Pool saturation index (LSI) calculator

Water always tries to reach equilibrium with calcium carbonate. If it holds less than it could, it takes calcium from wherever it can — plaster, grout, tile — and corrodes metal. If it holds more, calcium comes out as scale on tile lines, in salt cells and in heaters. The Langelier saturation index (LSI) says which way your water leans.

How the index is calculated

The calculator uses Langelier’s 1936 formula. It works out pHs — the pH at which your water would be exactly saturated with calcium carbonate — from temperature, calcium, alkalinity and dissolved solids, then compares it with your actual pH.

LSI = pH − pHs
pHs = (9.3 + A + B) − (C + D)
A = (log₁₀ TDS − 1) ÷ 10
B = −13.12 × log₁₀(°C + 273) + 34.55
C = log₁₀(calcium hardness) − 0.4
D = log₁₀(carbonate alkalinity)
carbonate alkalinity = total alkalinity − CYA ÷ 3

Pool test kits often use the factor-table version (pH + temperature factor + calcium factor + alkalinity factor − a constant), as on Taylor’s Watergram and Orenda’s calculator. It is the same idea in table form, but the two don’t agree exactly: at typical pool readings this formula comes out roughly 0.1 lower than the factor tables. With this page’s default readings it gives about −0.13, where the factor method gives about 0.0. Treat differences of a tenth either way as noise, and use the same method each time you compare. (The calcite saturation index, CSI, is a related calculation some pool chemists prefer; it isn’t offered here.)

Reading the result

Which reading moves it most

• pH counts one-for-one: raising pH by 0.2 raises the index by 0.2. It is the quickest lever, and the one that drifts most day to day.
• Calcium and alkalinity count by their logarithms: doubling either one raises the index by about 0.3.
• Temperature matters more than people expect: water that balances at 78°F is noticeably more scale-forming at 104°F in a spa or at the heater’s exchanger.
• Stabilizer lowers the useful alkalinity — a third of CYA is subtracted — so high-CYA pools read more corrosive than their TA suggests.
• Salt pools have high dissolved solids (salt plus everything else), which lowers the index a little.

To adjust: how much muriatic acid to lower pH in a 10,000 gallon pool, how much soda ash to raise pH in a 10,000 gallon pool, how much baking soda to raise alkalinity in a 10,000 gallon pool and how much calcium chloride to add to a pool. The order to do them in is in how to balance pool water.