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, so 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.

TDS mode, a rise above the fill water: TDS climbs over time (the 2024 Model Aquatic Health Code lists why the concentration of TDS increases over time among its operator-training topics, section 6.1.2.1.2.9). Reading the target as a rise above your fill water is a basis you choose for your own records, not one the code sets. To use it, pick TDS, set the target as a rise above fill water, and enter the fill-water TDS and the rise you allow. The target is built from them and the same identity runs:

target        = fillWater + allowedRise
drainFraction = (current − target) / (current − fillWater)
              = 1 − allowedRise / (current − fillWater)

Example with figures picked for the arithmetic, not as a recommendation: TDS 2,400 over fill water at 400, with 1,000 allowed above fill, gives a target of 1,400 and 1 − 1,000 / 2,000 = 0.5, so half of a 20,000 gal pool, 10,000 gal. This page asserts no allowed-rise figure: the CDC's 2024 Model Aquatic Health Code has TDS tested quarterly at all aquatic venues (section 5.7.5.9) and its code language sets no TDS limit, so the number is yours.

Public pools have a CYA ceiling and a closure line. The same code says the CYA concentration at all aquatic venues shall remain at or below 90 ppm (section 5.7.3.1.3.2), and that a venue with CYA at 300 ppm or higher shall remain closed until immediate remediation has occurred (5.7.3.1.3.3). A CYA reading is flagged against both lines; the drain math does not change. The MAHC is a model code for public venues, so the code your health department has adopted is the one that binds the site.

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); CDC, 2024 Model Aquatic Health Code, 5th edition (sections 5.7.3.1.3.2, 5.7.3.1.3.3, 5.7.5.9 and 6.1.2.1.2.9).