Pool chemistry chart
Every number a pool test gives you, the range to keep it in, and where that range comes from. Tap a fix to open the calculator that doses it.
| Test | Acceptable range | Aim for | If low | If high | Source |
|---|---|---|---|---|---|
| Free chlorine (FC) | ≥1 ppm (≥2 with CYA); keep ≥7.5% of CYA | CYA-based, e.g. 3–5 at 40 CYA | Add chlorine | Stop dosing; don’t swim above 10 ppm | CDC; FC/CYA chart |
| Combined chlorine (CC) | ≤0.5 ppm | 0 | — | Shock | CDC notes most health codes cap at 0.4 |
| pH | 7.0–7.8 | 7.4–7.6 | Raise pH | Lower pH | CDC |
| Total alkalinity (TA) | 60–120 ppm | 60–80 liquid/salt · 80–120 tabs | Baking soda | Acid + aeration | PHTA guidance / industry practice |
| Calcium hardness (CH) | 150–1,000 ppm | 200–400 plaster · 150–250 vinyl/fiberglass | Calcium chloride | Dilute; watch LSI | PHTA guidance |
| Stabilizer (CYA) | 0–100 ppm; none in hot tubs (CDC) | 30–50 chlorine · 60–80 salt | Add CYA | Drain & refill | CDC; Hayward & Pentair manuals |
| Salt (salt pools) | 2,700–3,400 ppm | ≈3,200 ppm | Add salt | Drain & refill | Hayward AquaRite manual |
| Saturation index (LSI) | −0.3 to +0.3 | 0 to +0.3 plaster | Raise pH/CH | Lower pH/TA | Langelier |
| Total dissolved solids | up to ~1,500 ppm above fill water | as low as practical | — | Dilute | Industry practice |
How the numbers fit together
Chlorine and stabilizer are a pair. CYA shields chlorine from sunlight but also holds most of it in reserve, so the free chlorine you need rises with CYA. A 3 ppm reading is plenty at 30 CYA and too low at 80.
pH and alkalinity are a pair. Alkalinity is the buffer; pH is the reading. High alkalinity makes pH drift up; low alkalinity makes it swing. Fix alkalinity first, then pH.
pH, alkalinity, calcium and temperature together decide whether water etches plaster or leaves scale. The LSI rolls them into one number.
Waiting out the half hour before a retest? ahaboo walks you through why the Moon has phases, a far tidier cycle than pH will ever be.
Testing schedule
| Test | How often |
|---|---|
| Free chlorine, pH | At least twice a day in use (CDC) |
| Combined chlorine | Weekly, or when it smells |
| Alkalinity, calcium | Weekly to monthly |
| Stabilizer, salt | Monthly, and after refills |
Or put a whole test into the pool chemical calculator and get every dose in order.
Questions pool owners ask
What are the ideal chemical levels for a pool?
Free chlorine: at least 1 ppm (CDC), or 2 ppm with stabilizer, and in practice about 7.5% of your CYA or more. pH: 7.0–7.8 (CDC), ideally 7.4–7.6. Total alkalinity 60–120 ppm. Calcium hardness 200–400 ppm for plaster, 150–250 for vinyl and fiberglass. Stabilizer 30–50 ppm (60–80 for salt). Combined chlorine 0.5 ppm or less.
How often should I test pool water?
CDC advises residential owners to test free chlorine and pH at least twice a day, and more often with heavy use. Total alkalinity and calcium change slowly: weekly or after big water changes is enough. Test stabilizer every few weeks and salt monthly.
What should I test first when water looks wrong?
Free chlorine and pH first, since they cause most problems. Then combined chlorine (for smell and irritation), then CYA, which decides how much chlorine you actually need.
Are test strips accurate enough?
Strips are fine for a quick daily check of chlorine and pH. For dosing decisions, and especially for CYA and chlorine above 5 ppm, a drop-based (titration) or photometer kit is far more precise. CDC recommends a DPD test kit for the most accurate chlorine results.