Pool Stabilizer Calculator

Almost everything in pool water can be moved in both directions. Chlorine is consumed continuously. pH drifts and is pushed back. Alkalinity goes up with bicarbonate and down with acid. Even calcium hardness, which also accumulates, at least does so slowly and predictably.

Cyanuric acid is different. It is not consumed by sunlight, by chlorine, or by anything living in the water. It does not evaporate. Sand, cartridge and DE filters all pass it straight through. There is no additive that reliably destroys it — the products sold for the purpose are enzyme or bacterial preparations with poor and inconsistent results.

So it accumulates, monotonically, for as long as anything keeps adding it. And the things that add it are the most convenient chlorination methods available: tablets in a floater and granules in a scoop.

The practical consequence is that cyanuric acid deserves more attention than its stability suggests. Chlorine wrong today is fixed today. Cyanuric acid wrong today is a drain.

Cyanuric acid earns its place. Free chlorine in an unstabilised outdoor pool has a half-life measured in tens of minutes under strong summer sun — a pool can lose most of its sanitiser in an afternoon. Cyanuric acid binds chlorine reversibly into compounds that ultraviolet light does not destroy, and 30 to 50 ppm cuts that loss dramatically.

The mechanism is also the limitation. At any moment most of the "free chlorine" a test reports is held in that bound reserve rather than acting as sanitiser. The active fraction — hypochlorous acid — is what kills things, and raising cyanuric acid lowers the proportion of the reading that is active.

That is why the CDC’s Model Aquatic Health Code specifies 2 to 4 ppm of free chlorine where cyanuric acid is present against 1 to 3 without it, why it caps CYA for fecal incident response at 15 ppm, and why 300 ppm triggers immediate remediation. The published disinfection times for chlorine assume little or no stabiliser; with a lot of it, the same reading takes considerably longer to do the same job.

For a residential pool the practical target is 30 to 50 ppm for a chlorine pool and 60 to 80 for a salt water pool, whose cell produces chlorine continuously and benefits from more protection. An indoor pool sees no ultraviolet at all and should have none: there is nothing to protect against and only the downside.

Granular cyanuric acid dissolves badly. It is a fine white powder that sinks, sits on the floor, and takes three to seven days to fully enter solution — longer in cool water. A test taken two or three days after dosing will read almost unchanged.

What happens next is predictable. The owner concludes the dose was too small and adds the same amount again. A week later the pool tests at double the target, and there it stays until several thousand gallons are replaced.

The fix is procedural rather than chemical. Put the powder in an old sock or a stocking, hang it in front of a return jet where the flow is strongest, and squeeze it a few times over the following days. Then leave the test kit alone for a week.

Liquid stabiliser avoids the problem entirely — it dissolves within hours and reads on the same day — at roughly 40% concentration and several times the price per ppm delivered. For anyone who has been caught by the granular trap once, it is frequently worth it.

One more detail: never broadcast granular stabiliser over a vinyl liner. It is mildly acidic and it will bleach the liner where it settles, permanently.

Since dilution is the only route, the arithmetic is simple and the logistics are not.

Replacing a fraction of the water replaces exactly that fraction of the cyanuric acid, so going from 120 to 50 ppm means replacing (120 − 50) ÷ 120 = 58% of the pool. On 20,000 gallons that is nearly 12,000 gallons out and back in.

Do it in stages. A pool drained empty can float out of the ground where the water table is high — a genuinely catastrophic and surprisingly common failure — and a vinyl liner that has dried and shifted rarely sits back correctly. Lowering a foot at a time and refilling between is slower and much safer.

Budget for the rebalancing. Fresh fill water arrives with its own alkalinity, hardness and pH, none of which will match the pool. On a heated pool, budget for reheating several thousand gallons as well, which in a cool month can exceed the cost of the water.

Two alternatives are worth checking before committing. Reverse osmosis pool water treatment is offered as a mobile service in some markets: it strips cyanuric acid, salts and total dissolved solids while keeping the water, which means keeping the heat and much of the balance. And in climates with heavy seasonal rain, a pool that overflows regularly dilutes itself for free — which is worth knowing before paying for a drain in October.