Ideal pool chemical levels: a chart and what each one does
Pool water chemistry comes down to six numbers: free chlorine, pH, total alkalinity, calcium hardness, stabilizer (CYA) and, in saltwater pools, salt. Here are the usual target ranges, what each one does, and the order to fix them in when several are off.
The chart
| Test | Common target | Acceptable range | Adjust with |
|---|---|---|---|
| Free chlorine (FC) | About 7.5% of CYA or more (e.g. 3+ ppm at 40 ppm CYA) | CDC minimum: 1 ppm, or 2 ppm with stabilizer | Chlorine calculator |
| Combined chlorine (CC) | 0 | 0.5 ppm or less | Raise FC (shock) until it clears |
| pH | 7.4–7.6 | CDC: 7.0–7.8 | pH calculator |
| Total alkalinity (TA) | 80–100 ppm (60–80 for salt pools) | 60–120 ppm | Alkalinity calculator |
| Calcium hardness (CH) | Plaster: 250–450 ppm. Vinyl/fiberglass: 150–250 ppm | Depends on surface and LSI | Calcium calculator |
| Stabilizer (CYA) | 30–50 ppm (salt pools often 60–80) | 0 indoors; keep under about 80–100 outdoors | CYA calculator |
| Salt (saltwater pools only) | Your generator's target, often about 3,200 ppm | Per the manual (commonly 2,700–3,600) | Salt calculator |
| Saturation index (LSI) | About 0 | −0.3 to +0.3 | LSI calculator |
The chlorine and pH minimums come from the US Centers for Disease Control and Prevention's guidance for home pools. The other ranges are common industry and pool-owner targets: test kit makers, chemical labels and pool professionals give similar but not identical numbers. Your equipment's manuals (especially for salt cells and heaters) take priority where they differ.
What each number does
Free chlorine: the sanitizer
Free chlorine kills germs and algae and breaks down swimmer waste. It's used up continuously by sunlight, swimmers and debris, so it needs topping up every day or two, or a steady supply from a salt cell or tablet feeder. The level you need depends on your stabilizer: CYA holds most of the chlorine in reserve, so higher CYA means you need more free chlorine for the same effect. Free chlorine and stabilizer explained covers this in detail.
pH: how acidic or basic the water is
pH affects comfort, how well chlorine works, and whether the water attacks or coats surfaces. As pH rises, chlorine becomes less effective. The CDC notes this is especially true above 8.0. Low pH stings eyes and etches plaster and metal.
Total alkalinity: the buffer
Alkalinity, mostly bicarbonate, resists pH changes. Too low and pH swings with every dose of chlorine or acid. Too high and pH keeps drifting upward, because the water gives off carbon dioxide.
Calcium hardness: protection for plaster
Water that's low in calcium pulls calcium out of plaster, grout and pebble finishes. Water that's high in calcium, especially when warm or at high pH, deposits it as scale.
Stabilizer (cyanuric acid)
Without stabilizer, sunlight can destroy most of the chlorine in an outdoor pool within a few hours. CYA protects it, but too much makes chlorine sluggish and shocking expensive.
Salt
Only for pools with a salt chlorine generator, which makes chlorine from dissolved salt. The cell needs salt within its design range to work properly and last.
The order to fix things in
- Make sure the volume is right. Every dose depends on it. Use the pool volume calculator.
- Stabilizer (CYA), because it sets how much chlorine you need. If it's far too high, a partial drain comes before anything else.
- Total alkalinity, because it controls how stable pH is.
- pH. Bring it into range after alkalinity.
- Calcium hardness. Slow-moving, and mostly matters for plaster and scale.
- Free chlorine last, at the level your CYA calls for. Exception: if the water is green, cloudy or unsafe, raise chlorine right away and balance the rest afterwards.
Add one chemical at a time, with the pump running, and give each an hour or more to circulate before adding the next. Never mix chemicals together. See pool chemical safety.
How often to test
| Test | How often |
|---|---|
| Free chlorine, pH | At least two or three times a week, daily in hot weather or heavy use. The CDC recommends testing home pools at least twice a day when in use. |
| Total alkalinity | Weekly |
| Stabilizer (CYA), calcium hardness | Monthly, and after refilling or heavy rain |
| Salt | Monthly, and after adding water |
Our pool water testing guide compares test strips and drop kits.
Frequently asked questions
What's the single most important number?
Free chlorine, in proportion to your stabilizer. Most green and cloudy pools come from chlorine running too low for the CYA level, not from pH or alkalinity being slightly off.
Do I need to keep every number in the "ideal" column?
No. Chlorine and pH need regular attention. The rest change slowly, and anywhere in the acceptable range is fine as long as the LSI is near zero and the water stays clear.
Are hot tub levels the same?
Not quite. The CDC recommends at least 3 ppm free chlorine in hot tubs, and the heat changes the water balance. This site focuses on pools.
Source: CDC, Home Pool and Hot Tub Water Treatment and Testing.