CSI Pool Calculator

This calculator uses a saturation-pH approach and then computes the index: CSI = pH − pHs. The saturation pH (pHs) is estimated using:

• pHs = (9.3 + A + B) − (C + D)
• A = (log10(TDS) − 1) / 10
• B = −13.12·log10(T°C + 273) + 34.55
• C = log10(Calcium) − 0.40
• D = log10(Carbonate Alkalinity)

Carbonate alkalinity is estimated from total alkalinity and reduced by stabilizer (CYA) and borates when provided.

1. Test pH, temperature, calcium hardness, and total alkalinity using a consistent method.
2. Enter the values and keep TDS at 1000 ppm if you do not have a reading.
3. Add CYA and borates if you use them, then calculate CSI.
4. If CSI is positive, reduce scale risk by lowering pH or calcium, or by adjusting alkalinity.
5. If CSI is negative, reduce corrosion risk by raising pH, alkalinity, or calcium as appropriate.

Why CSI matters for pool water near garden spaces

The Calcium Saturation Index (CSI) estimates whether water is likely to dissolve calcium from plaster or deposit calcium scale. When a pool sits close to garden beds, stable water balance also helps reduce splash-out residues on paving, pots, and irrigation edges. A CSI close to zero supports surface longevity, heater efficiency, and cleaner tile lines, especially during warm-weather use.

Inputs that drive the strongest CSI shifts

pH moves CSI quickly; small pH changes can swing results from slightly negative to strongly positive. Temperature increases CSI as water warms, which is why summer readings often show higher scale potential. Calcium hardness and alkalinity set the saturation baseline, while dissolved solids influence the TDS factor. Stabilizer (CYA) and borates are included to refine carbonate alkalinity for more practical decisions.

Interpreting results and action thresholds

This calculator flags CSI > +0.30 as scale risk and CSI < -0.30 as corrosion risk. Values between these bands are generally considered workable for day-to-day operation. If results are high, lowering pH is often the fastest lever. If results are low, raising alkalinity or calcium in controlled steps can reduce aggressiveness.

Operational tips for consistent readings

Test at the same time of day and use circulating water, not stagnant samples. Record temperature from water, not air. Recheck pH after chemical additions because pH drift can occur within hours. For salt systems, expect higher TDS and slightly higher CSI tendency. Save multiple runs in the history table to spot trends after rain, heavy bathing load, or seasonal changes.

Practical care guidance around plants and hardscape

Prevent overspray when adding acids or calcium products and rinse nearby leaves and stone borders if splashed. Keep chemical storage dry and away from fertilizers to avoid reactions. Maintain balanced CSI to reduce chalky deposits on coping and to limit aggressive water that can etch decorative finishes. Simple weekly checks usually prevent costly corrections later.