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Pool LSI Calculator — UK

Metric by default: °C in, mg/L readings

Water can have "perfect" pH and still eat plaster or scale a heater. LSI combines 5 readings into one verdict. Read-only planning number — it never doses anything.

Ideal 7.2–7.6. Biggest, fastest LSI lever.

Ideal 200–400 mg/L plaster. Low calcium is the top cause of corrosive LSI.

Ideal 80–120 mg/L.

Above 30 mg/L we auto-correct alkalinity (Taylor CYA/3 rule). Enter 0 if unstabilized.

Fresh pools ≈ 500. Salt pools ≈ 3000–4000. Smallest lever — estimate is fine.

Reading the result

  • • −0.3 to +0.3 — balanced, ideal near 0
  • • Below −0.3 — corrosive: etches plaster, pits metal
  • • Above +0.3 — scaling: coats tile, cells, heaters
  • • pH moves LSI fastest (±0.1 → ±0.1)
  • • Warm water pushes LSI up — recheck each season

Limits of LSI

  • • Doesn't predict metal corrosion — that's pH + salts + oxidants
  • • Never push pH/TA outside accepted ranges to chase 0.0
  • • Fiberglass/vinyl: LSI is secondary — keep CH low, don't add calcium to fix LSI

Next step

LSI tells you which lever to pull. Doses live in the chemical calculator.

Get exact doses →