Enter your water's numbers — get the saturation index, a plain-english verdict, and which lever to pull if it's off.
The index, in plain english
Water always wants a certain amount of dissolved calcium carbonate at a given pH, temperature, and alkalinity. Below that saturation point it takes calcium from wherever it can — plaster, grout, metal. Above it, it drops calcium as scale — on tile lines, inside heaters, on salt cell plates. The index measures how far you are from the happy middle, and every term in it is a lever you already know how to move.
The formula
index = pH + temp factor + log₁₀(CH) − 0.4 + log₁₀(TA − CYA/3) − 12.1
The CYA/3 correction removes stabilizer's fake contribution to alkalinity (cyanurate buffers register as TA but don't participate in calcium chemistry) — that's the CSI refinement. Salt pools use a slightly larger constant for their higher dissolved solids.
Which lever to pull
pH is the biggest, fastest lever — a 0.2 pH change moves the index 0.2. Temperature you don't control (but notice: the same water is more scale-prone in August than May, and heaters make their own hot spots). Calcium and alkalinity are the slow structural levers. The classic play for a scaling salt pool: hold pH at 7.5 with regular small acid doses and let borates slow the drift — see the borate calculator.
Frequently asked questions
What is the Langelier Saturation Index?
A score for whether your water wants to deposit calcium (scale) or dissolve it (etch surfaces, corrode). Between −0.3 and +0.3 is balanced; below −0.3 the water is aggressive toward plaster and metal; above +0.3 it scales surfaces, salt cells, and heaters.
What's the difference between LSI and CSI?
Same core idea — calcium carbonate saturation. CSI (Calcite Saturation Index) is the pool-community refinement that accounts for CYA's contribution to alkalinity, which this calculator also does. For everyday pool use the terms point at the same decision.
Do I need to care about LSI with a vinyl or fiberglass pool?
Much less — there's no plaster to etch. It still matters for heaters and salt cells (scale) at high positive values, but vinyl/fiberglass owners can mostly manage the individual levels and skip the index.
Why does my salt cell keep scaling up?
Persistent positive LSI — usually high pH plus warm water. The cell's plates locally raise pH even further, so they scale first. Keeping LSI near zero (often by holding pH ~7.5 and trimming alkalinity) is the actual fix, not repeated acid washing.