Pool saturation index calculator
pH, alkalinity and hardness can each sit inside their band and the water can still be eating the plaster. The saturation index folds all of them, plus temperature and dissolved solids, into one number — and tells you which single reading to move.
Pre-filled with a salt pool where every reading looks fine on its own. Type your own kit readings over it; salt pools put the salt reading in too.
= 27.8 °C
0 if the pool is unstabilised
From a TDS meter; 1,000 if unknown
0 for a chlorine pool
Saturation index
-0.65SI
Corrosive
Under-saturated: the water will take calcium from plaster, grout and pebble finishes, and attacks metal.
Move this one reading
pH: 7.4 → 7.78
pH is the fastest reading to move, and it can do this without leaving 7.2–7.8.
| Term | From | Value |
|---|---|---|
| pH | reading | 7.4 |
| + Temperature factor | 82 °F | 0.67 |
| + Calcium factor | 200 ppm | 1.90 |
| + Alkalinity factor | 80 − 20.8 CYA = 59.2 ppm | 1.77 |
| − TDS factor | 3,200 ppm | 12.40 |
| Saturation index | -0.65 |
The salt reading is higher than the TDS entered, so the salt figure is used as TDS — dissolved solids can never read lower than the salt in them.
To land on 0.00 by moving one reading alone: pH 8.09, CH 897 ppm, TA 286 ppm. Anything outside its own band (pH 7.2–7.8, TA 80–120, CH 200–400) is not a real option.
What it measures
Water carries a certain amount of dissolved calcium carbonate before it is full. Below that point it pulls calcium out of anything that has some — plaster, grout, pebble finish. Above it, the excess comes out as scale on tile, in the heater exchanger and on the salt cell plates. The saturation index is how far from that point the water sits. Zero is exactly full; a minus number is corrosive; a plus number is scale-forming.
This calculator uses the factor method a state health code prints for public pools: index = pH + temperature factor + calcium factor + alkalinity factor − TDS factor. Utah’s pool rule requires operators to hold the result between −0.3 and +0.3, and that is the band the bar shows. Taylor’s guidance is looser — no adjustment needed within ±0.5 — so a reading of −0.4 is a watch item, not an emergency.
How to use it
- Test pH, calcium hardness, total alkalinity and cyanuric acid off a drop kit, not a strip. The chemistry basics guide has the order.
- Take the water temperature at the return, not the air temperature. Cold water corrodes more: the same pool reads lower in March than in August.
- Enter TDS from a meter if you have one; 1,000 ppm is a fair stand-in for a chlorine pool on ordinary fill water.
- Salt pools: enter the salt reading. Salt is dissolved solids, so a 3,200 ppm salt pool moves the TDS factor from 12.1 to 12.4 on its own.
- Read the verdict, then the one reading to move. Correct it, circulate, re-test, and run the index again before touching anything else.
Worked example: the salt pool that looks fine
The calculator opens on this pool. pH 7.4, water at 82 °F, hardness 200, alkalinity 80, stabiliser 70, salt 3,200 ppm. Every reading is inside the band this site quotes for it.
| Step | Working | Result |
|---|---|---|
| Temperature factor | 82 °F, between the 76 °F (0.6) and 84 °F (0.7) rows | 0.68 |
| Calcium factor | log₁₀ 200 − 0.4 | 1.90 |
| Cyanurate alkalinity | 70 × 0.298 at pH 7.4 | 20.8 ppm |
| Alkalinity factor | log₁₀ (80 − 20.8) | 1.77 |
| TDS factor | 3,200 ppm salt, 3,000–3,999 row | 12.4 |
| Index | 7.4 + 0.68 + 1.90 + 1.77 − 12.4 | −0.65 |
| Same pool, no salt | TDS factor 12.2 at 1,000 ppm | −0.45 |
Corrosive, and the salt alone accounts for 0.2 of it. The fix the calculator picks is pH up to 7.78, which puts the index just inside the band at −0.30 without pH leaving 7.2–7.8. Use the pH-raising guide to get there by aeration first. Then, over the season, lift hardness into the upper half of its 200–400 band; at 400 ppm the same pool balances at pH 7.6 instead of needing the top of the range.
Why the CYA correction matters
The alkalinity test counts cyanurate as alkalinity, but it does nothing for calcium carbonate balance. The share it takes depends on pH: about 0.26 × CYA at 7.2, 0.33 at 7.6, 0.36 at 8.0. A tablet pool with 90 ppm of stabiliser loses about 30 ppm of its alkalinity reading before the index ever sees it.
Which reading to move, and why that one
pH first, whenever it can do the job without leaving 7.2–7.8: it moves in an afternoon with the acid or soda ash dose, and you can bring it back down just as fast. If pH cannot get there alone, the calculator looks at the slow readings. Corrosive water gets calcium hardness (calcium chloride, permanent). Scale-forming water gets alkalinity lowered with acid and aeration — the alkalinity calculator stages the dose. Hardness only comes down by draining and refilling, so it is the last resort.
Where this method is rough
The temperature column in the standard factor table steps unevenly; a 2006 analysis in Aquatics International found it runs about 0.1 high around 84–87 °F. Treat the second decimal as noise. If the index says −0.25 and the plaster is roughening, believe the plaster.
Notes
Heated spas and summer pools drift positive as they warm, which is when scale shows up first on the hottest surface: the heater exchanger and the cell. Cloudiness with a high index is calcium coming out of solution — cloudy water covers how to tell it from a filtration problem. And on a salt pool, run the index every time you add salt; the TDS factor moves with it.