What does the saturation index tell you?
The saturation index tells you whether your pool water is hungry for calcium or overloaded with it. Water below saturation (negative index) dissolves calcium carbonate from whatever it touches, including plaster, grout and pool tile mortar. Water above saturation (positive index) drops calcium carbonate out as scale and cloudiness. Balanced water, near 0, does neither.
You will see two names. LSI, the Langelier Saturation Index, is the classic formula. CSI, the Calcite Saturation Index, asks the same question with a more detailed correction for dissolved solids. For a typical backyard pool they usually agree closely, and the fixes are identical.
How to use the pool saturation index calculator
- Enter pH from a fresh test.
- Enter water temperature and choose °F or °C. Use the temperature the water will actually reach, not just today’s reading, if you run a heater.
- Enter calcium hardness and total alkalinity in ppm.
- Enter stabilizer (CYA). The calculator subtracts the cyanurate share of alkalinity automatically.
- Tick the saltwater box if you have a salt chlorinator or high dissolved solids.
- Read the index and the verdict: corrosive, balanced, or scale-forming.
The example table above shows how different combinations land. If you need doses to move any of these readings, use the pool chemical calculator.
What is the formula, and what does each factor do?
SI = pH + TF + CF + AF − 12.1 (use 12.2 for salt pools or water above about 1,000 ppm dissolved solids).
| Factor | What it is | How it is calculated | Effect of a change |
|---|---|---|---|
| pH | Acidity of the water | Your test reading | +0.1 pH = +0.1 SI |
| TF | Temperature factor | From a temperature table (76°F ≈ 0.6, 84°F ≈ 0.7) | 50°F to 90°F adds about 0.5 |
| CF | Calcium factor | log10(calcium hardness) − 0.4 | Doubling calcium adds about 0.3 |
| AF | Alkalinity factor | log10(carbonate alkalinity) | Doubling carbonate alkalinity adds about 0.3 |
| Constant | Accounts for dissolved solids | 12.1, or 12.2 for salt | Salt pools read about 0.1 lower |
Carbonate alkalinity is total alkalinity minus the part contributed by cyanuric acid. At pH 7.5, that correction is roughly CYA ÷ 3. Ignoring it makes stabilized pools look more scale-forming than they are.
Worked example: is this pool balanced?
A plaster pool tests at pH 7.5, 82°F (28°C), calcium hardness 300 ppm, total alkalinity 90 ppm and CYA 40 ppm.
- Carbonate alkalinity: 90 − about 12.5 = 77.5 ppm, so AF = log10(77.5) = 1.89
- CF = log10(300) − 0.4 = 2.08
- TF at 82°F = 0.67
- SI = 7.5 + 0.67 + 2.08 + 1.89 − 12.1 = +0.04, balanced
If the same water were in a salt pool, the 12.2 constant would make it −0.06, still balanced.
Why does water temperature matter so much?
Calcium carbonate becomes less soluble as water warms, so the same water drifts toward scaling when it heats up and toward corrosion when it cools down. Take the balanced pool from the example and change only the temperature:
| Water temperature | Saturation index | Verdict |
|---|---|---|
| 50°F (10°C) | −0.38 | Corrosive |
| 82°F (28°C) | +0.04 | Balanced |
| 90°F (32°C) | +0.13 | Balanced, leaning positive |
Heaters and heat pumps. Water inside a heater’s heat exchanger is hotter than the pool, so it scales first. If you heat to 88–90°F, run your index closer to 0 or slightly negative at pool temperature.
Salt cells. Chlorinator plates run at a locally higher pH, so scale collects there before you see it on tile. A positive index shows up as crust on the cell plates.
Winter and cold climates. Cold water is more aggressive. If you close the pool for winter or run it in cool weather, check the index at the low temperature the water will actually sit at, and lean slightly positive rather than negative to protect plaster.
How can you tell if your water is corrosive or scaling?
| Symptom | Likely index | What is happening |
|---|---|---|
| Rough, etched plaster; exposed aggregate | Negative | Water dissolving calcium from the surface |
| Grout washing out between tiles | Negative | Same process at tile joints |
| Blue-green or brown stains, metallic taste | Negative | Copper or iron dissolving from heaters or fittings |
| White crust at the waterline tile | Positive | Calcium carbonate scale forming |
| Gray, sandpaper-like deposits on plaster | Positive | Scale on the surface |
| Persistent cloudiness that filtering will not clear | Positive | Fine calcium particles in suspension |
| Scaled salt cell plates or heater “kettling” noise | Positive | Scale on hot or high-pH surfaces |
Taylor Technologies, the test-kit maker, notes that values below −0.3 may cause corrosion and values above +0.5 may lead to cloudiness and scaling. This calculator warns beyond ±0.3 so you get an earlier signal.
How do you correct an out-of-balance index?
Start with pH, because it moves the index one for one and is quick to adjust. Use calcium and alkalinity for longer-term corrections.
If the index is too negative (corrosive):
- Raise pH toward 7.6–7.8 with soda ash, within the ideal range.
- Raise calcium hardness with calcium chloride, especially in plaster pools (250–450 ppm is common there).
- Raise total alkalinity with baking soda if it is below about 70–80 ppm.
If the index is too positive (scaling):
- Lower pH toward 7.2–7.4 with muriatic acid. Add half the calculated dose, circulate 30 minutes, retest.
- Lower total alkalinity with acid and aeration if it is above about 120 ppm.
- If calcium hardness is the cause, replace part of the water; there is no chemical that removes it.
Adjust one thing at a time, retest, then recalculate. For the full set of target ranges, see ideal pool water chemistry levels.
What are the most common saturation index mistakes?
- Using the wrong temperature. A spring reading does not describe a heated summer pool.
- Skipping the CYA correction. It overstates alkalinity and hides corrosive water.
- Chasing perfect zero. Anything within ±0.3 is fine; small daily swings are normal.
- Fixing calcium with pH. Pushing pH outside 7.2–7.8 to compensate for very low calcium creates other problems.
Safety note: add acid and other chemicals to water, never the reverse, keep acid away from chlorine, and wear gloves and eye protection. The temperature table and constants this calculator uses are listed on our methodology page.