity test also titrates cyanurate, so CYA makes your TA read higher than the carbonate alkalinity the LSI actually depends on. The fix is to subtract the cyanurate portion before computing the alkalinity factor — roughly a third of your CYA at typical pH, and the exact share rises with pH. Enter your CYA above (it’s optional) and this calculator does that correction for you, which matters most for high-stabilizer pools (CYA 80+ ppm), where ignoring it makes the water look more scaling than it really is.
LSI rises with higher pH, alkalinity, calcium, and temperature, and falls when any of those drop. To raise a corrosive (negative) LSI, the usual levers are increasing alkalinity (sodium bicarbonate) or calcium hardness (calcium chloride). To lower a scaling (positive) LSI, lower pH with acid first — it’s the fastest, safest lever. Change one factor at a time and re-test.
Calcium carbonate is less soluble in warm water, so warmer water scales more easily — a heated spa at 100 °F can be scaling at the same chemistry that’s perfectly balanced in a 60 °F pool. That’s why the same pool can swing from corrosive in winter to scaling in summer, and why temperature is part of the formula.
The chemistry is identical, but saltwater pools carry more total dissolved solids, which slightly shifts the constant in the formula (12.2 instead of 12.1). Toggle “Saltwater” above and the calculator accounts for it. Salt cells also tend to drive pH up over time, so salt pools are especially prone to scaling if you don’t keep pH and alkalinity in check.
How many gallons (or liters) your pool holds — any shape, sloped depths, spas included. The number you need before dosing anything.
Open toolHow much liquid chlorine, bleach, or shock to add to hit your target free chlorine.
Open toolDial in cyanuric acid and the ideal FC/CYA ratio so your chlorine actually works.
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