Pool Chlorine Calculator

Everything on a pool store shelf marked "chlorine" delivers hypochlorous acid to the water, and that is where the similarity ends. They differ in strength by a factor of nine, in how they move pH, and — most consequentially — in what they leave behind.

Liquid chlorine is sodium hypochlorite, usually 10 or 12.5% trade strength. It adds chlorine and a small amount of salt, and nothing else. Cal-hypo is calcium hypochlorite at 65 or 73%, and every dose adds calcium hardness. Dichlor and trichlor are chlorinated isocyanurates: they carry their own stabiliser, which is either the feature or the problem depending on how much is already in the pool.

For a single dose the choice barely matters. Across a season it decides how the pool behaves, because two of the three side effects accumulate without limit and neither can be removed by any means except replacing water.

The practical consequence is that the right product depends on what the pool already has. A soft-water vinyl pool benefits from cal-hypo’s calcium. A pool at 30 ppm of cyanuric acid can use tablets for a while. A pool already at 80 ppm cannot use tablets at all without making the problem worse.

Cyanuric acid does something genuinely useful: it binds chlorine reversibly, shielding it from ultraviolet light. Without it an outdoor pool loses most of its free chlorine in a few hours of summer sun. With 30 to 50 ppm, that loss falls dramatically.

The cost is that most of the chlorine is bound at any given moment. Free chlorine as measured by a test kit includes both the bound reserve and the small active fraction, and the ratio between them is set by the CYA level. Raise CYA and the same reading represents progressively less working sanitiser.

This is why the CDC’s Model Aquatic Health Code specifies a higher minimum free chlorine — 2 to 4 ppm rather than 1 to 3 — once cyanuric acid is present, and why it treats 300 ppm as requiring immediate remediation. It is also why a pool that tested fine all June starts growing algae in August with the same numbers on the test strip: the numbers did not change, but what they mean did.

The trap is that the most convenient chlorination method is the one that causes it. Tablets in a floater require no attention, dissolve steadily, and add 0.62 ppm of stabiliser for every ppm of chlorine. Four months of that on a pool consuming 2 ppm a day is around 150 ppm of cyanuric acid — enough to take a well-stabilised pool well past any useful level.

And it is one-way. Cyanuric acid is not consumed, does not evaporate, and is not removed by any filter. Draining and refilling is the correction, occasionally supplemented by reverse osmosis water treatment where that service exists.

Two habits make dosing reliable, and both are about measurement rather than chemistry.

The first is knowing the volume. Every calculation on this page is directly proportional to it, and estimates are routinely 20% out — which means doses are routinely 20% out in the same direction all season. Measuring a pool once, properly, from its shape and depths, is worth more than any amount of care with the scoop.

The second is testing with something that resolves the range you care about. Test strips are adequate for a rough check and unreliable above about 5 ppm, which is exactly where you need accuracy when correcting a problem. A drop-based FAS-DPD kit reads free chlorine in 0.2 ppm increments to any level, and it also measures combined chlorine, which strips cannot do at all.

Beyond that, the mechanics are simple: pump running, broadcast over the deep end rather than poured in one spot, one product at a time, and a full turnover before retesting. Granular products should be pre-dissolved in a bucket for vinyl pools, where undissolved granules sitting on the liner will bleach it.

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