FC/CYA chart: look up your exact FC target by CYA level
As stabilizer increases, required chlorine increases proportionally. Running a fixed FC target without knowing your CYA is the most common reason pools go green on schedule. Use the chart and table below for your specific minimum, target, and shock values.
What is the FC/CYA relationship?
FC/CYA describes how much free chlorine is required at different stabilizer (CYA) levels to maintain effective sanitization. As CYA rises, required FC rises proportionally — higher CYA binds more chlorine into less active forms, so more total FC is needed to keep enough active sanitizer (HOCl) in solution.
The chart shows four data series. If you prefer a simpler view, the table below has the same numbers.
| CYA (ppm) | Minimum FCRegular chlorine | Minimum FCSaltwater (SWG) | Target FC | Shock Level |
|---|---|---|---|---|
| 20 | 1.5 | 1 | 2–4 | 10 |
| 30 | 2.25 | 1.5 | 3–5 | 12 |
| 40 | 3 | 2 | 4–6 | 16 |
| 50 | 3.75 | 2.5 | 5–7 | 20 |
| 60 | 4.5 | 3 | 5–8 | 24 |
| 70 | 5.25 | 3.5 | 6–8 | 28 |
| 80 | 6 | 4 | 7–9 | 31 |
| 90 | 6.75 | 4.5 | 7–10 | 35 |
| 100 | 7.5 | 5 | 8–10 | 39 |
Minimum FC — the minimum safe FC. Do not drop below it. Target FC — your daily operating range (temperate conditions). Shock level — the FC target needed to clear an active algae bloom. In hot or sunny climates, maintain FC near the top of the target range — UV and heat accelerate depletion. Use the calculator below for climate-adjusted targets.
Safe FC levels by CYA — quick lookup
Target range is what to aim for daily. Don't let FC fall below the minimum. Full numbers (including shock levels) are in the table above.
FC target calculator
Enter your CYA and climate — minimum FC is fixed, target range adjusts for UV and temperature.
Minimum FC is the same in all climates — it's set by CYA chemistry. The target range is wider in hot, sunny climates because UV and heat deplete chlorine faster, requiring more buffer above the minimum.
Minimum FC ≈ CYA × 5% (saltwater / SWG — industry guideline)
What "shock level" really means
The shock-level column is the FC concentration used as the target for breakpoint chlorination — the point where chlorine destroys combined chloramines and algae faster than they form. This is a commonly used operational guideline (FC ≈ CYA × 40%), not a precise stoichiometric law. Hold FC at this level, retesting every few hours and re-dosing as needed, until the pool passes an overnight chlorine loss test of less than 1 ppm. Saltwater pool owners should add liquid chlorine manually rather than driving the salt cell at maximum, which accelerates cell wear. If your pool turns cloudy during or after shocking, see why pools go cloudy after shocking.
Why does CYA affect chlorine?
CYA (stabilizer/conditioner) protects chlorine from UV breakdown. That's its job and it does it well. But CYA also binds to chlorine molecules, reducing the amount of "active" chlorine available to sanitize.
Think of it this way: at any given moment, only a small percentage of your FC is actually active. The rest is bound to CYA, held in reserve. As CYA increases, a smaller percentage of your total FC is active. To compensate, you need more total FC.
The numbers behind it
At typical pool pH, only a small fraction of FC exists as free hypochlorous acid (HOCl). CYA doesn't change the pH-driven HOCl equilibrium, but it does bind HOCl into less active chlorocyanurate forms — so the higher your CYA, the more total FC you need to maintain adequate active sanitizer. That's why the same FC reading can mean "well sanitized" at CYA 30 and dangerously low at CYA 80.
Why does my free chlorine drop so fast?
Fast FC loss usually comes from one or more of these factors:
- CYA is higher than you think. If CYA has crept up but you're dosing the same amount of chlorine, you're fighting a higher minimum and burning through it faster.
- High bather load or organic debris. Sweat, sunscreen, and plant matter consume chlorine quickly. Heavy use days need more frequent dosing.
- pH too high. At pH 8.0, chlorine is roughly 3× less effective than at pH 7.4. More chlorine gets consumed doing the same job — keep pH in the 7.4–7.6 range.
- Early algae growth. If FC is disappearing overnight with no obvious cause, algae may be starting. Anything more than 1 ppm overnight loss is a red flag.
How CYA builds up
CYA doesn't break down, evaporate, or get filtered out. It only leaves the pool through water removal (splash-out, backwashing, or draining). The most common cause of high CYA is stabilized chlorine tablets (trichlor). Every tablet you dissolve adds more CYA to the pool.
| Chlorine Source | Adds CYA? |
|---|---|
| Trichlor tablets | Yes — significant amount |
| Dichlor granules | Yes — moderate amount |
| Liquid chlorine (bleach) | No |
| Cal-hypo (granular shock) | No |
| Salt water generator (SWG) | No |
If you use trichlor tablets as your primary chlorine source, test CYA monthly. It will creep up over the season. Many pool owners switch to liquid chlorine once CYA reaches their target to prevent further buildup. Learn more about which chlorine types affect CYA.
What happens when CYA is too high
As CYA climbs above 80–90 ppm, problems compound:
- FC minimums get impractical. At 100+ ppm CYA, you need FC of 8+ just to stay sanitized. That's expensive and hard to maintain — and if you fall behind, your chlorine drops too low fast.
- Shock levels become extreme. Shocking a pool with CYA at 100 means holding FC at 39 ppm. That burns through chlorine fast.
- Test accuracy drops. Most FC test kits lose precision above 10 ppm, making it hard to verify you're at the right level.
- Algae risk increases. The margin between "sanitized" and "algae bloom" gets razor thin.
What CYA level is too high?
Most pool experts put the practical upper limit at 80–90 ppm. Above 100 ppm CYA, you need to hold FC at 39–40 ppm for effective breakpoint chlorination — roughly a gallon of liquid chlorine per 10,000 gallons, every few hours, until the pool clears. Most pool owners don't realize CYA has crept this high because it builds up slowly over the season, one trichlor tablet at a time.
What to do when CYA is too high
The only way to lower CYA is dilution:
- Partial drain and refill. Drain 25–30% of your pool water and refill with fresh water. This proportionally reduces CYA (and all other dissolved solids).
- Retest after refilling and let the water circulate for at least a full pump cycle before testing.
- Switch to liquid chlorine going forward to prevent CYA from climbing again.
Is there a chemical that removes CYA?
No. Several products are marketed as "CYA reducers" but none have shown consistent, reliable effectiveness. The only dependable method is dilution — drain some water and refill with fresh. Use the CYA dilution calculator to find exactly how many gallons to drain. If CYA is extremely high (above 100 ppm), a 50% drain-and-refill is often the fastest reset.
Pool chlorine chart for saltwater (SWG) pools
SWG pool owners
Saltwater pools tend to run CYA in the 60–80 range intentionally. The salt cell produces chlorine slowly and CYA helps it last longer between generation cycles. This is fine as long as you adjust your FC target to match. At 70 CYA, target FC of 6–8.
What is the ideal CYA for a saltwater pool?
Most SWG pool owners target 60–80 ppm. The higher range is intentional: more CYA helps the chlorine your salt cell produces last longer between generation cycles so the cell doesn't have to run as hard. The tradeoff is a higher FC minimum — at 70 ppm CYA, you need at least 3.5 ppm FC. Test CYA at the start of each season and after significant water refills, since fresh water dilutes CYA and may require you to add stabilizer.
Hypochlorous acid (HOCl): the active sanitizer
Free chlorine isn't a single thing — it's an equilibrium between two forms in solution:
- HOCl (hypochlorous acid) — a neutral molecule that penetrates bacterial cell membranes and kills pathogens. This is the active sanitizer.
- OCl− (hypochlorite ion) — negatively charged, 50–80× less effective than HOCl at the same concentration.
The ratio between these two forms is controlled almost entirely by pH. At pH 7.4, roughly 65% of your FC is active HOCl. At pH 8.0, that drops to about 20%. This is why pH management matters so much — the same FC reading provides very different levels of sanitization depending on where your pH sits.
CYA adds a third layer
CYA binds to HOCl, forming chlorocyanurate compounds that are essentially inactive as sanitizers. The higher the CYA, the greater the fraction of FC held in this bound, less-active form. The widely used minimum guideline (FC ≈ CYA × 7.5%) is calibrated to ensure enough free HOCl remains in solution at typical pool pH. Running below the guideline means too little active sanitizer is available — regardless of what your total FC test reads.
This three-way interaction — total FC, pH, and CYA — is why the old "keep FC between 1–3 ppm" rule fails for stabilized pools. The number that matters is HOCl concentration in solution, which you can't measure directly. The FC/CYA chart gives you the total FC target that produces adequate HOCl at normal pH. Keep both FC above the CYA-based minimum and pH in the 7.4–7.6 range, and you've got the sanitization side covered.
Chloramines and breakpoint chlorination
When chlorine reacts with nitrogen-containing compounds — ammonia from sweat, urine, fertilizer, and decaying organics — it forms combined chlorine (CC), primarily chloramines:
- Monochloramine (NH2Cl) — the dominant form in pool water; only mildly irritating
- Dichloramine (NHCl2) — the source of the characteristic "pool smell" and eye irritation
- Trichloramine (NCl3) — a respiratory irritant; forms in heavily loaded water and off-gasses at the surface
Combined chlorine (CC = Total Chlorine − Free Chlorine) is nearly inactive as a sanitizer and actively harmful to swimmers. A healthy pool doesn't smell like chlorine — that smell is chloramines, not chlorine. If your pool smells strongly of "chlorine," the problem is usually too little free chlorine relative to the nitrogen load, not too much.
What is breakpoint chlorination?
Breakpoint chlorination is the FC concentration at which the reaction kinetics flip: chlorine begins destroying chloramines faster than they form. Below the breakpoint, adding more chlorine temporarily increases combined chlorine as new chloramines form. Above it, combined chlorine plummets and free active chlorine accumulates.
A commonly used operational guideline puts that threshold at FC ≈ CYA × 40% — which is why the shock levels in the chart use this value. At CYA 50, you need to raise FC to 20 ppm to cross the breakpoint. You hold at that level, testing every few hours and re-dosing as needed, until:
- The water clears visually
- Combined chlorine drops to 0.5 ppm or below
- An overnight chlorine loss test shows less than 1 ppm drop
Starting a shock treatment and not reaching breakpoint is counterproductive. You'll burn through a lot of chlorine, transiently increase chloramines, and end up back where you started within a day. Get above the breakpoint threshold and hold it.
Ideal CYA ranges
| Pool Type | Recommended CYA | Why |
|---|---|---|
| Chlorine pool (tablets) | 30–50 ppm | Good UV protection without requiring excessive FC |
| Chlorine pool (liquid) | 30–50 ppm | Add CYA separately at season start. Stays stable all year |
| Saltwater (SWG) pool | 60–80 ppm | Higher CYA helps the cell's chlorine output last longer |
| Indoor pool | 0–20 ppm | No UV exposure, so minimal CYA needed |
Let the app do the math
The app can automatically adjust your chlorine target based on your CYA reading. Log your results, get the right dosing recommendation, and keep FC where it needs to be.
Frequently asked questions
What is the minimum free chlorine level for my pool?
The widely used guideline for minimum free chlorine is approximately CYA × 7.5% for regular chlorine pools. At CYA 30, that's about 2 ppm; at CYA 50, about 4 ppm; at CYA 80, about 6 ppm. Saltwater pools use CYA × 5% because the salt cell continuously replenishes consumed chlorine. Below the minimum, the fraction of active sanitizer is too low to reliably kill algae and bacteria.
What does shock level mean?
Shock level is FC = CYA × 40% — see the shock level breakdown above for exactly how to run a shock treatment.
Why does CYA affect how much chlorine I need?
CYA binds to chlorine molecules, reducing the active fraction — see the full explanation above.
Which chlorine products add CYA to pool water?
Trichlor tablets add significant CYA with every use. Dichlor granules add a moderate amount. Liquid chlorine (sodium hypochlorite), cal-hypo granular shock, and saltwater generators add no CYA. If trichlor tablets are your primary chlorine source, test CYA monthly — it accumulates steadily and doesn't leave the pool except through water removal.
Can FC be too high?
Yes. Above 10 ppm most swimmers find the water irritating to skin and eyes. Above 20 ppm it's generally not safe to swim until levels drop. That said, holding high FC during shock treatment is normal and intentional — at CYA 50 that means raising to 20 ppm and holding there. The rule is simple: don't swim while actively shocking. Once FC drops back below 10 ppm (typically 12–24 hours after the final dose), it's safe to swim.
On the other end: FC that's perpetually higher than your target range isn't a problem by itself, but it can indicate you're overdosing — and that the CYA-adjusted minimum is higher than you realize. If you're consistently at 10+ ppm and still seeing algae, the issue is usually high CYA making your chlorine less effective, not too much FC.
What FC should I maintain at CYA 30?
At CYA 30, the minimum FC is 2.25 ppm and the target range is 3–5 ppm. Shock target is 12 ppm. This is the easiest CYA level to manage — the minimum is low, the shock level is achievable, and your chlorine lasts well between doses. If your CYA is at 30, you're in good shape: keep dosing with liquid chlorine to hold it there.
What FC should I maintain at CYA 50?
At CYA 50, the minimum FC is 3.75 ppm and the target range is 5–7 ppm. Shock target is 20 ppm. This is the upper end of the recommended range for standard chlorine pools. Keep FC at or above 3.75 ppm at all times — at CYA 50, dropping to 2–3 ppm (the old "normal" advice) leaves you well below the sanitization threshold. Test FC at least 2–3 times per week.
Is there a chemical that lowers CYA?
No product has shown consistent, reliable effectiveness at removing CYA. The only dependable method is dilution: drain 25–50% of your pool and refill with fresh water. After refilling, switch to liquid chlorine or cal-hypo to prevent CYA from climbing again.
Pool Chlorine Series
Related reading
- Why Is My Pool Water Cloudy? — low effective FC (relative to CYA) is the #1 cause
- Pool Chlorine Too Low? — what to do when FC drops below the CYA minimum
- Pool Turned Green Overnight? — what happens when FC drops below the CYA minimum
- Free Chlorine Dose Calculator — enter pool size and CYA-based target FC for exact doses
- Free Pool Shock Calculator — find your CYA-based shock target and exactly how much chlorine to reach it
- Does Liquid Chlorine Raise CYA? — common myth, clear answer
- Desert Pool Chemistry — why desert pools need CYA 60–80 ppm and higher FC targets