To lower cyanuric acid (CYA), replace the fraction of water given by 1 − (target CYA ÷ current CYA). Going from 100 ppm to 50 ppm means you replace 50% of the water: 5,000 gallons in a 10,000-gallon pool. Aim for the PHTA ideal of 30–50 ppm. Most local codes cap CYA at about 100 ppm.
That fraction assumes your fill water has no CYA and that you drain once, then refill. Several smaller drains need more water in total. The safe way to move that water depends on your pool type, covered below.
How much to drain: the quick table
| Current CYA | Target CYA | Water to replace |
|---|---|---|
| 100 ppm | 50 ppm | 50% |
| 120 ppm | 50 ppm | about 58% |
| 80 ppm | 40 ppm | 50% |
The pattern is simple: halving the CYA means replacing half the water. Going from 80 ppm to 40 ppm is the same 50% job as 100 to 50.
Worked example: a 20,000-gallon pool at 120 ppm
Say your test reads 120 ppm and you want 50 ppm. The fraction is 1 − (50 ÷ 120) = 0.583, so you replace about 58% of the water. In 20,000 gallons that is roughly 11,700 gallons out and the same amount of fresh water back in.
If you don’t know your volume, the pool volume calculator gives a starting figure: the volume calculator estimates gallons from length, width and average depth. Steps, benches and slopes make the real number different, so treat the result as an estimate.
Why one big exchange beats several small ones
Each partial exchange dilutes what is left after the previous one. Two separate 25% exchanges from 100 ppm leave about 56 ppm, not 50, because the second exchange removes a quarter of an already-diluted pool. If your pool type forces you to work in smaller steps, plan on extra rounds, and retest after each one once the water has circulated.
The formula also assumes the fill water has no CYA. City water normally has none. If you fill from another source, test it first: any CYA in the fill water means you need to replace more.
Know your pool’s limits before you drain
Pentair warns that an emptied in-ground pool can pop out of the ground where the water table is high, and that fiberglass or in-ground vinyl liner pools should never be empty, because the liner or shell can bow and crack. For those pools, do the replacement as several partial exchanges and refill straight away, accepting the extra water the table above does not show.
- Plaster and concrete pools: a large single exchange is possible, but not on a high water table. If you don’t know your groundwater conditions, ask the builder before taking the level far down.
- Pump: keep the water above the skimmer while the pump runs, or switch the pump off and drain with a separate submersible pump.
- Discharge: where pool water may go is a local rule. Check before pumping it into a street drain.
Stop the CYA from climbing back
Taylor’s numbers explain most high readings. Every pound of trichlor added to 10,000 gallons raises CYA by 7 ppm, and two 3-inch tablets weigh about a pound. Too much stabilizer leaves chlorine overstabilized, so it disinfects less effectively. Taylor’s fix is the one in this article: partial draining with fresh water, plus a temporary switch to unstabilized chlorine (liquid chlorine or cal hypo) for daily sanitizing.
After refilling, circulate, retest CYA, and adjust chlorine and pH. With stabilizer in the water, CDC recommends at least 2 ppm free chlorine and pH 7.0–7.8. Salt-generator pools are covered by the same 30–50 ppm PHTA range unless the generator manual says otherwise. Our CYA guide covers what stabilizer does, and the water testing guide covers the rest of the routine.
Sources
- Pool & Hot Tub Alliance (PHTA), published by Taylor Technologies: Recommended Water Chemistry Parameters
- Taylor Technologies: Why Monitoring Cyanuric Acid (CYA) in a Pool is Important
- Pentair: What to do with your pool after a power outage
- CDC: Home Pool and Hot Tub Water Treatment and Testing
Research checked 2026-10-08.
