CYA reduces UV chlorine loss
FC targets must scale with CYA
Lowering CYA = water replacement
What CYA does in a chlorine pool
CYA protects chlorine from sunlight, but too much CYA means you need a higher free chlorine target. This guide explains how to read CYA, connect it with FC, and avoid under-chlorinating a pool that looks clear but is not stable.
CYA (cyanuric acid), also called stabiliser or conditioner, helps outdoor pools because it slows how fast free chlorine (FC) is destroyed by sunlight. At the same time, higher stabiliser levels mean the same FC reading is less active. That is why a pool can look clear, show a familiar chlorine number, and still struggle with algae if the daily target has not been adjusted.
Main takeaway: pick a sensible stabiliser level for your pool type, then keep free chlorine proportional to that reading. Many “chlorine disappears” complaints come from either too little stabiliser for sun exposure or rising stabiliser while the chlorine target stays too low.
Scope note: this guide is written for residential outdoor pools. Commercial, public, body corporate and high-bather-load pools may have stricter testing, record-keeping and chemical limits under local health requirements.
Why pools lose chlorine in the sun
Low stabiliser
FC can drop sharply through the day during bright, warm weather.
Heat and swimming
More swimmers, sunscreen, sweat and debris increase chlorine demand.
Right operating band
Stabiliser helps chlorine last longer, making daily control easier.
Wrong chlorine target
Clear water can still be unstable if FC is too low for the current CYA reading.
Choosing practical CYA operating ranges
These ranges are practical residential operating ranges, not universal rules for every pool. Always consider local requirements, equipment instructions, bather load and the pool’s ability to maintain the matching chlorine level.
Outdoor residential pools
- Liquid/manual dosing: typically 30–50 ppm.
- Salt chlorinator (SWG), residential pools: often 60–80 ppm, only where local requirements allow it and the system can reliably maintain the matching FC target.
Covered or low-UV exposure
- Often 20–40 ppm, depending on sun exposure and chlorine demand.
If you are unsure, start conservative, for example 30–40 ppm, stabilise the daily chlorine level, then adjust. Raising CYA is easy; lowering it usually requires dilution.
How to test CYA correctly
- Use bright indirect light and read at eye level. Black-dot tests are visual, so lighting matters.
- Follow the kit mix/shake timing precisely. If unsure, repeat the test and use the middle reading.
- After adding stabiliser, allow circulation time before re-testing, commonly 24–48 hours.
- Large CYA changes usually come from dilution: overflow, backwash, vacuum-to-waste, leaks, drain/refill or major top-ups.
Helpful next steps
If the issue is tablets, phosphates, uncertain test results or ongoing maintenance, these pages explain the next step in more detail.
Stabilised vs unstabilised chlorine
How tablets affect stabiliser and how to avoid CYA creep.
Phosphates in pools
When phosphates matter, when they do not, and why chlorine control still comes first.
Professional water testing & balancing
For CYA, FC, pH, stabiliser and water balance checks.
Pool maintenance plans
Regular testing, cleaning and water balance support.
FC/CYA quick reference for normal maintenance
Use Min FC as the floor. Aim for Target FC so normal daily loss does not drop the pool below the minimum. If algae appears while chlorine “looks normal”, confirm the stabiliser reading and compare it with the target range.
In practice: higher stabiliser protects chlorine from UV, but it also requires a higher daily chlorine target to keep sanitation strength consistent.
Maintenance targets — CYA 20 to 50
| CYA (ppm) | Min FC (ppm) | Target FC (ppm) |
|---|---|---|
| 20 | 2 | 3 |
| 30 | 2 | 4 |
| 40 | 3 | 5 |
| 50 | 4 | 6 |
Maintenance targets — CYA 60 to 100
| CYA (ppm) | Min FC (ppm) | Target FC (ppm) |
|---|---|---|
| 60 | 5 | 7 |
| 70 | 5 | 8 |
| 80 | 6 | 9 |
| 90 | 7 | 10 |
| 100 | 8 | 12 |
At very high stabiliser levels, daily chlorine targets can become difficult to maintain consistently. If the pool cannot hold its target or algae pressure increases, plan dilution rather than chasing the problem with repeated chemical additions.
Algae cleanup FC reference
When algae or heavy organic demand is confirmed, a one-time chlorine spike is often not enough. The pool usually needs a higher FC level that matches the current CYA, held with circulation and brushing until the water clears and chlorine loss becomes stable.
| CYA (ppm) | Cleanup FC (ppm) |
|---|---|
| 20 | 8 |
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 70 | 28 |
| 80 | 32 |
| 90 | 36 |
| 100 | 40 |
Safety note: high-FC cleanup is not a swimming condition. Do not swim during cleanup treatment. Follow chemical label directions, maintain circulation, avoid mixing chemicals, and confirm FC has returned to an appropriate operating range before swimming.
Common causes of CYA creep
- Tablets all the time: trichlor and dichlor add stabiliser as they add chlorine, so CYA slowly rises. See the guide to stabilised vs unstabilised chlorine.
- Rare stabiliser testing: CYA is checked only after algae, dull water or chlorine loss becomes obvious.
- Fixed chlorine targets: the pool is kept at generic numbers while stabiliser climbs into a range that needs a higher target.
How to raise CYA: stabiliser dosing formula
In metric pool math, 1 ppm = 1 gram per 1,000 litres. The simplest dosing formula is:
Stabiliser grams ≈ (Pool litres × Δppm) ÷ 1,000
If your product is not 100% cyanuric acid, divide by its fraction. Example: for a 96% product, divide by 0.96.
If your product is not 100% cyanuric acid, divide by its fraction. Example: for a 96% product, divide by 0.96.
Add stabiliser gradually and allow time to dissolve and circulate before re-testing, commonly 24–48 hours.
Dosing cheat sheets
Raise CYA by 10 ppm
| Pool volume | Stabiliser needed |
|---|---|
| 30,000 L | 300 g |
| 40,000 L | 400 g |
| 50,000 L | 500 g |
| 60,000 L | 600 g |
| 70,000 L | 700 g |
| 80,000 L | 800 g |
Raise CYA by 20 ppm
| Pool volume | Stabiliser needed |
|---|---|
| 30,000 L | 600 g |
| 40,000 L | 800 g |
| 50,000 L | 1,000 g |
| 60,000 L | 1,200 g |
| 70,000 L | 1,400 g |
| 80,000 L | 1,600 g |
How to lower CYA with dilution math
CYA does not evaporate. The dependable way to reduce it is water replacement. Plan the percentage first:
Fraction to replace ≈ 1 − (Target CYA ÷ Current CYA)
Examples
| Current → Target | Replace approx. |
|---|---|
| 100 → 50 ppm | 50% |
| 80 → 50 ppm | 37.5% |
| 70 → 40 ppm | 42.9% |
| 60 → 40 ppm | 33.3% |
After meaningful water replacement, re-test stabiliser and reset the daily chlorine target from the FC/CYA table.
When CYA is too low vs too high
Too low
Chlorine can disappear quickly in sun. Test CYA and FC on the same day before increasing daily chlorine use.
Too high
The pool needs a higher chlorine target. If that target is hard to maintain, dilution may be more reliable than repeated dosing.
Symptoms: what you see and what to test first
- FC crashes to near-zero late afternoon: test stabiliser and free chlorine on the same day. The pool may be under-stabilised for its sun exposure.
- Algae despite a familiar chlorine number: compare the FC result with the current CYA reading. Also see why FC/CYA usually matters more than phosphates.
- Dull water, irritation or persistent smell: test FC, combined chlorine if available, pH and stabiliser; also check filter condition.
- SWG runs longer but FC stays low: confirm stabiliser, output percentage, pump time and cell condition.
- CYA appears to drop suddenly: look for dilution events such as overflow, backwash, vacuum-to-waste, leaks or major top-ups.
When testing is inconsistent
If stabiliser, free chlorine or pH readings do not make sense, professional testing can confirm the numbers before you add chemicals or plan water replacement.
FAQ
1) How often should I test CYA?
For outdoor residential pools in summer, about monthly is a useful routine. Re-test after overflow, backwashing cycles, vacuum-to-waste, leaks or partial drain/refill.
2) What is considered high CYA?
Many residential pools become harder to manage once CYA is around 80–100 ppm unless the matching chlorine target is raised and the chlorination system can maintain it consistently.
3) Is high CYA dangerous?
The usual problem is not that CYA is instantly dangerous by itself. The problem is that high CYA requires a higher free chlorine target. If the pool is managed with a low generic chlorine number, sanitation becomes less reliable and algae prevention can fail.
4) Why do I get algae when FC looks normal?
Because free chlorine targets depend on stabiliser. If CYA rises but the chlorine target does not, the pool can look clear for a while but become unstable.
5) Can I lower CYA quickly without draining?
The most dependable method is water replacement, either partial drain/refill or controlled exchange. Plan the percentage first so you do not over-drain.
6) What is different about salt chlorinator pools?
Residential salt pools often run a higher stabiliser level to reduce UV loss, but chlorine targets must rise with CYA and the cell plus pump schedule must keep up.
7) Do I need CYA in winter?
If the pool remains chlorinated and gets sun exposure, some stabiliser can still help. UV and chlorine demand are usually lower, so FC may hold longer.
8) What happens to CYA after heavy rain or overflow?
Dilution lowers CYA. After overflow or backwashing, re-test stabiliser and adjust the chlorine target and SWG settings if applicable.
9) When should I call for professional help?
Call for help if algae keeps returning, CYA is very high and needs a careful dilution plan, pool volume is uncertain, there is a suspected leak, or test results are inconsistent.