Pool chemistry is taught with simple targets: maintain FC between 1–3 ppm, CYA between 30–80 ppm, pH between 7.2–7.8. The problem is that these numbers are not independent. A pool at 1 ppm FC and 30 ppm CYA is well-disinfected. The same pool at 1 ppm FC and 80 ppm CYA may be on the edge of inadequate disinfection. And a pool at 1 ppm FC and 150 ppm CYA is biologically unsafe regardless of what the FC test shows. This is the FC:CYA ratio, and it is one of the most practically important relationships in pool chemistry.
Free chlorine in water exists in two forms: hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Only HOCl is a strong disinfectant — OCl- is much weaker. The ratio of HOCl to OCl- is controlled by pH (lower pH = more HOCl).
Cyanuric acid (CYA) complicates this by forming a third chlorine species: chlorinated cyanurates (CNCl forms). These are essentially a chlorine reservoir — not actively disinfecting. CYA holds chlorine in this reserve, releasing it slowly as HOCl is consumed. This is why CYA extends chlorine life in sunlight — it protects the chlorine reserve from UV degradation.
The problem: as CYA increases, a larger fraction of your "free chlorine" reading is actually tied up in these cyanurate complexes and not actively disinfecting. The DPD test your kit uses measures total free chlorine (HOCl + OCl- + cyanurate-bound chlorine). It cannot distinguish between "killing" chlorine and "reserve" chlorine.
The research-based recommendation (from the Recreational Water Illness and Injury Prevention Work Group and subsequent studies) is:
Minimum FC = 7.5% of CYA level. This is the minimum to maintain adequate HOCl concentration for routine disinfection and algae prevention.
| CYA Level | Minimum FC (7.5%) | Recommended FC Range | Shock FC (SLAM) |
|---|---|---|---|
| 20 ppm | 1.5 ppm | 2–4 ppm | 12 ppm |
| 30 ppm | 2.3 ppm | 3–5 ppm | 15 ppm |
| 40 ppm | 3.0 ppm | 4–6 ppm | 20 ppm |
| 50 ppm | 3.75 ppm | 5–7 ppm | 24 ppm |
| 60 ppm | 4.5 ppm | 6–8 ppm | 28 ppm |
| 80 ppm | 6.0 ppm | 7–10 ppm | 31 ppm |
| 100 ppm | 7.5 ppm | Drain recommended | 40 ppm |
A pool at 80 ppm CYA needs at least 6 ppm FC for adequate disinfection — but many service programs are still targeting 1–3 ppm FC as a blanket rule. This is why pools with high CYA often develop chronic algae issues despite apparently "passing" chemistry test results. The FC number looks fine; the actual disinfection does not match.
Trichloro tablets (chlorine pucks) contain approximately 57% CYA by weight. Every pound of trichlor added to a residential pool adds about 6 oz of CYA. Pools on a consistent trichlor program accumulate CYA throughout the season. By late summer, a pool that started at 30 ppm CYA in May may be at 80–100 ppm CYA — which is why algae blooms are most common in August even on pools with regular service.
Test CYA monthly on trichlor-dosed pools. For pools using liquid chlorine (no CYA added), test quarterly — CYA will not accumulate but may drop slowly if significant dilution events occur (rain, partial drain).
CYA above 100 ppm requires draining 25–30% of the pool and refilling to restore manageable chemistry. Do not try to "chlorinate through" very high CYA — the required FC levels become impractical and corrosive to equipment at sustained 10+ ppm FC.
PoolLens logs both CYA and FC readings so you can monitor the ratio over time. Spot CYA creep early and schedule a partial drain before algae season hits. Free for pool service professionals.
Open PoolLens Free →The FC:CYA ratio describes the minimum amount of free chlorine needed to maintain effective disinfection at a given CYA level. As CYA rises, it binds more of the free chlorine into a less active form, reducing killing power. The minimum recommended FC is approximately 7.5% of the CYA level for normal disinfection.
High CYA (above 80–100 ppm) causes chlorine lock — a condition where the free chlorine reading appears adequate but actual disinfecting power is severely reduced. Algae can bloom in a pool testing 3 ppm FC if CYA is 150 ppm because the effective hypochlorous acid (HOCl) concentration is far too low to control biology.
The only reliable way to lower CYA is through dilution — draining a portion of the pool and refilling with fresh water. CYA is very stable and does not degrade significantly under normal pool conditions. Drain 25–30% of the pool volume to reduce CYA by approximately 25–30%.
For outdoor pools, CYA below 20 ppm provides insufficient UV protection, causing rapid chlorine degradation in sunlight. For an outdoor pool, 30–40 ppm is the minimum effective CYA level. Indoor pools do not need CYA (no UV exposure), and many indoor commercial facilities operate without CYA entirely.