Updated: 18 Aug 2026

What is cyanuric acid (CYA) in a pool?

Cyanuric acid (CYA), commonly called pool stabiliser, is a chemical used in outdoor chlorinated pools to reduce chlorine loss to ultraviolet light. A CYA result cannot be judged on its own because CYA changes chlorine availability; the appropriate operating range depends on the sanitiser system, chlorine-management method, equipment instructions and the free chlorine the pool can maintain.

In pool water, CYA reacts reversibly with free-chlorine species and forms chlorinated cyanurates. This creates a dynamic equilibrium: some chlorine remains in more active forms while some is associated with cyanurate. Increasing CYA can improve protection from sunlight, but it also changes the relationship between the measured free-chlorine residual and the chlorine available for rapid disinfection.

That is why a free-chlorine reading should be interpreted with the current CYA result and the way chlorine is being supplied. A manually dosed pool, a tablet-fed pool and a salt-chlorinated pool are all chlorine pools, but the dosing pattern, equipment output and manufacturer requirements can be different.

Residential scope: this guide concerns privately used outdoor residential pools. Public and other regulated aquatic facilities can operate under different water-quality, testing and record-keeping requirements.

Why CYA and free chlorine need to be assessed together

UV exposure

Outdoor chlorine is degraded by sunlight. CYA can slow that loss, which can make an outdoor pool easier to manage through sunny weather.

Free chlorine

The FC test reports the free-chlorine residual. The same FC result does not represent identical chlorine conditions when CYA concentrations are different.

Chlorine source

Liquid chlorine, stabilised chlorine products and electrolytic chlorinators all provide chlorine, but they affect the pool and its CYA level differently.

System setup

Chlorinator output, pump runtime, automation, bather load, sunlight and equipment instructions all influence the residual the pool can maintain.

A saltwater chlorinator produces chlorine

A salt chlorinator is an electrolytic chlorine-generation system. The cell uses chloride in the pool water to produce chlorine, so a saltwater pool should not be treated as a separate “non-chlorine” sanitising category.

The relevant differences are the chlorinator's production capacity, programmed output, filtration runtime, salt concentration, control method and the water-chemistry limits specified for the equipment. CYA should therefore be set with reference to the actual chlorinator rather than a universal “salt pool” number.

Liquid chlorine

Unstabilised liquid chlorine increases chlorine without deliberately adding CYA, allowing stabiliser to be adjusted separately.

Stabilised chlorine

Trichlor and dichlor provide chlorine while also adding cyanuric acid to the pool, so regular use can increase CYA over time.

Salt chlorinator

The cell generates chlorine during operation. Its required runtime and output depend on the unit, pool demand and operating conditions.

Automated chlorine control

Sensor-controlled or integrated systems should be operated within the chemistry and calibration requirements specified for that equipment.

Manufacturer example: the Zodiac EL Series installation manual lists nominal chlorine production of 25 g/h for the EL25 and 35 g/h for the EL35. Its water-chemistry table lists cyanuric acid up to 50 ppm for that model family. This is equipment-specific guidance, not a target for every salt chlorinator.

Test CYA before changing the dose

  • Follow the test-kit procedure, including sample preparation, mixing time, lighting and viewing position.
  • Test free chlorine and CYA close enough together that the results describe the same operating condition.
  • Repeat an unexpected CYA result before making a large chemical correction or planning substantial water replacement.
  • After adding stabiliser, follow the product label for application, circulation, filter operation and re-testing.
  • If CYA changes unexpectedly, check for backwashing, vacuum-to-waste, overflow, leaks, partial drain/refill work and recent use of stabilised chlorine products.
Pool volume matters. A dosing calculation is only as reliable as the pool-volume estimate and the test result used as its inputs.

How to use an FC/CYA chart correctly

FC/CYA charts are operating tools that link free chlorine to measured CYA. They are useful because they prevent chlorine from being managed as one fixed number regardless of stabiliser concentration.

This page does not present a third-party FC/CYA chart as a mandatory Australian residential standard. If a pool is managed with a published FC/CYA method, use the values, definitions and treatment procedure from that method rather than combining numbers from different charts.

Trouble Free Pool FC/CYA method

Trouble Free Pool publishes a residential FC/CYA operating chart that distinguishes routine chlorine management from its separate algae-cleanup procedure. If that method is being used, refer to its source chart rather than treating an isolated FC/CYA percentage as a universal rule.

https://www.troublefreepool.com/blog/2019/01/18/chlorine-cya-chart/

What research tells us about the relationship

Peer-reviewed work has also examined CYA and FC as linked variables rather than independent readings. A 2019 risk-model study recommended regulating FC and CYA jointly and proposed a maximum CYA-to-FC ratio of 20 within the conditions modelled in that study. That research finding is not the same thing as a complete residential maintenance chart.

https://www.mdpi.com/2073-4441/11/6/1314

Practical order: identify the sanitiser system, check its manufacturer requirements, measure CYA and FC, then apply one defined operating method consistently.

Which guidance takes priority?

Situation Primary reference What to check
Specific chlorinator or controller Manufacturer instructions CYA range, salt level, chlorine output, flow, runtime and sensor requirements.
Private residential pool using an FC/CYA method The selected published method Its minimum, normal operating target, test method and treatment procedure.
Public or regulated aquatic facility Applicable regulatory requirements Required disinfectant residuals, CYA limits, testing frequency and record keeping.

What low or high CYA can change

Low CYA in an outdoor pool

Chlorine may be lost rapidly during strong sun exposure. Confirm the CYA reading, FC result and chlorine demand before increasing chlorinator output or daily dosing.

Higher CYA

The relationship between measured FC and active chlorine changes. Check whether the current level suits the equipment and chlorine-management method before adding more stabiliser.

Unexpected chlorine loss

CYA is only one possible cause. Organic contamination, algae, bather load, circulation, cell output and dosing errors can also increase chlorine demand.

Repeated algae

Test FC and CYA together, then check circulation, filtration and the sanitation method rather than relying on water clarity or one familiar chlorine reading.

CYA stabiliser dose calculator

This calculator estimates the mass of dry cyanuric-acid stabiliser required to raise CYA. Use it only when the product label states the active cyanuric-acid concentration by mass, such as a percentage by weight or grams per kilogram.

It is not a volume calculator for liquid conditioner products. Liquid products can use different concentration declarations and may require product density or a manufacturer-specific dosing instruction.

Enter pool volume, current CYA, target CYA and dry-product strength.
Calculation Pure CYA required (g) = pool litres × (target CYA − current CYA) ÷ 1,000 Dry product required (g) = pure CYA required ÷ (active CYA % ÷ 100)
The result is an estimated dose based on the values entered. Check the product label and re-test after the product has been applied and circulated as directed before deciding whether another correction is required.

CYA dose example

A 1 ppm concentration is 1 mg/L. Raising a 50,000-litre pool by 10 ppm therefore requires approximately 500 grams of pure CYA.

100% active CYA

50,000 L × 10 ppm ÷ 1,000 = 500 g of dry product.

96% active CYA

500 g ÷ 0.96 = approximately 521 g of dry product.

The exact amount added should still follow the product label. If pool volume or the test result is uncertain, avoiding an intentional overshoot makes the next correction easier.

Why product strength matters

A stabiliser dose should be based on the amount of active cyanuric acid in the product, not simply the total package weight.

For example, Zodiac UV Blockout lists an active constituent of 1000 g/kg cyanuric acid. For that dry product, 1000 g/kg is equivalent to 100% active CYA by mass. A product labelled 960 g/kg would be entered as 96%.

Read the label first. If concentration is not stated as % w/w or g/kg of cyanuric acid, use the manufacturer's own dosing instruction rather than converting the value through this calculator.

How dilution lowers CYA

When CYA is above the level chosen for the pool's sanitising system, controlled water replacement is the usual practical way to reduce its concentration.

If replacement water contains negligible CYA and the pool is fully mixed after replacement, a theoretical one-stage replacement fraction can be estimated with:

Theoretical dilution estimate Fraction to replace = 1 − (target CYA ÷ current CYA)
Current CYA Target CYA Theoretical replacement
100 ppm 50 ppm 50%
80 ppm 50 ppm 37.5%
70 ppm 40 ppm 42.9%
60 ppm 40 ppm 33.3%
These percentages are dilution calculations, not instructions to drain that percentage of every pool in one operation. Pool construction, groundwater conditions, hydrostatic risk, discharge requirements and local water restrictions can affect how water replacement should be carried out.

Common causes of rising CYA

Regular trichlor or dichlor use

Trichlor and dichlor provide chlorine while also adding cyanuric acid to the pool, so regular use can increase CYA over time.

Infrequent CYA testing

FC may be checked often while CYA is checked less frequently, so changes in the relationship can go unnoticed.

Repeated stabiliser additions

Adding stabiliser after a doubtful low reading can create an unnecessary correction that later has to be reduced through dilution.

Fixed chlorine targets

Managing FC from habit without checking the current CYA can disconnect the chlorine target from the actual condition of the pool.

For more detail on chlorine products that also contribute CYA, see stabilised vs unstabilised chlorine .

CYA troubleshooting by symptom

FC falls during sunny weather

Test CYA and FC, then compare the result with chlorine demand, sun exposure and the output or dose being supplied.

Clear water but FC falls every day

Clear water does not identify the cause. Check CYA, chlorine demand, chlorinator production, circulation and recent contamination.

Algae keeps returning

Confirm FC and CYA together and inspect filtration and circulation before assuming that another one-off chlorine dose will solve the problem.

Salt chlorinator cannot hold FC

Check cell condition, output setting, pump runtime, flow, salt level, CYA, pool volume and current chlorine demand.

If the main problem is a pool that stays visually clear but loses chlorine every day, see Clear Pool but Chlorine Drops Every Day .

Related testing and maintenance guides

Pool water testing and balancing Confirm CYA, free chlorine and the wider water balance before a major chemical correction. Clear pool but chlorine drops every day Work through UV exposure, chlorine demand, circulation and chlorinator output. Stabilised vs unstabilised chlorine See how different chlorine products affect CYA during routine dosing. Pool maintenance plans Regular testing, cleaning and equipment checks for residential pools.

CYA and pool stabiliser FAQ

Is CYA the same as pool stabiliser?
Yes. Cyanuric acid is commonly called chlorine stabiliser or conditioner. It is mainly used in outdoor chlorinated pools to reduce chlorine loss caused by ultraviolet light.
Can I judge CYA without looking at free chlorine?
No. CYA changes chlorine chemistry, so the CYA result should be considered with free chlorine, the sanitising system and the operating method being used.
Is a saltwater pool still a chlorine pool?
Yes. A salt chlorinator produces chlorine electrolytically from chloride in the pool water. Salt is part of the chlorine-generation process; it does not replace chlorine as the sanitiser.
What CYA should a salt chlorinator use?
Check the instructions for the actual chlorinator. Manufacturer recommendations differ by system, so one CYA number should not be assigned to every salt-chlorinated residential pool.
Is the Trouble Free Pool FC/CYA method an Australian standard?
No. It is a third-party residential pool-management method. It can be used as a defined operating approach, but it should not be described as an Australian regulatory requirement or a universal manufacturer specification.
Can I use the calculator for liquid stabiliser?
Not unless the liquid product can validly be calculated from an active CYA percentage by mass and the required product mass is useful for dosing. For liquid conditioner, follow the manufacturer's concentration and dosing instructions.
Does the calculated dose guarantee the final CYA reading?
No. It is a mass calculation based on pool volume, test results and product strength. Re-test after application and circulation according to the product instructions before making another correction.
How is high CYA reduced?
Controlled water replacement is the usual practical correction. The theoretical amount depends on current CYA, target CYA and the CYA concentration of the replacement water, but the actual draining method must also suit the pool and site conditions.
When is professional water testing useful?
Professional testing is useful when CYA results are inconsistent, pool volume is uncertain, a substantial dilution is being considered or the chlorine system cannot maintain its expected residual. See pool water testing and balancing .

Technical references

Litra PoolCare services

Relevant pool services

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