Chlorine Lock in a Pool: What It Really Is and How to Fix It

The calculator below shows the minimum and target FC for your CYA. It opens with a typical "locked" pool: CYA 80 ppm and FC 2 ppm. At that CYA, the minimum is 6 ppm and the target is 6–8 ppm, so a reading of 2 ppm is far too low, even though it would be fine at low CYA. And 6 ppm is already above the 4 ppm swim limit on many labels. That gap is the "lock."

Chlorine level calculator

Inputs

Stabilizer (CYA)ppm

Chlorine sourceLiquid chlorine, bleach, or tabletsSalt water generator

WaterPoolHot tub / spa

Your FC now (optional)ppm

Result for example values. Enter your own numbers.

Minimum FC

6 ppm

Target range

6–8 ppm

Shock

Follow your product’s label directions

Active chlorine (HOCl) at the low target, rough estimate

≈ 0.03 ppm (at pH 7.5)

Your FC 2 ppm: Act now

Stabilizer (CYA) is above the recommended range, so chlorine targets are high. Partial draining is the only way to lower it.

Pool product labels (EPA) often say not to swim above 4 ppm. The CDC model code links the minimum to CYA. Keep CYA at 30–40 ppm so both fit.

Estimates, not guarantees. Always follow the product label. How we calculate

What people mean by "chlorine lock"

People use the term for two different experiences:

• the test shows chlorine, but the water still turns cloudy or green, as if the chlorine has stopped working;
• you add chlorine, and the reading barely moves or is gone by the next day.

Neither one means the chlorine is trapped for good. The first is usually too much stabilizer for the amount of chlorine in the water. The second is usually something using the chlorine up: algae, ammonia, sweat and urine, or strong sun with too little stabilizer. A bad test can mimic both. The table below sorts them out.

How stabilizer (CYA) holds chlorine

Stabilizer shields chlorine from sunlight by binding to it loosely. The bond is reversible: bound chlorine is released as the active part is used. But only the small unbound part, hypochlorous acid, kills germs and algae quickly. The CDC's Model Aquatic Health Code shows that the active chlorine level stays about the same when the ratio of CYA to FC stays the same. So the more CYA you have, the more FC you need to get the same effect.

That is why a reading of 3 ppm can be plenty in one pool and too little in another. At CYA 40, 3 ppm is our minimum. At CYA 80, the minimum is 6 ppm. Our minimum is 7.5% of CYA (5% for salt water generators), but never below 2 ppm when the water has stabilizer. It is rounded up to the next 0.5 ppm; see the methodology. The full table is below.

Free chlorine (FC) targets by stabilizer (CYA), ppm

CYA	Minimum FC	Target FC	Salt pool minimum	Salt pool target
0 ppm	1	2–4	1	2–4
10 ppm	2	2–4	2	2–4
20 ppm	2	2–4	2	2–4
30 ppm	2.5	2.5–4.5	2	2–4
40 ppm	3	3–5	2	2–4
50 ppm	4	4–6	2.5	2.5–4.5
60 ppm	4.5	4.5–6.5	3	3–5
70 ppm	5.5	5.5–7.5	3.5	3.5–5.5
80 ppm	6	6–8	4	4–6
90 ppm	7	7–9	4.5	4.5–6.5
100 ppm	7.5	7.5–9.5	5	5–7

How we apply the CDC/MAHC principle that minimum chlorine rises with CYA: minimum ≈ 7.5% of CYA (5% for salt water generators), at least 2 ppm with CYA. To shock, follow the directions on your product’s label. See methodology.

Download this data (CSV)

Now compare the table with your product label. Many pool chlorine labels prohibit swimming above 4 ppm; some say 3 ppm. Check yours.

• CYA 40 or lower: the target range starts at or below 4 ppm, so you can keep FC at the minimum and still stay within a 4 ppm label limit when people swim.
• CYA 50: the minimum is 4 ppm, the same as the limit. There is no room left.
• CYA 60 and higher: the minimum in our model is above a 4 ppm limit. Follow the label, not the model, and lower CYA.

You can't solve the last case by adding more chlorine, because the label wins. You solve it by bringing CYA down. Salt water pools use a lower minimum, so the problem starts at a higher CYA; see pool chlorine levels.

Five causes that look like chlorine lock

Test FC, total chlorine (TC), pH, and CYA with a drop kit if you can. Then find your row:

More than one row can apply. High CYA makes algae more likely, and algae then uses up chlorine. Start with the test you trust most and work down the list.

Fix 1: keep FC at the minimum for your CYA

If your CYA is 50 ppm or lower, the fix is usually steady chlorine. Keep FC at or above the minimum in the table every day, not only right after you add chlorine. Most problems start when FC dips below the minimum overnight or during a hot week.

• Test FC at the same time each day for a week to see how much you lose.
• Add chlorine in the evening, so the sun doesn't burn it off before it mixes.
• Stay within your label: don't raise chlorine above its limit for swimming while people use the pool, and don't swim until FC is back at or below that limit.

The chlorine dose calculator turns the gap between your FC and your target into fluid ounces or pounds of your product. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.

Fix 2: lower CYA by replacing water

Stabilizer doesn't break down in normal pool use, and no common chemical removes it. The CDC's model code makes the point plainly: draining alone doesn't lower the level; the refill with fresh water does. So the fix is to drain part of the pool and refill it.

The share to replace comes from one formula: (CYA now − CYA target) ÷ (CYA now − CYA in your fill water). With fill water at 0 ppm CYA, our engine gives:

You don't have to do it all at once. Several smaller drains and refills reach the same level, which matters for vinyl liners and above-ground pools: check your pool's manual before draining low. The CYA drain calculator works out gallons for your pool, and how often to drain a pool covers timing, partial drains, and when to call a pro. Retest CYA after refilling and running the pump for a few hours.

Products sold as stabilizer reducers use bacteria and can be slow or inconsistent. We could not confirm reliable results for them, so we don't count on them here.

Prevent it: watch stabilized chlorine

CYA usually builds up from stabilized chlorine. Trichlor tablets add about 0.6 ppm of CYA for every 1 ppm of chlorine, and dichlor adds about 0.9 ppm (PHTA). Liquid chlorine, bleach, and calcium hypochlorite (cal-hypo) add none. A season on tablets alone can push CYA well past 80.

• Test CYA every month or two in season. How to test cyanuric acid shows how to get a clean reading.
• When CYA reaches about 50 ppm, switch to unstabilized chlorine, such as liquid chlorine or cal-hypo, until the next water change.
• See how fast your own routine adds CYA with the CYA buildup calculator.

FAQ

Is chlorine lock real?

Not as a permanent lock. Stabilizer binds chlorine loosely and releases it as it is used. The real problem is that high CYA raises the amount of free chlorine you need, often above the limit on the label. Lowering CYA by replacing water fixes it.

Can you get chlorine lock with low CYA?

Not the stabilizer kind. If CYA is low and chlorine still disappears, look for demand instead: algae, ammonia, or heavy use. With CYA under 20, sunlight alone can burn off most of a day's chlorine. Run the overnight loss test to tell them apart.

Can a hot tub get chlorine lock?

Yes, if you use dichlor for a long time. Each dose adds stabilizer, and a hot tub holds little water, so CYA builds fast. The CDC recommends not using CYA in hot tubs. Changing the water resets it; see how often to change hot tub water.

Does cal hypo cause chlorine lock?

No. Cal-hypo adds no stabilizer, so it can't raise CYA. It adds calcium instead, about 0.7 ppm for each 1 ppm of chlorine, so watch calcium hardness if you use it all season.

Sources

• CDC, Model Aquatic Health Code 2024, Annex (CYA and free chlorine ratio, CYA lowered only by refilling with fresh water): cdc.gov (PDF)
• CDC, Home pool and hot tub water treatment and testing (FC at least 2 ppm with CYA; no CYA in hot tubs): cdc.gov
• PHTA fact sheet, Dichlor (CYA added per ppm of chlorine): phta.org
• U.S. EPA, examples of pool chlorine labels with a re-entry limit of 4 ppm and 3 ppm: EPA label (PDF), EPA label (PDF)
• FC minimum and target by CYA, and the drain shares: our engine; see methodology.