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Hold pool pH between 7.2 and 7.8, ideally 7.4 to 7.6, and free chlorine at 2 to 4 ppm. The CDC's healthy swimming guidance sets the operating floor at 1 ppm free chlorine for pools and 3 ppm for hot tubs, with pH 7.2 to 7.8 in both. Free chlorine must scale with cyanuric acid: hold roughly 7.5 percent of your stabilizer reading, so 50 ppm cyanuric acid needs about 4 ppm chlorine. Also target total alkalinity 80 to 120 ppm, calcium hardness 200 to 400 ppm, and cyanuric acid 30 to 50 ppm.
What are the full target ranges?
| Reading | Target | Acceptable range | Test how often |
|---|---|---|---|
| Free chlorine | 2 - 4 ppm | 1 - 10 ppm (scale to cyanuric acid) | 2 - 3x per week |
| Combined chlorine | 0 ppm | Below 0.5 ppm | Weekly |
| pH | 7.4 - 7.6 | 7.2 - 7.8 | 2 - 3x per week |
| Total alkalinity | 80 - 120 ppm | 60 - 180 ppm | Weekly |
| Calcium hardness, plaster | 250 - 350 ppm | 200 - 400 ppm | Monthly |
| Calcium hardness, vinyl | 175 - 250 ppm | 150 - 300 ppm | Monthly |
| Cyanuric acid, chlorine pool | 30 - 50 ppm | 20 - 80 ppm | Monthly |
| Cyanuric acid, salt pool | 60 - 80 ppm | 50 - 90 ppm | Monthly |
| Salt, salt pool | 3,000 - 3,400 ppm | 2,700 - 3,900 ppm | Monthly |
| Total dissolved solids | Below 1,500 over fill | — | Seasonally |
| Phosphates | Below 200 ppb | Below 500 ppb | If algae recurs |
Hot tub targets differ: free chlorine 3 to 5 ppm or bromine 4 to 6 ppm, calcium hardness 150 to 250 ppm, and the same pH band.
Why does chlorine target depend on cyanuric acid?
Because stabilizer binds chlorine to protect it from sunlight, and bound chlorine is not immediately available to sanitize. The higher the cyanuric acid, the more total chlorine you need to keep the same amount of active chlorine in circulation. The widely used working rule is to hold free chlorine at about 7.5 percent of the cyanuric acid reading.
| Cyanuric acid | Minimum FC | Target FC | Shock level FC |
|---|---|---|---|
| 0 ppm (indoor) | 1 ppm | 2 - 3 ppm | 10 ppm |
| 30 ppm | 2 ppm | 3 - 4 ppm | 12 ppm |
| 40 ppm | 3 ppm | 4 - 5 ppm | 16 ppm |
| 50 ppm | 4 ppm | 5 - 6 ppm | 20 ppm |
| 60 ppm | 5 ppm | 6 - 7 ppm | 24 ppm |
| 80 ppm | 6 ppm | 7 - 8 ppm | 31 ppm |
| 100 ppm | 8 ppm | 9 - 11 ppm | 39 ppm — dilute instead |
Two consequences follow. A pool testing at 3 ppm chlorine is well sanitized at 30 ppm stabilizer and badly undersanitized at 90 ppm, which is why the same reading means different things in different pools. And once cyanuric acid climbs past about 90 ppm, the required chlorine level becomes impractical, and the fix is a partial drain rather than more chemicals. This is the most common hidden cause of a pool that keeps losing chlorine.
In what order should I adjust things?
- Total alkalinity first. Alkalinity buffers pH, so adjusting pH before alkalinity means it drifts straight back. Raise with sodium bicarbonate, lower with muriatic acid added slowly with the pump running.
- pH second. Once alkalinity is in band, pH becomes easy to hold. Aim for 7.4 to 7.6.
- Calcium hardness third. Raise with calcium chloride; the only way to lower it is dilution.
- Cyanuric acid fourth. Add through the skimmer with the pump running. It dissolves slowly, so do not re-test for several days.
- Chlorine last, to the level your stabilizer reading requires.
- Salt, if applicable, once everything else is settled.
Never add two chemicals at once. Add one, run the pump for at least one full turnover so it mixes, then re-test before adding the next. Chlorine and acid in particular must never meet in concentrated form.
What is water balance and does the LSI matter?
Water balance is the combination of pH, alkalinity, calcium hardness and temperature that determines whether the water is aggressive, neutral or scale-forming. The Langelier Saturation Index expresses it as a single number, with roughly -0.3 to +0.3 considered balanced. Negative means the water is hungry for calcium and will take it from plaster, grout and metal. Positive means it will deposit calcium as scale on the tile line, the heater exchanger and the salt cell.
For a vinyl pool the index matters less, since there is no plaster to dissolve, but the corrosion side still applies to heaters, ladders and pump seals. For a plaster or pebble pool it matters a great deal, and it is why the same pH reading is acceptable in one pool and not another. Free online LSI calculators take the five inputs and are worth using seasonally, particularly as water temperature swings.
What kit gives readings you can trust?
Test strips are convenient and adequate for a routine free chlorine and pH check. They are unreliable above about 5 ppm chlorine and for cyanuric acid, which are precisely the readings that drive expensive decisions. A drop-based FAS-DPD kit measures free and combined chlorine separately, which is the distinction that tells you whether you have a sanitizer problem or an organic load problem.
Replace reagents annually and store them cool and dark, because degraded reagents read low. And watch for DPD bleach-out at high chlorine: the sample flashes pink then goes clear, which looks like zero. Dilute half and half with distilled water, retest, and double the result. More on diagnosing persistent problems in why your pool keeps losing chlorine and why pool water is cloudy after shocking.
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