Home CYA (Cyanuric Acid) Calculator
CYA (Cyanuric Acid) Calculator — Free Chlorine Targets
Compare your measured CYA and free chlorine using a clearly labeled FC/CYA operating method. The calculator shows Trouble Free Pool (TFP) targets for outdoor pools and keeps those separate from CDC's residential minimum-chlorine guidance. No sign-ups. Instant results.
💡 Quick answer — two different guidance frameworks
TFP manually chlorinated outdoor pool: minimum FC = 7.5% of CYA; target FC = 11.5%; SLAM/Shock FC = 40%
TFP salt chlorine generator: minimum FC = 5% of CYA; this calculator uses 7.5% as a practical operating target
CDC residential guidance with CYA/stabilized chlorine: at least 2 ppm FC and pH 7.0–7.8
Use the calculator below to keep the TFP operating method separate from the CDC residential baseline.
FC/CYA ratios are operating targets for algae prevention used by TFP. CDC residential guidance is a separate baseline: pH 7.0–7.8 and at least 2 ppm free chlorine when CYA or stabilized chlorine is used.
Test your CYA and chlorine accurately:
- Liquid reagent test kit — accurate CYA, FC, pH, and alkalinity readings
- FAS-DPD chlorine test kit — useful when measuring elevated FC during a SLAM process
- Liquid chlorine — raise FC to your target level instantly
Example calculation
For a manually chlorinated outdoor pool with CYA of 50 ppm, the TFP ratio model gives:
| TFP level | Formula | Result |
|---|---|---|
| Minimum FC | 50 × 7.5% | 3.8 ppm |
| Target FC | 50 × 11.5% | 5.8 ppm |
| SLAM FC | 50 × 40% | 20 ppm |
If the current FC were 2 ppm, it would be below the TFP minimum for CYA 50. That does not mean this table by itself determines whether swimming is safe: CDC's residential guidance uses a different baseline of at least 2 ppm FC when CYA is present, together with pH 7.0–7.8. Follow local rules and any active treatment instructions as well.
CYA FC Chart — TFP targets for manually chlorinated pools
This table applies the ratios documented by Trouble Free Pool for a manually chlorinated outdoor pool: minimum FC = 7.5% of CYA, target FC = 11.5% of CYA, and shock/SLAM FC = 40% of CYA.
| CYA | TFP minimum FC | TFP target FC | TFP SLAM FC |
|---|---|---|---|
| 20 ppm | 1.5 ppm | 2.3 ppm | 8 ppm |
| 30 ppm | 2.3 ppm | 3.5 ppm | 12 ppm |
| 40 ppm | 3.0 ppm | 4.6 ppm | 16 ppm |
| 50 ppm | 3.8 ppm | 5.8 ppm | 20 ppm |
| 60 ppm | 4.5 ppm | 6.9 ppm | 24 ppm |
| 70 ppm | 5.3 ppm | 8.1 ppm | 28 ppm |
| 80 ppm | 6.0 ppm | 9.2 ppm | 32 ppm |
| 90 ppm | 6.8 ppm | 10.3 ppm | 36 ppm |
| 100 ppm | 7.5 ppm | 11.5 ppm | 40 ppm |
TFP documents a lower 5% minimum FC/CYA ratio for salt chlorine generator pools; the interactive calculator applies that minimum and uses 7.5% as a practical operating target. These are TFP operating targets, not CDC regulatory limits.
CDC baseline vs. FC/CYA operating targets
These two frameworks answer different questions and should not be presented as if they are the same standard.
| Framework | What it says | Use |
|---|---|---|
| CDC residential guidance | pH 7.0–7.8; at least 1 ppm FC without CYA, or at least 2 ppm FC when CYA/stabilized chlorine is used | Public-health baseline for home pools |
| TFP manually chlorinated method | Minimum FC 7.5% of CYA; target 11.5%; SLAM 40% | Residential operating method intended to prevent algae while accounting for CYA |
| TFP salt-generator method | Minimum FC/CYA ratio can be maintained at 5% | Operating method for continuously chlorinated SWG pools |
Do not use an FC/CYA ratio alone as a “safe to swim” test. Water clarity, pH, sanitizer level, local rules, active treatment instructions, and the chemical product label all matter. CDC also notes that DPD chlorine tests can bleach out above about 10 ppm and show a falsely low or zero result.
What the TFP minimum, target, and SLAM FC values mean
Minimum FC — TFP operating floor
For a manually chlorinated outdoor pool, TFP documents a minimum FC of 7.5% of CYA. This is an algae-prevention operating threshold within that method, not a CDC swim-safety cutoff.
Target FC — daily operating target
TFP documents a target of 11.5% of CYA for manually chlorinated pools. The buffer above the minimum helps prevent normal daily chlorine loss from carrying FC below the operating floor before the next dose.
SLAM FC — treatment level within the TFP method
TFP documents shock/SLAM FC at 40% of CYA. A SLAM is a specific treatment process, not a routine daily target. Use the SLAM calculator for the treatment workflow and use a suitable chlorine test method at elevated FC.
Why CYA changes how much chlorine you need
🔬 CYA changes active chlorine
Cyanuric acid forms reversible chlorine-CYA species, reducing the fraction of measured free chlorine present as rapidly active hypochlorous acid.
☀️ CYA protects outdoor chlorine
CYA is used to reduce sunlight-driven chlorine loss, which is why stabilizer is useful in outdoor pools.
⚖️ Different methods use different targets
CDC gives a minimum residential sanitizer baseline, while TFP uses FC/CYA ratios as an operating method for algae prevention. This page keeps those frameworks labeled separately.
📈 Stabilized chlorine can raise CYA
Trichlor and dichlor add CYA with each dose. Water replacement is the predictable way to reduce CYA when it becomes higher than the target used by your maintenance program.
Use the stabilizer calculator to add or dilute CYA and the chlorine calculator to calculate the product dose needed for a chosen FC target.
What to do when CYA is above your target
First verify the CYA reading and decide which operating method or equipment guidance you are following. HTH's current stabilizer guidance uses 30–50 ppm for outdoor trichlor/dichlor pools and 60–80 ppm for salt chlorine generator pools, and warns that levels above 100 ppm can reduce chlorine effectiveness.
If your measured CYA is above the target you actually intend to maintain, partial water replacement reduces it predictably. The required fraction depends on current CYA, target CYA, and the CYA in the replacement water. Use the stabilizer calculator or pool dilution calculator rather than assuming every high reading requires the same 30–50% drain.
If trichlor or dichlor is the reason CYA keeps climbing, an unstabilized chlorine source stops adding more CYA. Verify the trend with a reliable test before making a large water replacement.
CYA and free chlorine calculator FAQs
What is the relationship between CYA and free chlorine?
Cyanuric acid protects chlorine from sunlight but also changes the amount of rapidly active chlorine available at a given FC reading. That is why some residential pool-care methods use an FC/CYA ratio rather than one fixed FC target for every outdoor pool.
How much free chlorine do I need for my CYA level?
It depends on the framework you are following. TFP documents a 7.5% minimum and 11.5% target FC/CYA ratio for manually chlorinated pools, plus a 40% SLAM level; it also documents a 5% minimum ratio for SWG operation. CDC residential guidance is different: at least 2 ppm FC when CYA or stabilized chlorine is used, with pH 7.0–7.8.
Why can a pool develop algae even when chlorine is measurable?
A measurable FC reading does not by itself describe the whole operating condition. CYA, sunlight, organic demand, circulation, testing accuracy, and the maintenance method all matter. If you use a CYA-based method such as TFP, compare the FC reading with that method's FC/CYA target rather than assuming that any detectable FC is sufficient.
What CYA level is typical for an outdoor pool?
Targets vary with the chlorination method. HTH currently lists 30–50 ppm for outdoor pools using trichlor or dichlor and 60–80 ppm for salt chlorine generator pools. Other systems or maintenance methods may specify different targets, so use the applicable product or equipment guidance.
Can CYA get too high, and how do I lower it?
Yes. If CYA is above the target used by your system or maintenance method, verify the test before acting. Water replacement is the predictable way to lower CYA; the amount to replace depends on current CYA, target CYA, and replacement-water CYA. Use the Stabilizer Calculator for the dilution estimate.
Does CYA affect the SLAM method?
Yes. Within the TFP method, SLAM FC is set at 40% of CYA, so a higher CYA produces a higher SLAM target. That is a TFP treatment rule, not a CDC routine-chlorine standard. Use the SLAM Calculator for the treatment workflow.
Do saltwater pools use the same FC/CYA ratio?
Not exactly. TFP documents that SWG chlorination can be maintained at a 5% minimum FC/CYA ratio, lower than its 7.5% minimum for manually dosed pools. The interactive calculator applies the SWG minimum separately instead of using the manual-pool ratio for both systems.
Do I need CYA in an indoor pool?
CYA's main benefit is protection from sunlight, so that UV-protection benefit is absent indoors. Indoor pools can also be governed by local commercial or facility rules. The calculator therefore does not apply the outdoor TFP FC/CYA ratios to the indoor option; it shows the relevant CDC residential baseline and advises following local or operator requirements.