How we calculate our numbers
Every target range and formula on this site is derived from published standards — not invented. Here is exactly where the numbers come from.
Our sources
Where the pool industry has an established standard, we follow it. Where practitioners disagree, we say so and explain which convention we use and why.
- ANSI/APSP/ICC-11 — Water Quality in Public Pools and SpasThe American National Standard maintained by the Pool & Hot Tub Alliance (PHTA). Basis for our water-balance approach and LSI usage, including the guidance against manipulating inputs to reach a target index.
- CDC Model Aquatic Health Code (MAHC)Public-health guidance for disinfection and contamination response — the basis for our shock and fecal/vomit-contamination recommendations.
- Chlorine / cyanuric-acid equilibrium chemistryThe FC-depends-on-CYA relationship (minimum FC and shock levels scale with CYA) comes from the well-documented HOCl / chlorocyanurate equilibrium.
- Manufacturer dosing dataProduct strengths (e.g. 10% and 12.5% liquid chlorine, 31.45% and 14.5% muriatic acid) and dose math use published product concentrations.
Free chlorine and CYA
The single most important rule on the site: safe free chlorine is not a fixed number — it depends on your stabilizer (CYA) level.
Shock FC = CYA × 40% Example: at CYA 50, minimum FC = 3.75 ppm, target 5–7 ppm, and shock = 20 ppm.
| CYA (ppm) | Minimum FC | Target FC | Shock FC |
|---|---|---|---|
| 30 | 2.25 ppm | 3–5 ppm | 12 ppm |
| 50 | 3.75 ppm | 5–7 ppm | 20 ppm |
| 80 | 6 ppm | 7–9 ppm | 32 ppm |
These values define every FC minimum, target, and shock number used across the site. The complete chart at every CYA value from 0–100 is on the FC/CYA chart, and the underlying reason CYA changes the target is explained in FC/CYA ratio.
pH, alkalinity, and calcium targets
Our standing target ranges, used consistently on every guide and calculator:
| Parameter | Target | Notes |
|---|---|---|
| pH | 7.4–7.8 | 7.5–7.6 sweet spot. Fix alkalinity first when both are off. |
| Total alkalinity | 60–120 ppm | 60–80 ppm for saltwater pools to slow pH rise. |
| Calcium hardness | 200–400 ppm (plaster) | 150–250 ppm for vinyl/fiberglass. Biggest single LSI lever. |
Langelier Saturation Index (LSI)
We calculate LSI from the values you actually test — pH, calcium hardness, total alkalinity, water temperature, and total dissolved solids (TDS) — using the full formula, not a simplified approximation. We also apply the CYA correction to carbonate alkalinity:
The full equation, the temperature and TDS factors, and a per-factor breakdown are shown on the LSI explainer and computed live by the LSI calculator. Two calculators can return different LSI values for the same water; the four reasons why are documented in why LSI calculators disagree.
We do not let you "tune" inputs to hit a perfect LSI. The correct method is to test your water first, then calculate LSI from those real measurements. The Pool & Hot Tub Alliance specifically warns against adjusting inputs just to reach a desired index, because that is inconsistent with the ANSI/APSP/ICC-11 standard. Our tools follow that approach.
Dosing assumptions and limits
- Doses assume well-mixed water and the product strength you select (e.g. 10% vs 12.5% liquid chlorine, 31.45% vs 14.5% muriatic acid).
- Target ranges are general guidance, not a substitute for testing your own water. Always test before dosing, and re-test after.
- Climate matters. High-evaporation, hard-fill-water, and high-UV pools (e.g. desert pools) often need different CYA and calcium targets than national averages; those cases are called out where relevant.
- Calculators round to practical measuring increments; small differences from a hand calculation are expected.
What this site is and isn’t
This is plain-language pool-owner education built on established chemistry and industry standards. It is not professional water-treatment consulting, and it is not medical or safety advice. For commercial pools, unusual water sources, or health-related questions, consult a licensed pool professional or your local health authority.
Who writes and maintains this
PoolChem Tracker is written and maintained by Jeffrey Thompson, a pool owner and developer who has published 50+ guides on pool chemistry. I am not a chemist and do not claim to be one — everything here is built on established pool-chemistry principles, the industry standards listed above, and years of testing, tracking, and maintaining a real saltwater pool in the California desert.
Found something that looks wrong or out of date? I read every email and correct errors: support@poolchemtracker.com
See the chemistry in action
PoolChem Tracker applies these same formulas to your actual readings and tracks them over time.
