Pool LSI Calculator (Saturation Index)
Salt pools: add the salt level.
Saturation index (LSI)
−0.06
Your water is balanced: it will neither etch plaster nor deposit scale.
- Status
- Balanced (−0.3 to +0.3)
- Saturation pH (pHs)
- 7.56
- Carbonate alkalinity
- 77 ppm
- Temperature factor
- 0.67 at 82 °F
- Calcium factor
- 2.08
- Alkalinity factor
- 1.89
- TDS constant
- 12.2
- Never mix pool chemicals with each other, wet or dry.
- Always add chemicals to water, never water to chemicals.
- Follow the manufacturer's label. It overrides any calculator, including this one.
How it's calculated
This is the pool-industry form of the Langelier index, as published by Taylor Technologies:
- The cyanurate correction uses cyanuric acid's pKa of 6.88: at pH 7.5 about 81% is ionized, so roughly a third of the CYA level shows up in the TA test.
- The result below −0.3 means the water is undersaturated with calcium carbonate and will dissolve it from plaster and grout; above +0.3 it will deposit it as scale.
- The saturation pH (pHs) is the pH at which this water would be exactly balanced: pHs = pH − LSI.
Worked example
pH 7.5, water 82 °F, CH 300 ppm, TA 90 ppm, CYA 40 ppm, TDS 1,000 ppm:
- TF at 82 °F: between 76 °F (0.6) and 84 °F (0.7) → 0.67.
- CF: log10(300) − 0.4 = 2.08.
- Cyanurate alkalinity: 40 × 0.388 × 0.81 = 12.5 ppm, so carbonate alkalinity = 90 − 12.5 = 77.5 ppm.
- AF: log10(77.5) = 1.89.
- LSI = 7.5 + 0.67 + 2.08 + 1.89 − 12.2 = -0.06: balanced.
Temperature factors
| Water temp | °C | Factor |
|---|---|---|
| 32 °F | 0 | 0 |
| 37 °F | 3 | 0.1 |
| 46 °F | 8 | 0.2 |
| 53 °F | 12 | 0.3 |
| 60 °F | 16 | 0.4 |
| 66 °F | 19 | 0.5 |
| 76 °F | 24 | 0.6 |
| 84 °F | 29 | 0.7 |
| 94 °F | 34 | 0.8 |
| 105 °F | 41 | 0.9 |
| 128 °F | 53 | 1 |
Target ranges that usually give a balanced LSI
| Parameter | Range | Ideal |
|---|---|---|
| pH | 7.2–7.8 | 7.4–7.6 |
| Total alkalinity (TA) | 60–120 ppm | 80–100 ppm (60–80 for salt pools) |
| Calcium hardness (CH) | 150–400 ppm | 250–350 ppm plaster; 150–250 ppm vinyl/fiberglass |
| Cyanuric acid (CYA) | 30–80 ppm | 30–50 ppm (chlorine); 60–80 ppm (salt) |
Taylor Technologies temperature factors and TDS constant (Pool & Spa Water Chemistry); balanced range ±0.3 per PHTA guidance.
Frequently asked questions
What is a good LSI for a pool?
Between −0.3 and +0.3 is considered balanced. Many owners aim for slightly negative to zero in summer, because LSI rises as water warms, and especially near zero for plaster pools.
How do I raise the LSI?
Raise pH, total alkalinity or calcium hardness. pH has the biggest effect per step; calcium is the most stable fix for plaster pools with soft fill water. Warmer water also raises LSI.
How do I lower the LSI?
Lower pH or total alkalinity with acid. If calcium hardness is very high, partial drain and refill is the only lasting fix.
Why does the LSI change with temperature?
Calcium carbonate is less soluble in warm water, so the same chemistry becomes more scale-forming as it heats up. With the example values on this page, LSI is -0.06 at 82 °F but -0.48 at 50 °F. That is why heaters scale first.
Why does CYA matter for the LSI?
Part of a total alkalinity reading is cyanurate from stabilizer, which does not take part in calcium carbonate balance. The calculator subtracts it: at pH 7.5, 40 ppm CYA accounts for about 13 ppm of TA.
What is the difference between LSI and CSI?
The Calcite Saturation Index (CSI) is a more detailed version that uses ionic strength and activity corrections instead of the simplified factors in the LSI. For residential pools the two usually point the same way; the CSI matters most in high-TDS or salt pools.
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