CYA Reduction Calculator

Cyanuric acid is a stable triazine ring. Chlorine does not consume it, sunlight does not break it down at any useful rate, and it does not evaporate. Every ppm you have ever added is still in the pool unless water left.

That makes the arithmetic unusually honest for pool chemistry. Replace half the water and you halve the CYA. There is no dose, no shock level and no run time that changes it.

It also means the number only ever goes up on its own. A pool on trichlor tablets gains about 0.6 ppm of CYA for every 1 ppm of chlorine delivered, so a pool using 2 ppm a day picks up roughly 36 ppm a month. That is a season taking a pool from 30 ppm to past 100 without anybody doing anything wrong.

This is the part that costs people thousands of gallons, and almost nothing says it.

Running a hose while you drain is tempting. It keeps a vinyl liner wet, it keeps the water level up, and it feels efficient. But the moment you mix fresh water in, you start diluting the water you are still pumping out — so every gallon you remove carries less CYA than the last, and the exchange stops being a straight swap.

The difference is large. Taking a pool from 110 ppm to 40 means replacing 64% of the volume if you drain and then refill. Do it with the hose running and you need to exchange 101% — the entire pool over again — for the same result. On 20,000 gallons that is 12,700 gallons against 20,200, and 7,500 gallons of the difference is water you paid for and threw away.

The penalty grows the further you are going: 100 to 50 costs 39% extra, 120 to 30 costs 85% extra. Drain to your mark, stop, refill, retest. The calculator above assumes you do it that way.

The water has to come out in stages, and the reason is structural rather than chemical.

A pool is held down by the water in it. Take too much out at once and groundwater pressure can float a fiberglass shell or lift a concrete one, and a vinyl liner can shrink, wrinkle or pull out of its track. The full explanation of why draining is the dangerous move applies here in full — this is a partial drain done deliberately, not an empty pool.

A third at a time is the usual rule. Lower by a third, refill, let it circulate for a few hours, retest, repeat if the number is still high. Slower, and it never puts the shell at risk.

Two practical notes. Check your utility before you start — many charge sewer based on metered water, and many will credit or exempt a pool fill if you ask first, which can be a meaningful share of the bill. And expect the refill to bring its own chemistry: fresh water usually arrives with lower calcium hardness and different alkalinity, so plan to rebalance rather than assuming you are back to normal.

A drain fixes the number. It does not fix what produced it, and a pool that needed this once will need it again in about the same time unless something changes.

The cause is almost always stabilized chlorine. Trichlor tablets and dichlor granules both carry CYA as a passenger, and they carry it in every single dose. Nothing about that is a defect — it is what stabilized chlorine is — but it means tablets are a product with a budget rather than a routine.

The fix is to run tablets while you have headroom and switch to liquid chlorine once you are near 50 ppm. Liquid adds no stabilizer at all, which makes it the only product you can use indefinitely without arriving back here.

Test CYA monthly rather than annually. It moves slowly and predictably, which means a monthly reading gives you months of warning — and the whole point of knowing early is that partial dilution over a season is far cheaper than a big drain in August.