The Langelier Saturation Index (LSI) is a calculated number that predicts whether pool water is in equilibrium, tending to corrode, or tending to deposit scale. It combines five variables into a single actionable score.
Key Facts
- LSI target range: -0.3 to +0.3. Zero is perfect balance.
- Negative LSI (below -0.3): water is aggressive and will corrode pool surfaces and equipment.
- Positive LSI (above +0.3): water is over-saturated and will deposit calcium carbonate scale.
- Temperature has a large effect on LSI — the same chemistry at 90°F scores higher than at 70°F.
How the LSI Is Calculated
LSI = pH + Temperature Factor (TF) + Calcium Factor (CF) + Alkalinity Factor (AF) - 12.1. Each factor is a number derived from a lookup table based on the actual measurement. Temperature factor increases with rising temperature (hotter water has a higher scaling tendency). Calcium factor increases with higher calcium hardness. Alkalinity factor increases with higher total alkalinity. The constant 12.1 is the saturation constant for calcium carbonate at typical pool chemistry conditions. The result is a dimensionless index that tells you the balance state of the water.
Using the LSI Practically
Calculate the LSI every time you do a full water test, using the current water temperature. If the LSI is below -0.3, the most common corrections are raising pH, raising calcium hardness, or raising alkalinity. If the LSI is above +0.3, lower pH (largest effect), lower calcium hardness (requires partial drain), or lower alkalinity slightly. Of the adjustable variables, pH has the largest impact on LSI because the pH factor in the formula is the raw pH value, not a lookup number — a 0.1 pH change shifts LSI by 0.1 directly.
LSI for Heated Spas
Hot tub water is typically maintained at 98–104°F. At these temperatures, the temperature factor in the LSI formula is significantly higher than for a pool at 78°F. This means that spa water requires lower pH, lower alkalinity, and lower hardness than pool water to achieve the same LSI target. A spa with pH 7.6, TA 100 ppm, and hardness 250 ppm at 102°F may have an LSI of +0.6 — actively scaling. The same water at 78°F in a pool would have an LSI of about +0.2 — well within range. Always calculate LSI at actual water temperature, not a standard assumption.
Examples
Pool test data: pH 7.4, temperature 82°F, calcium hardness 250 ppm, total alkalinity 100 ppm, TDS 1,500 ppm. Using the LSI calculator: TF = 0.6 (at 82°F), CF = 1.9 (at 250 ppm), AF = 1.9 (at 100 ppm). LSI = 7.4 + 0.6 + 1.9 + 1.9 - 12.1 = 1.7 - nope, let me recalculate. pH 7.4, TF for 82°F is approximately 0.6, CF for 250 ppm is 1.9, AF for 100 ppm is 2.0. LSI = 7.4 + 0.6 + 1.9 + 2.0 - 12.1 = -0.2. Slightly negative but within the acceptable range. No adjustment needed.
Common Mistakes to Avoid
- Calculating the LSI without entering the actual current water temperature, which is the most commonly overlooked variable.
- Ignoring calcium hardness in the LSI calculation because it changes slowly — it still has a large effect on the index.
- Treating any negative LSI as dangerous — LSI between -0.3 and 0 is normal for many well-managed pools, especially those using salt systems.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference