The quick answer: high cyanuric acid (CYA) doesn't make chlorine stop working. It means you need proportionally more chlorine: free chlorine (FC) of at least about 7.5% of CYA, and 40% of CYA to shock. Above roughly 80–100 ppm, that gets impractical. The only reliable way down is to replace part of your water.
To work out how much: share to replace = 1 − (target CYA ÷ current CYA). Going from 100 to 50 ppm means replacing 50% of the water. The calculator handles partial drains in steps.
Why high CYA is a problem
CYA protects chlorine from sunlight by binding most of it in a reserve, which releases active chlorine (HOCl) as it's needed. The more CYA there is, the smaller the active share at any given FC. That's why your FC target rises with CYA.
| CYA | Minimum FC (7.5%) | Shock FC (40%) |
|---|---|---|
| 50 | 3.8 | 20 |
| 80 | 6 | 32 |
| 100 | 7.5 | 40 |
Above about 130 ppm CYA, 7.5% of CYA is more than 10 ppm, the most anyone should swim in (CDC). At that point you can't keep the water both sanitized and swimmable: replace water first.
So high CYA isn't toxic or "locked". It's expensive and fragile:
- Daily chlorine must stay higher.
- Shocking an algae outbreak needs huge doses.
- A day or two of missed dosing turns the pool green.
That's the real meaning behind the "chlorine lock" myth.
First, confirm the reading
CYA tests are easy to misread, and a wrong number leads to an unnecessary drain.
- Use a turbidity (black-dot) test from a drop kit, not test strips.
- Read it outdoors in shade, with your back to the sun:
- In very bright light the dot stays visible longer, so you add more sample and read falsely low.
- In dim indoor light the dot vanishes early, so you read high.
- Sample temperature: cold samples read high. Let the sample warm toward room temperature.
- Above about 90 ppm, most kits can't read accurately. Dilute the sample 1:1 with tap water that has no CYA, test, then double the result.
- Store tests vary. If a store says 150 and your kit says 70, retest with dilution.
Why it got high
Stabilized chlorine is almost always the reason:
- Trichlor tablets (pucks): every 1 ppm of FC adds about 0.6 ppm of CYA. A pool using 3 ppm a day from tablets adds about 1.8 ppm of CYA a day, which is roughly 50 ppm a month.
- Dichlor granules ("stabilized shock"): every 1 ppm of FC adds about 0.9 ppm of CYA.
- Adding stabilizer to correct a reading that was actually fine, or adding it twice because it dissolves slowly. Wait a week before retesting after adding stabilizer.
CYA doesn't evaporate and barely breaks down. It only leaves through water loss: splash-out, backwashing, rain overflow and drains.
How to lower CYA: partial drain and refill
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Test your fill water for CYA. It should be 0, but some areas use stabilized chlorine in municipal supplies, and some wells have surprises.
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Work out the share to replace: 1 − (target ÷ current).
From → to Replace 100 → 50 50% 90 → 60 33% 120 → 60 50% 150 → 50 67% -
Check whether it's safe to drain your pool type:
- Vinyl liners can shrink or float if drained too low. Many owners keep water at least 12 inches deep, and drain and refill in several smaller steps.
- In-ground plaster or fiberglass pools can pop out of the ground with a high water table. Never fully drain these without professional advice.
- Several smaller drains get you to the same place. Two drains of 30% each remove about 51% of the CYA (0.7 × 0.7 = 0.49 remaining).
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Drain: use a submersible pump or the filter's waste setting. Check local rules on where pool water can go.
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Refill, then retest everything. Dilution lowers CYA, salt, calcium, TA and chlorine equally. Rebalance, starting with FC for your new CYA.
Tip: in rainy climates, letting rain dilute the pool by repeatedly lowering the water level works the same way, just slowly.
Do CYA reducer products work?
Some products use bacteria or enzymes to break down cyanuric acid. They need warm water, specific conditions, and often low chlorine for a period, which carries its own algae risk. Results reported by pool owners are inconsistent. Reverse osmosis (RO) water treatment services also exist in some areas and avoid wasting water, but they cost more.
Dilution is the method that always works. Try a reducer only if water is scarce or very expensive, and retest carefully.
Can you live with high CYA?
Yes, if you're disciplined. With 90 ppm of CYA, keep FC at 6.8 ppm or more every evening, and accept that a SLAM means holding 36 ppm, with nobody in the pool until it is back in range. A salt water generator makes this easier, because it adds chlorine all day. Many SWG pools are run at 70–80 ppm on purpose.
Preventing it from coming back
- Switch daily chlorine to liquid chlorine or bleach once CYA is in range. They add no CYA.
- Use tablets only if you plan for the CYA they add, and test monthly.
- Shock with liquid chlorine or cal-hypo, not dichlor.
- Log CYA monthly. A rising line means it's time to change something.
Track CYA in Pool Calc
Pool Calc recalculates every chlorine target when CYA changes. It warns you when a stabilized product will push CYA up, and its dilution calculator tells you how much water to replace. The log shows CYA drift over the season, so you can stop it early.
One pool? Every feature, free. No account, no ads, and iCloud backup is included.
Follow local rules for draining pool water and your pool manufacturer's guidance on water levels.
Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.