Tell it what's too high and your target — get the percentage to replace, gallons, water cost, and hose time.
The dilution math
% to replace = (current − target) ÷ (current − fill water level)
Dissolved levels dilute in exact proportion to the water you swap — the one pool problem with genuinely simple math. The fill-water term matters mostly for calcium: if your tap runs 200 ppm calcium, no amount of swapping gets the pool below that.
A worked example
CYA at 120, target 50, in 15,000 gallons: (120−50)÷120 = 58% — about 8,750 gallons. At $6 per thousand that's ~$53 of water and roughly 15 hours of hose time, best done as two 30% stages a day apart. Fifty dollars to make chlorine work properly again is the best deal in pool care — the alternative is running triple chlorine doses all season against muted sanitizer.
Order of operations after the refill
Fresh water dilutes everything, so after refilling run the full report card: you'll typically re-dose some salt or alkalinity you didn't mean to lower. Budget the whole project with the fill cost calculator if your utility charges sewer on top.
Frequently asked questions
How much water do I drain to lower CYA?
Dilution is linear: to cut any dissolved level (CYA, salt, calcium) by X percent, replace X percent of the water. CYA at 120 targeting 60 means replacing 50% of the pool.
Is it safe to drain my pool halfway?
For most vinyl and fiberglass pools, avoid draining below the shallow-end floor without professional guidance — liners shift and shells can float with high groundwater. The safer pattern for big corrections is two or three partial swaps of 25–33% each.
Can I lower CYA without draining?
Practically, no — no additive removes it at pool scale reliably. Water replacement is the fix, which is why not overshooting CYA (and not using stabilized chlorine pucks endlessly) matters so much.
Do I drain and refill at the same time?
The simultaneous method (drain from the deep end while the hose refills) avoids low-water risks and keeps the pool swimmable, but it's less efficient — fresh water mixes with old and some goes straight down the drain. Budget ~20% extra replacement volume if you do it that way.