CYA Drain / Dilution Calculator

Lowering CYA, calcium, salt, or TDS has no chemical "reducer" — the only lever is to drain a fraction of the water and refill with fresh. Dilution is linear, so the fraction to drain is:

drainFraction = (current − target) / (current − fillWater)
drainPercent  = drainFraction × 100
gallonsToDrain = poolVolume × drainFraction        (rounded, raw fraction)
resultingLevel = fillWater + (current − fillWater) × (1 − drainFraction)

Worked example (CYA 180 → 60 in 15,000 gal): (180 − 60) / 180 = 0.6667 → drain 66.7% → 15,000 × 0.6667 = 10,000 gal. Resulting level 180 × (1 − 0.6667) = 60.0 ppm ✓. We multiply the raw fraction, not a rounded 0.667 — rounding only the displayed percentage and gallons keeps the math drift-free.

Fill-water generalization: if your refill water already carries some of the parameter (hard well water for calcium/TDS), the denominator becomes current − fillWater — a higher fill-water level needs a bigger drain. If the fill water is at or above your reading, dilution can't reach the target.

Staged plan: when one pass would drain more than your cap (default 50%), we split it into partial drain-and-refill cycles using the (1 − f) recurrence — each cycle's resulting level feeds the next. The last cycle drains only the fraction needed to land exactly on target, so the plan never overshoots.

Ideal ranges shown are advisory, not enforced: CYA 30–50, calcium 200–400 (pool), salt ≈ 3,000–3,500 (SWG), TDS freshwater 500–2,500. TDS alone is rarely the reason to drain — confirm a specific problem first. Before you drain for CYA, check whether the reading is even a problem yet: the FC to CYA ratio calculator tells you what free chlorine that stabilizer level demands, and the cyanuric acid calculator covers the opposite move when a reading is too low. If trichlor tablets are what pushed CYA up in the first place, the tablet feeder calculator shows how quickly a given tab load reruns the same climb, and the fill and drain time calculator turns the gallons above into the hours the pump or hose will actually need. Sources: Founders Pool Service (drain formula + CYA/calcium/TDS examples); Orenda (TDS / mixing identity).