LSI / Langelier Saturation Index Calculator

The Langelier Saturation Index combines six readings into one signed number that says whether your water is balanced, corrosive (etches plaster, dissolves grout, attacks metal), or scale-forming (deposits calcium on surfaces and equipment). Each parameter can read "in range" while the water as a system is still out of balance — that's exactly what the index catches.

carbonate alkalinity = total alkalinity − (CYA ÷ 3)   (CYA 0 ⇒ no correction)

LSI = pH + TF + CHF + CAF − TDSF

  TF   = temperature factor   (84°F → 0.7)
  CHF  = calcium-hardness factor (300 ppm → 2.1)
  CAF  = carbonate-alkalinity factor (70 ppm → 1.7)
  TDSF = TDS factor (~1,000 ppm → 12.1; ~12.2 low; higher for salt pools)

Verdict bands:  < −0.30 corrosive · −0.30 to +0.30 balanced (0.00 ideal) · > +0.30 scaling

The four factors come from the published CPO / Taylor Langelier factor table — not bare log10 expressions, which land about 0.15 too high. We validate the table against Orenda's worked example (pH 7.6, 84°F, TA 90, CH 300, CYA 60, TDS 1,000 → carbonate alkalinity 70 → LSI +0.00) and the three standard CPO sample problems (−0.9, +0.2, +1.1). The CYA correction — subtracting CYA ÷ 3 from total alkalinity at pool pH — is shown explicitly in the breakdown; it's the single most common reason a naive five-input calculator disagrees with a pro. The index is a log scale: −2.0 is about ten times more corrosive than −1.0.

This is a single-test water-balance index. It does not output a chemical dose or "what to change by how much" — the index is a diagnosis; the linked dosing calculators turn the knobs. Nothing is saved: this is one test, computed in your browser, never a per-pool history log.

Sources: Orenda Technologies — Understanding LSI; AquaChek — Langelier Saturation Index; Pool Research — Saturation Index Calculator; Hasa — What is the LSI in pool water (carbonate alkalinity = TA − CYA ÷ 3). Factor table cross-checked against the CPO sample problems (Pool Operation Management).