Free Chlorine and CYA: Why Your Chlorine Target Depends on Stabilizer

What cyanuric acid does

Sunlight breaks down chlorine quickly. In an unstabilized outdoor pool, a large part of the chlorine can disappear on a single sunny day. Cyanuric acid, sold as "stabilizer" or "conditioner" and also contained in dichlor and trichlor, slows that loss by binding to chlorine.

That binding has a cost. Chlorine attached to CYA is held in reserve. It still shows up on a free chlorine (FC) test, because the test reagent pulls it loose, but at any moment most of it is not the active form that kills germs and algae quickly. As the reserve gets used, more is released, so it is not wasted. But the active share at any instant depends on the ratio of free chlorine to CYA.

Why the target moves with CYA

Think of the FC reading as total spending money and CYA as a savings account that locks some of it away. With little CYA, most of the free chlorine is available. With a lot of CYA, most of it is locked up, so you need a larger FC reading to have the same amount available.

That is why a single fixed range like "1 to 3 ppm" can be too much for an unstabilized indoor pool and too little for an outdoor pool with high stabilizer.

Two kinds of guidance

Public-health minimums. CDC recommends free chlorine of at least 1 ppm in pools and at least 2 ppm in pools that use cyanuric acid or stabilized chlorine, at least 3 ppm in hot tubs, and pH 7.0–7.8. CDC recommends not using cyanuric acid or stabilized chlorine products in hot tubs at all. These are floors for disinfection, written for a wide range of pools.

Ratio rules used by many pool owners. The Trouble Free Pool community publishes FC targets that scale with CYA. Its chart's minimum free chlorine is roughly 7.5% of the CYA level for manually chlorinated pools (a lower ratio is used for salt-water generators), and its recommended targets sit above that minimum so the level doesn't fall below it between doses. Trade press has described the same 7.5% relationship.

These are minimums, not targets. Your target is the minimum plus your pool's typical daily chlorine loss, so that the level doesn't fall below the minimum before your next dose. A water log tells you that daily loss.

Practical consequences

• High CYA makes everything harder. At 100 ppm CYA, the ratio rule calls for at least 7.5 ppm FC every day and much more to clear algae. Most owners find it easier to replace some water and bring CYA down.
• Stabilized chlorine raises CYA constantly. Each 1 ppm FC from dichlor leaves about 0.9 ppm CYA; from trichlor, about 0.6 ppm. Over a summer of tablets, CYA can climb a long way. Our chlorine calculator shows this side effect.
• Indoor pools and hot tubs get little or no sunlight, so they gain little from CYA. CDC advises against it in hot tubs.
• CYA only goes down by dilution. Draining and refilling 25% of the water lowers CYA by about 25%, if the fill water has none.

Worked example

Testing CYA

Most home kits measure CYA by how cloudy a sample becomes when reagent is added (a turbidity test). It is less precise than chlorine tests, especially at low levels, so take the reading in good light, repeat it, and treat it as approximate. Strips are a rough guide only. See how to test pool water.

Common questions

What is a good CYA level for an outdoor pool?

Recommendations vary. Many residential pool owners using liquid chlorine keep CYA around 30 to 60 ppm, and salt-water pools often run higher. Whatever level you choose, raise free chlorine to match.

Why does my pool get algae even though chlorine reads 2 ppm?

If your CYA is high, 2 ppm free chlorine may leave very little active chlorine. At 60 ppm CYA, the 7.5% rule of thumb suggests at least 4.5 ppm.