The Free Chlorine to Cyanuric Acid (FC:CYA) Relationship
In modern pool chemistry, there is no single static chlorine number. The true sanitizing power of hypochlorous acid is governed by the concentration of Cyanuric Acid (CYA).
For safe swimming, Free Chlorine must always be maintained at approximately 7.5% of your CYA level (minimum 2–4 ppm). If your CYA is 50 ppm, your target daily FC is 4 to 6 ppm. If your CYA is 80 ppm (typical for salt pools), your target FC is 6 to 9 ppm.
Comparison of Common Pool Chlorine Products
| Sanitizer Product | Active Strength | Effect on pH | Side Effects / Byproducts |
|---|---|---|---|
| Liquid Chlorine (Sodium Hypo) | 10% - 12.5% | Neutral (net zero) | Zero CYA, Zero Calcium |
| Cal-Hypo Granular (Calcium Hypo) | 65% - 73% | Slightly raises pH | Adds 0.7 ppm Calcium / 1 ppm FC |
| Dichlor Shock (Granular) | 55% - 62% | Slightly lowers pH | Adds 0.9 ppm CYA per 1 ppm FC |
| Trichlor 3-Inch Pucks | 90% | Strongly Acidic | Adds 0.6 ppm CYA per 1 ppm FC |
Frequently Asked Questions
How much liquid chlorine should I add to shock my pool?
The shock dose depends directly on your Cyanuric Acid (CYA) level. At 40 ppm CYA, your shock target is 16 ppm Free Chlorine (FC). For a 15,000-gallon pool with 1 ppm current FC, you need approximately 240 fl oz (1.88 gallons) of 12.5% liquid chlorine to reach 16 ppm FC.
What is the difference between Free Chlorine and Combined Chlorine?
Free Chlorine (FC) is active, unbonded sanitizer available to eliminate bacteria and algae. Combined Chlorine (CC) is chlorine bonded with swimmer waste, sweat, and nitrogen compounds (chloramines), which cause strong chemical odors and eye sting. CC should always be below 0.5 ppm.
Why does my pool turn green even though test strips show high chlorine?
This condition is known as "Chlorine Lock" or CYA over-stabilization. If you use trichlor pucks or granular dichlor shock, Cyanuric Acid accumulates. At CYA levels over 100 ppm, chlorine molecules become bound to stabilizer and cannot sanitize effectively, allowing algae to flourish despite high total chlorine readings.
Can I swim immediately after adding liquid chlorine?
For routine maintenance doses, you can swim once the chlorine has circulated for 15 to 30 minutes and Free Chlorine is within the safe 2 to 5 ppm range. If you performed a SLAM shock treatment with chlorine above 10 ppm, wait until FC falls below the shock threshold and water is crystal clear.
Why is Liquid Chlorine preferred over granular shock?
Liquid chlorine (10% to 12.5% strength) adds zero Cyanuric Acid and zero Calcium Hardness to pool water. Granular Cal-Hypo adds calcium (which can cause scale), while Dichlor and Trichlor add CYA (which can cause chlorine lock). Liquid chlorine delivers pure, fast-acting sanitation.
When is the best time of day to add chlorine to a pool?
Always add chlorine after dusk or in the evening. Direct ultraviolet sunlight rapidly degrades unprotected free chlorine. Adding sanitizer at night allows it to work uninterrupted for 8 to 10 hours without photolytic degradation.