Calcium hardness is one number. The saturation index is that number plus four others. Pool owners often watch the first and miss the second.
The short answer: watch both, for different reasons. Calcium hardness has its own range for your surface (Taylor: 200 to 400 ppm). The LSI tells you whether the water as a whole is corrosive or scaling (Taylor's ideal: −0.30 to +0.30). The same calcium level can be fine in one pool and scaling in another.
Part of: Pool Water Balance: Calcium, Scale and the LSI, our overview of water balance.
| Calcium hardness | Saturation index (LSI) | |
|---|---|---|
| What it is | One test result | pH, temperature, calcium, alkalinity and dissolved solids combined |
| Target | Taylor 200 to 400 ppm; TroubleFreePool by surface | Taylor −0.30 to +0.30 |
| Changes | Slowly (evaporation, fill water, cal-hypo) | Whenever any input changes, especially pH and temperature |
| Tells you | Whether calcium itself is too low or too high | Whether the water will etch or scale |
Same calcium, opposite answers
Calcium 300 ppm in both pools, about 1,000 ppm dissolved solids (our calculation with Utah's LSI formula; temperature and dissolved-solids factors from its Table 2, calcium and alkalinity as exact log values):
| Pool | pH | Temp | Alkalinity | LSI |
|---|---|---|---|---|
| A | 7.2 | 76 °F | 70 ppm | −0.48 |
| B | 7.8 | 84 °F | 120 ppm | +0.46 |
Pool A is below Taylor's ideal range on the corrosive side; pool B is above it on the scaling side, close to Taylor's action level of +0.50. Exactly the same calcium.
As an Amazon Associate we earn from qualifying purchases. We have not tested these products.
Test kit A test kit that reads calcium hardnessCalcium hardness is one of the five numbers the saturation index needs, together with pH, alkalinity, temperature and dissolved solids.
How to use both
- Keep calcium inside the range for your surface (calcium hardness in pools).
- Check the LSI whenever you change pH, alkalinity or calcium, and when the season changes; Taylor says to check it in summer and winter and after refilling.
- If the LSI is off, adjust in Taylor's order: alkalinity, pH, then calcium.
Tool: pool LSI calculator.
Sources
- Taylor Technologies: LSI calculator and target ranges (ranges, check in summer and winter, order of adjustment)
- Utah Admin. Code R392-302-25 (public pools) (LSI formula and factors)
- TroubleFreePool: Recommended levels (calcium ranges by surface)
Sources read on November 6, 2026, unless a date is given. Product labels change: the label on the product you buy always wins.
