Pool LSI calculator: is your water corrosive, balanced or scaling?
Your pH, alkalinity and calcium can each read "fine" while the combination etches plaster or scales a heater. Enter your numbers to get the index, the one change that fixes it, and the exact dose.
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Show the math
LSI = pH − pHs, where pHs = (9.3 + A + B) − (C + D)
Warm water holds less calcium carbonate, so the index climbs with temperature: water near the top of the band in spring can tip into scaling in August or at a heater's outlet, and cold winter water leans aggressive. Only temperature changes in this row.
Balanced is −0.3 to +0.3. Below that the water is aggressive — it pulls calcium out of plaster, grout, and tile and etches those surfaces. Above it, the water scales — cloudy water, crust on tile, a struggling heater and cell. pH is the fastest lever: a 0.2 pH change moves the index about 0.2. This is the Langelier index with the pH-dependent CYA and borate corrections to alkalinity (the CSI refinement); it matters most on plaster/gunite and for anything with a heater or salt cell.
Why one index beats five ranges
The standard advice hands you five target ranges and stops there. But water chemistry isn't five independent numbers — it's one equilibrium. A pool at pH 7.5, alkalinity 60, and calcium 150 has every number "in range" and water that's still aggressive enough to slowly dissolve plaster and corrode a heater. The Langelier Saturation Index (and its modern refinement, the CSI) captures that interaction in a single value, so you're balancing the actual water, not chasing four gauges independently.
What actually moves it
pH is the biggest, fastest lever — a 0.2 change moves the index about 0.2, and it's cheap to adjust. Temperature matters (warm water scales more; a heated spa swings), and calcium hardness and alkalinity move it slowly. The one people miss: CYA inflates your alkalinity reading without helping balance, so we subtract the cyanurate share to get true carbonate alkalinity. That share depends on pH (about a third of the CYA reading at pH 7.5, a bit more at 8.0), and borates count too, so the calculator works it out from your own numbers instead of using a flat CYA ÷ 3. That's the CSI correction, and it's why a high-stabilizer pool can look balanced yet run aggressive.
Who needs to care (and who mostly doesn't)
It matters most on plaster, gunite, and tile — those surfaces are calcium carbonate, so aggressive water literally dissolves them and scaling water crusts them. On vinyl and fiberglass the wall itself isn't calcium-based, so it's lower-risk — but the same water runs through your heater and salt cell, which corrode or scale either way. So balance still protects the expensive equipment, whatever your pool is made of. And unbalanced water — not a pool robot — is one of the real things that wears a finish over time.
The same water in August
The calculator shows your water at five temperatures because the index climbs as water warms. Water that sits comfortably in range at 70°F (21°C) in spring moves toward the scaling side at 90°F (32°C) in a Florida August, and further at a gas heater's outlet or in a spa. If you heat the pool, balance for the hot end.
What to do with your number
See the five numbers and their target ranges → ·Get the exact dose to move a level → ·The full water-balance guide →
FAQ
What is the Langelier Saturation Index (LSI) for a pool?
The LSI (and its refinement, the CSI) is a single number that says whether your water is corrosive, balanced, or scaling with respect to calcium carbonate. It combines pH, temperature, calcium hardness, and carbonate alkalinity. Between -0.3 and +0.3 the water is balanced. Below -0.3 it is aggressive — it dissolves calcium out of plaster and grout and attacks metal, heaters, and salt cells. Above +0.3 it deposits scale on surfaces, tile, heaters, and cells. It matters because your five numbers can each read "in range" while the combination is still attacking or scaling your pool.
Is my pool water corrosive or scaling?
Enter your pH, alkalinity, calcium hardness, CYA, and water temperature into the calculator above. If the index lands below -0.3 the water is aggressive (corrosive) and you should nudge it up; above +0.3 it is scaling and you should bring it down; between the two it is balanced. pH is the biggest, fastest lever — a 0.2 change in pH moves the index by about 0.2.
How does CYA (stabilizer) affect water balance?
Cyanuric acid registers on a total-alkalinity test but does not participate in calcium-carbonate chemistry, so it inflates your alkalinity reading. To balance water correctly you subtract the cyanurate share — about a third of the CYA reading at pH 7.5, a little more at higher pH, and borates count too — to get true carbonate alkalinity. The calculator works that share out from your own pH. That is the difference between the older LSI and the CSI refinement, and it is why a high-CYA pool can look balanced on paper while actually running aggressive.
What is the fastest way to fix unbalanced water?
pH. It is the biggest and fastest lever in the index — a 0.2 pH change moves the saturation index about 0.2, and pH is cheap and quick to adjust with acid (to lower) or aeration/soda ash (to raise). Calcium hardness and alkalinity move it too but change slowly and cost more. Fix pH first, then fine-tune alkalinity and calcium if needed.
Does water balance matter for vinyl and fiberglass pools?
It matters most for plaster, gunite, and tile pools, whose calcium-carbonate surfaces are directly dissolved by aggressive water or scaled by over-saturated water. Vinyl and fiberglass surfaces are not calcium-based, so the wall itself is less at risk — but the same water still runs through your heater and salt cell, which corrode when the water is aggressive and scale when it is over-saturated. So balance still protects your equipment even in a vinyl or fiberglass pool.
More pool chemistry calculators
Chemical Dosing Calculator · Chlorine Calculator · Shock (SLAM) Calculator · pH Calculator · Alkalinity Calculator · Calcium Hardness Calculator · CYA (Stabilizer) Calculator · Salt Calculator · Pool Volume Calculator · Pool Pillow Calculator · Pool Tracker
This uses the Langelier Saturation Index with pH-dependent CYA and borate corrections to alkalinity (the CSI refinement), and a TDS term you can set. It's a guide to balance, not a substitute for testing — measure your real numbers with a drop-based kit first. Pool School is free and independent; we never sell you a bottle you don't need.