Enter your pool volume, current CYA, and target CYA to find out exactly how many gallons to drain and replace with fresh water. Target CYA is 30–50 ppm for most pools.
Cyanuric acid (CYA) is extremely stable in pool water. Unlike chlorine, it is not consumed by UV light, oxidation, or bather use. It doesn't volatilize, settle out, or degrade over the pool season. Once it's in the water, it stays — the only way to reduce it is to physically remove water that contains CYA and replace it with water that doesn't.
If you've been using trichlor tablets or dichlor granules, CYA has been building up with every dose. Each pound of trichlor adds approximately 0.6 ppm of CYA per 10,000 gallons. Switching to liquid chlorine stops further buildup, but it won't lower the CYA that's already there — only draining will do that.
Use a submersible pump at the lowest point to drain the most CYA-concentrated water first. Refill from a hose away from the drain. Run the circulation pump for at least 24 hours after refilling before retesting — CYA takes time to distribute evenly throughout the pool.
CYA dilution follows a simple mass balance: if you remove X% of the water and replace it with zero-CYA tap water, you remove X% of the CYA. The math assumes your tap water contains no CYA, which is true for virtually all municipal and well water supplies.
A 15,000-gallon pool has CYA at 90 ppm. Target is 40 ppm.
Drain fraction = 1 − (40 ÷ 90) = 55.6%. Gallons to drain = 15,000 × 0.556 = 8,333 gallons. Since 56% exceeds 50%, a staged approach is easier:
Yes. Cyanuric acid is chemically inert in pool water — there is no product that breaks it down. The only effective method is dilution: drain a portion of the pool and replace with fresh water. Partial drains work proportionally; a 50% drain cuts CYA in half.
For standard chlorine pools: 30–50 ppm. At this range, CYA protects chlorine from UV degradation while keeping your minimum FC requirement manageable (FC minimum = CYA × 7.5%). Above 80 ppm, the required FC level climbs high enough that most pool owners end up chronically underchlorinated without realizing it. For saltwater/SWG pools, 60–80 ppm is typical.
No. Municipal tap water does not contain cyanuric acid. Neither does well water or reclaimed water in typical cases. CYA in your pool comes exclusively from chlorine products — trichlor tablets, dichlor granules, or stabilized shock. Switching to liquid chlorine after the drain prevents CYA from climbing again.
Run the pump for at least 24 hours after completing the refill before testing. CYA distributes slowly through the water column and takes time to reach a uniform concentration. CYA test kits also have a wide margin of error near the edges of their readable range — confirm with a second reading a day later if you're close to your target.
Retest all your chemistry after refilling and adjust from there — don't assume shock is needed. A partial refill dilutes everything: CYA, calcium hardness, total alkalinity, and chlorine. Check FC first; with a lower CYA level your minimum FC target is lower, so your existing chlorine may be sufficient. Rebalance TA and pH before adding chlorine if they've shifted.
The most common cause of high CYA is trichlor tablet use over multiple seasons. Each tablet adds a small amount of CYA, and it accumulates with no natural exit. Once you've drained to target, switch to liquid chlorine (10% or 12.5% sodium hypochlorite) for routine dosing — it adds only chlorine, no CYA, no calcium. Reserve trichlor for situations where liquid storage is impractical.
PoolChem Tracker logs every reading so you can see how CYA builds across the season — and get a heads-up before it climbs high enough to require another drain.
