What is the FC/CYA ratio?

The FC/CYA ratio is the relationship between your free chlorine level and your cyanuric acid level. Because CYA binds to chlorine and reduces its sanitizing strength, you need more total FC as CYA increases. The minimum safe ratio is FC ≥ CYA × 7.5% for standard pools, or FC ≥ CYA × 5% for saltwater/SWG pools.

How much chlorine do I need at CYA 50?

At CYA 50 ppm, your minimum FC is 3.75 ppm (50 × 7.5%). In practice, aim for 4–6 ppm to maintain a buffer above the minimum. Below 3.75 ppm, algae can establish in as little as 24–48 hours.

What happens if FC is too low relative to CYA?

When FC drops below the CYA-based minimum, the pool is effectively under-chlorinated even if the FC number looks reasonable. Algae can establish quickly, and the water can turn green or cloudy despite a 'normal' chlorine reading. This is the most common cause of algae in pools that are being dosed regularly.

Why does high CYA cause pool problems?

As CYA increases, a larger fraction of your chlorine gets bound up in an inactive form. At CYA 100+, effective chlorine is so diminished that you'd need extremely high FC levels to stay safe — levels that are impractical to maintain. The only fix is to lower CYA by draining and refilling.

How do I calculate my FC target from CYA?

Multiply your CYA by 0.075 (7.5%) for a standard pool, or by 0.05 (5%) for a saltwater/SWG pool. That gives your minimum FC. For a practical target, add 1–2 ppm above the minimum as a safety buffer. Example: CYA 40 × 7.5% = 3 ppm minimum; aim for 4–5 ppm.

Pool Chemistry Explained

The FC/CYA Ratio

4 min read · Chlorine & CYA

In one sentence

The FC/CYA ratio is the relationship between free chlorine and cyanuric acid that determines your pool's actual sanitizing power — because as CYA rises, a larger share of your chlorine is held inactive, and you must raise FC to compensate.

Why the ratio exists

Cyanuric acid (CYA) works by binding to free chlorine molecules, shielding them from UV destruction by sunlight. That binding is the same mechanism that reduces chlorine's killing power. A CYA-bound chlorine molecule is temporarily inactive — it will eventually release and sanitize, but far more slowly than unbound chlorine.

The result: two pools can have identical FC readings but completely different sanitizing capacity. A pool with FC 3 ppm and CYA 30 is well protected. A pool with FC 3 ppm and CYA 100 is effectively under-chlorinated and prone to algae — even though both test kits show the same number.

The 7.5% rule (the number to remember)

Free chlorine must be at least 7.5% of CYA for a standard pool. For saltwater/SWG pools, 5% is sufficient because generators run continuously. Below these thresholds, algae can establish within 24–48 hours even if FC looks "fine."

FC target by CYA level

CYA (ppm)	Minimum FC (standard pool)	Minimum FC (SWG pool)	Shock level
0–10	1 ppm	1 ppm	10 ppm
20	2 ppm	1 ppm	8 ppm
30	2–3 ppm	2 ppm	12 ppm
40	3 ppm	2 ppm	16 ppm
50	4 ppm	3 ppm	20 ppm
60	5 ppm	3 ppm	24 ppm
70	5–6 ppm	4 ppm	28 ppm
80	6 ppm	4 ppm	32 ppm
100	8 ppm	5 ppm	40 ppm
120+	9+ ppm	6+ ppm	48+ ppm

Shock level is CYA × 40% — the FC needed to break through active algae or chloramines. At high CYA, these shock doses become difficult to achieve and maintain. See the full FC/CYA chart for detailed ranges.

What happens when the ratio is off

FC too low relative to CYA: This is the most common pool problem. Algae establishes, water turns hazy or green, and adding more chlorine doesn't seem to help — because it is all being consumed faster than you add it. The fix is to raise FC to meet the minimum, and hold it there until the pool clears.

CYA too high: When CYA climbs above 80–100 ppm (common in pools using stabilized tablets all season), the minimum FC target becomes impractically high. You'd need 8–10 ppm FC just to stay safe, and shocking requires 40+ ppm. At this point the only real fix is draining and refilling to lower CYA.

How to use the ratio in practice

Know your CYA. Test it at the start of the season and after any major refill. Many pool owners haven't tested CYA in years.
Calculate your FC target. Multiply CYA by 0.075 (standard pool) or 0.05 (SWG). That's your minimum. Add 1–2 ppm as a buffer.
Dose to that target. Use the chlorine dose calculator to find exactly how much liquid chlorine brings you to your CYA-adjusted target.
If CYA is above 80 ppm, consider a partial drain. The CYA dilution calculator shows how much water to replace.

Want to...

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