Cyanuric Acid Calculator: Pool Stabilizer to Add or Drain

Cyanuric acid, sold as stabilizer or conditioner, slows the sunlight that would otherwise strip chlorine out of an outdoor pool in a matter of hours. It is also the one reading on a drop kit that nothing this site doses will bring down. So this calculator answers in whichever direction your water needs. Enter a reading under your target and it weighs out the stabilizer to add; enter one above your target and it works out the share of the water to drain and refill instead. Either way it prints the lowest everyday free chlorine that your reading now, and the target you typed, each call for, so the two readings are settled together rather than one at a time. Every dose is sized from the gallons you are treating, which the pool volume calculator works out from a tape measure.

Worked example: raising CYA from 20 to 40 ppm

The figures below are the ones the calculator opens with, written out so the arithmetic can be followed. They are an example, not a recommendation for anybody's pool. A 15,000-gallon pool reads 20 ppm cyanuric acid and 3 ppm free chlorine, and the target is 40 ppm.

• The answer is 40.1 oz (2.50 lb) of stabilizer that is 100% cyanuric acid — the assay on HASA's Pool Conditioner safety data sheet, which is where the Cyanuric acid (stabilizer): % cyanuric acid on your label field gets its starting value. A weaker granule needs proportionally more.
• Both chlorine rows read 2.0 ppm. That is CDC's minimum for a home pool once cyanuric acid is in the water, and section 5.7.3.1.1.2.2 of MAHC 2024, a public-pool code, puts its own minimum there too. Dividing each reading by 45 gives 0.4 ppm at 20 ppm CYA and 0.9 ppm at 40 ppm, so neither division comes close to lifting that floor.
• The last row sets one reading against the other: 20 ppm of stabilizer to 3 ppm of chlorine is 6.7 : 1, well inside the 45:1 line described further down.

How much CYA

PHTA's technical bulletin on cyanuric acid, which is industry guidance rather than any kind of code, keeps the working band narrow: "The ideal range for cyanuric acid is 30-50 ppm" when it is used at all. The same bulletin puts maximum stabilization between 50 and 100 ppm and reports no measured gain past that, so a higher reading buys no more protection from the sun than a middling one.

Two harder lines come from MAHC 2024, which is a public-pool code and is quoted here for its figures, not because it governs a backyard pool. Section 5.7.3.1.3.2 reads: "The CYA concentration at all AQUATIC VENUES shall remain at or below 90 ppm (mg/L)." MAHC 5.7.3.1.3.3 goes further: "If the CYA level is determined to be 300 ppm (mg/L) or higher, the AQUATIC VENUES shall remain closed until immediate remediation has occurred." The calculator turns each into a warning row rather than a verdict, and the drain share it prints is the only lever either row can be answered with.

CYA and free chlorine

Two published figures decide the everyday floor for free chlorine once there is stabilizer in the water, and the calculator shows whichever of the two is higher. The table under it lists both, side by side, for a range of readings.

• CDC's home-pool page covers a pool running on stabilized chlorine, dichlor or trichlor included: "CDC recommends pH 7.0–7.8 and a chlorine concentration of at least 2 ppm in pools." The same 2.0 ppm minimum, wherever cyanuric acid is in use, appears at section 5.7.3.1.1.2.2 of MAHC 2024, a public-pool code.
• The imminent health hazards listed at MAHC 6.6.3.1, in that same public-pool code, include "AQUATIC VENUES using CHLORINE STABILIZERS where the CYA:DPD-FC ratio exceeds 45:1", which is a second floor at the reading divided by 45.

Below 90 ppm of cyanuric acid that division stays under 2 ppm, so the CDC figure is the one doing the work; at exactly 90 ppm the two meet; only above it does the ratio take over. Keep the two apart when you read the rows: one is a public-health recommendation for a home pool, the other the line at which a public venue can be ordered corrected or closed. They are quoted here because they are the clearest published figures this site was able to check, not because a backyard pool is inspected against either.

Trichlor and dichlor add CYA

PHTA's bulletins put a figure on each stabilized product. Of the tablets: "When trichlor is used 0.6 ppm of cyanuric acid is added". Of the granules: "When dichlor is used 0.9 ppm of cyanuric acid is added". Both are per ppm of available chlorine the product delivers.

The molecular weights give the same thing to another decimal place. A trichlor molecule carries three chlorine equivalents on one cyanurate ring and a dichlor molecule two, so the ring arrives at 0.607 ppm of cyanuric acid per ppm of chlorine from trichlor and 0.910 ppm from dichlor. Every tablet in the floater and every scoop of granules therefore pushes this reading up a little, which is why the chemical calculator folds that by-product into its stabilizer step instead of treating the two chemicals as unrelated.

This calculator doses no chlorine of its own, so it never prints a re-entry row. The shock calculator and the chemical calculator both do, and dichlor's is the fiddly one: the EPA Reg. No. 7124-31 label sets its limit at 3 ppm, but the saved scan of that label runs three columns together and breaks every sentence in them, so both tools attribute the figure to the label instead of quoting it. One panel of the label allows swimming at exactly 3.0 ppm and another does not, and the stricter panel is the one this site follows, so the row shows up at exactly 3.0 ppm, not only past it.

Every row this calculator can print

The first row carries the answer, in one of four forms.

• A weight of stabilizer, where the target sits above the reading. Whether there is anything to add is decided on the tenth of a ppm the row would print, so a target a few hundredths above the reading counts as no change and falls through to the fourth form below.
• A share of the water to drain and refill, where the reading sits above the target. This branch is decided on hundredths of a ppm, the precision readings are entered at, and the gallons are shown beside the percentage.
• Nothing, with the gap named, where the reading is above the target by so little that the drain would print as fewer than 1 gal or as 0.0% of the pool. The row gives the overshoot in hundredths and says it is too little to drain for, rather than offering a drain that rounds away to nothing.
• Nothing to do, where the reading is at the target, or under it by less than the tenth of a ppm a dose row would show: the row says CYA is at the target.

Under the answer sit the two free-chlorine floors, one for the reading and one for the target, each naming who sets it, and then the ratio of stabilizer to chlorine. With a chlorine reading of zero there is nothing to divide by, and that row reads No free chlorine reading above 0 instead of a number.

Three warning rows can follow, each quoting the line behind it. CYA more than 45 times free chlorine appears whenever the reading is past that ratio, and also whenever there is stabilizer in the water and no free chlorine at all, because 45 times nothing is still nothing. CYA above 90 ppm needs a reading past 90, so a pool sitting at exactly 90 ppm does not raise it; CYA at 300 ppm or higher takes its own label literally and shows at 300 as well as above it. All three quote MAHC 2024, a public-pool code, and none of them is a compliance check on your pool; they are there because the water behaves the same way whoever owns it.

Hot tubs

CDC is blunt about spas: "CDC recommends not using cyanuric acid or chlorine products with cyanuric acid in hot tubs." Elsewhere on the same page CDC says disinfection takes longer where cyanuric acid is in the water. This site sizes no doses for spas or hot tubs at all, and this calculator is written for pools.