Free chlorine and stabilizer: how much chlorine your pool really needs
"Keep chlorine at 1–3 ppm" is the most repeated advice in pool care, and it's the main reason pools turn green. The chlorine level you need depends on how much stabilizer (cyanuric acid, CYA) is in the water. This guide explains why, and gives a simple rule for finding your number.
What stabilizer does
Chlorine in water exists mostly as hypochlorous acid and hypochlorite. Ultraviolet light from the sun breaks both down quickly: in an unstabilized outdoor pool, much of the chlorine can be gone within a few hours of bright sun. Cyanuric acid latches onto chlorine loosely and shields it from UV. The bound chlorine isn't lost: as the small amount of active chlorine is used up, more is released from the CYA. It acts like a reservoir.
The catch is that only the unbound part is actively killing germs and algae. At a given free chlorine reading, more CYA means a smaller active fraction. The CDC notes this too: its chlorine minimum doubles from 1 ppm to 2 ppm when stabilizer is used, and it points out that disinfection takes longer in the presence of cyanuric acid.
The FC/CYA ratio
Because the active chlorine depends mainly on the ratio of free chlorine (FC) to CYA, pool owners can target a ratio rather than a fixed number. A guideline widely used in the pool-owner community, popularized by the Trouble Free Pool forum's chlorine/CYA chart, is:
Shock level ≈ 40% of CYA
Salt pools can run a little lower because the cell adds chlorine continuously through the day, instead of in one daily dose that has to last. The minimum is a floor: aim a couple of ppm above it so normal daily losses don't take you under.
| CYA (ppm) | Minimum FC (7.5%) | Minimum FC, salt cell (5%) | A comfortable target | Shock (40%) |
|---|---|---|---|---|
| 20 | 2* | 2* | 4–5 | 8 |
| 30 | 2.3 | 2* | 4.5–5.5 | 12 |
| 40 | 3 | 2 | 5–6 | 16 |
| 50 | 3.8 | 2.5 | 6–7 | 20 |
| 60 | 4.5 | 3 | 6.5–7.5 | 24 |
| 70 | 5.3 | 3.5 | 7.5–8.5 | 28 |
| 80 | 6 | 4 | 8–9 | 32 |
* Where the percentage gives less than 2 ppm, the CDC's minimum for stabilized pools (2 ppm) applies instead.
The "comfortable target" column is simply the minimum plus a margin of 2–3 ppm, so a day of sun and swimming doesn't take you under. Above about 30 ppm CYA, the guideline asks for more than the CDC's 2 ppm. It's stricter because it's designed to prevent algae, not just to meet a health minimum.
Chlorine labels and some local codes set an upper limit on free chlorine for swimming. Follow your product label, and don't swim at shock levels.
Why high CYA causes trouble
Stabilizer doesn't get used up. It leaves only with water that's drained, splashed out or backwashed. Every trichlor tablet and every scoop of dichlor adds some:
- Trichlor adds about 0.6 ppm CYA for each 1 ppm of chlorine.
- Dichlor adds about 0.9 ppm CYA for each 1 ppm of chlorine.
A pool that uses 2 ppm of chlorine a day from tablets gains about 1.2 ppm CYA a day, more than 30 ppm a month. By late summer, CYA of 80–100 ppm or more is common. At that point the chlorine level needed rises out of reach, algae appears "even though there's chlorine in the water", and shocking takes gallons of chlorine. The fix is replacing water (the drain and refill calculator shows how much), then switching some or all of your chlorine to liquid chlorine, bleach or a salt cell, which add no CYA.
How to choose your CYA level
- Liquid chlorine or bleach, added daily or by a pump: 30–50 ppm.
- Salt chlorine generator: often 60–80 ppm. It means the cell can run at a lower output. Check the generator's manual.
- Mostly shaded or indoor pool: little or none.
- Tablets in a feeder: start low, around 30 ppm, because the tablets will keep raising it.
The stabilizer calculator works out how much to add.
A worked example
Your pool has 50 ppm CYA and you add liquid chlorine. Minimum FC: 0.075 × 50 ≈ 3.8 ppm, so a target of 6–7 ppm after each dose is comfortable. If the pool loses about 2–3 ppm per sunny day, you'd add chlorine daily to bring it back to 6–7. The chlorine calculator gives the dose. With 12.5% liquid chlorine in 15,000 gallons, raising FC by 3 ppm takes about 46 fl oz.
Frequently asked questions
Is 7 ppm of chlorine safe to swim in?
The CDC gives minimum chlorine levels and tells pool owners not to exceed the product label's upper limits. With stabilizer in the water, 7 ppm free chlorine is much less active than 7 ppm in unstabilized water, which is why stabilized pools can run higher readings without the "chlorine smell" people associate with overchlorination. That smell is usually combined chlorine (chloramines), a sign that more chlorine is needed, not less. Follow your chlorine product's label for swimming limits.
Do I need CYA in an indoor pool?
Usually not, since there's little UV to protect against. Check local rules for indoor or public pools.
Can I test CYA with strips?
Strips give a rough idea. A turbidity drop test, read in daylight, is more reliable. See how to test pool water.
Sources: CDC, Home Pool and Hot Tub Water Treatment and Testing. Trouble Free Pool, CYA and chlorine (community guideline).