Is your pool's free chlorine adequate for its cyanuric acid level? Calculate the FC/CYA ratio instantly to ensure effective sanitation based on MAHC and CDC guidelines.
Enter your chlorine and stabilizer levels below to see whether your pool is properly protected against pathogens.
Typical range: 1-5 ppm
Typical range: 30-50 ppm
Commercial pools may have stricter CYA limits
7.5%
Adequate Protection
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Your chlorine level is adequate.
The FC/CYA ratio is the relationship between your pool's free chlorine (FC) level and its cyanuric acid (CYA, also called stabilizer or conditioner) level. It is expressed as a percentage: FC divided by CYA times 100. This ratio is the single most important indicator of whether your chlorine can actually kill pathogens in the water.
Cyanuric acid protects chlorine from UV degradation, which is essential for outdoor pools. However, CYA also binds to chlorine molecules, reducing the amount of "active" hypochlorous acid (HOCl) available to kill bacteria, viruses, and algae. The higher the CYA, the less effective each ppm of chlorine becomes. A pool with 2 ppm FC and 30 ppm CYA has far more killing power than a pool with 2 ppm FC and 150 ppm CYA, even though the FC test reads the same.
The widely-accepted minimum FC/CYA ratio is 7.5% (FC = CYA x 0.075). Below this threshold, chlorine cannot maintain adequate sanitation, and the pool becomes vulnerable to algae blooms and pathogen survival. The recommended target is 10% (FC = CYA x 0.10), which provides a safety margin for fluctuations in chlorine demand.
When you add chlorine to pool water, it forms hypochlorous acid (HOCl), the active sanitizer. HOCl is a powerful oxidizer that destroys bacteria, viruses, and organic contaminants on contact. However, when CYA is present, most of the chlorine binds to cyanuric acid molecules, forming chlorinated isocyanurates. These bound chlorine molecules are in constant equilibrium with HOCl, but only a small fraction is "free" and active at any given moment.
Think of it like a reservoir: CYA acts as a slow-release mechanism. While this protects chlorine from sunlight, it also dramatically reduces the instantaneous killing power. At typical pool conditions (pH 7.5, 80 degrees F):
This is why simply reading "3 ppm FC" on a test kit does not tell the whole story. The CYA level determines how much of that 3 ppm is actually working.
Free chlorine in water exists in three forms, and the balance between them determines sanitizing power:
The FC/CYA ratio accounts for this binding effect. A 7.5% ratio ensures enough HOCl is available at any given moment to maintain a minimum CT (concentration x time) value sufficient for pathogen inactivation. Research by the CDC and others has confirmed that this ratio corresponds to approximately 0.05 ppm active HOCl, which is sufficient to inactivate most common pool pathogens within acceptable timeframes.
pH also affects the HOCl/OCl- balance. At pH 7.2, about 63% of free chlorine (excluding CYA effects) is HOCl. At pH 7.5, it drops to about 50%. At pH 7.8, only about 33% is HOCl. This is why maintaining proper pH (7.4-7.6) is important alongside the FC/CYA ratio. The calculator uses pH 7.5 as the standard reference point for the effectiveness bar.
The Model Aquatic Health Code (MAHC), developed by the CDC, provides guidelines for commercial aquatic facilities. Key provisions related to FC and CYA include:
If your commercial pool operates under MAHC guidelines, maintaining the FC/CYA ratio above 7.5% is not just best practice, it is a regulatory requirement. CYA levels above 90 ppm typically require partial drain and dilution.
CYA regulations vary significantly across the United States. Many states and counties set their own maximum CYA levels for commercial pools:
Always check your local health department regulations for the specific CYA limits that apply to your pools. Residential pools are generally not subject to CYA regulations, but the science behind the limits still applies to home pool sanitation.
As CYA accumulates in a pool (it does not break down or evaporate), you face a choice: keep raising FC to maintain the ratio, or dilute the CYA. Here are guidelines:
Draining 25-30% of the pool water and refilling with fresh water reduces CYA by roughly 25-30%. You may need to repeat this process or drain more aggressively for very high CYA levels.
"Chlorine lock" is one of the most persistent myths in the pool industry. The claim is that at a certain CYA level, chlorine becomes completely "locked up" and cannot sanitize at all. This is false. The chemistry does not support a lock mechanism at any CYA level.
What actually happens is a gradual, proportional reduction in chlorine effectiveness as CYA increases. There is no magic threshold where chlorine suddenly stops working. At 200 ppm CYA, chlorine is severely reduced in effectiveness, but it still works. You would simply need a very high FC level (15+ ppm) to maintain the 7.5% ratio, which is impractical.
The myth likely arose because pool owners with high CYA see algae blooms despite "normal" FC readings. They conclude chlorine has stopped working entirely. In reality, the FC/CYA ratio was simply too low, and there was not enough active HOCl to prevent algae growth. Raising FC to match the CYA level (or lowering CYA) resolves the issue every time.
Bottom line: there is no chlorine lock. There is only an insufficient FC/CYA ratio.
What FC/CYA ratio should I target?
For residential pools, target a minimum of 7.5% (FC = CYA x 0.075) and ideally 10% or higher (FC = CYA x 0.10). The 10% target provides a buffer for chlorine consumption between service visits. For commercial pools, follow your local health code, which may require specific FC levels based on CYA concentration per the MAHC table.
My FC tests fine at 3 ppm but I keep getting algae. Why?
This almost always indicates high CYA. If your CYA is 80+ ppm, then 3 ppm FC gives you a ratio of only 3.75%, well below the 7.5% minimum. Your chlorine test reads "fine" but there is not enough active HOCl to prevent algae. Either raise FC to match your CYA (at 80 ppm CYA, you need at least 6 ppm FC) or drain and dilute to lower CYA.
Can I use this calculator for saltwater pools?
Yes. Saltwater pools produce the same chlorine chemistry (HOCl) as any other chlorine pool. The FC/CYA ratio applies equally. Saltwater pools that use stabilized chlorine for supplemental dosing or have CYA added separately should monitor CYA levels just like any other pool. One advantage of salt systems is that the generated chlorine does not add CYA, so CYA levels tend to be more stable.
Does CYA break down over time?
CYA is extremely stable in pool water. It does not break down from UV exposure, chlorine oxidation, or normal chemical processes. The only practical ways to reduce CYA are dilution (draining and refilling), backwashing (if using a DE or sand filter), and splash-out. Some specialty products claim to break down CYA biologically, but results are inconsistent. Plan on dilution as your primary CYA reduction method.
Why does using trichlor tabs raise my CYA so fast?
Trichlor (trichloroisocyanuric acid) is approximately 54% available chlorine and 46% cyanuric acid by weight. Every time you add trichlor, you add CYA. A typical residential pool using trichlor as the sole sanitizer can see CYA rise 10-15 ppm per month during swimming season. This is why many pool professionals recommend liquid chlorine (sodium hypochlorite) or cal-hypo as the primary sanitizer, with CYA added separately to maintain the target level.
What if my pool has zero CYA?
If CYA is 0, the FC/CYA ratio is undefined. Without CYA, your chlorine will be fully active (no CYA binding), but it will also degrade rapidly from UV sunlight. An outdoor pool with no CYA can lose 90% of its chlorine in 2-3 hours of direct sunlight. For outdoor pools, add CYA to 30-50 ppm. For indoor pools, CYA is generally not needed since there is no UV exposure, and lower FC levels (1-2 ppm) are sufficient.
PoolDial logs every chemical reading, flags FC/CYA ratio issues automatically, and recommends exact dosing adjustments. Stop guessing and start managing water chemistry with precision.
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