The FC/CYA ratio: how stabilizer sets your chlorine level

What FC and CYA actually are

FC (free chlorine) is the chlorine your test kit reads as available to sanitize. CYA (cyanuric acid, "stabilizer" or "conditioner") is a chemical that protects chlorine from sunlight. Without it, the sun can destroy most of the chlorine in an outdoor pool within hours.

The catch: CYA does not just shield chlorine, it binds to it. Most of the free chlorine in a stabilized pool sits attached to CYA as a reserve. Only a small fraction is in the active form, hypochlorous acid (HOCl), which is what actually kills germs and algae. As the bound chlorine is used up, the reserve releases more.

Why the ratio matters more than the number

Because CYA holds chlorine back, the same FC reading means very different things at different CYA levels. 3 ppm FC with no stabilizer is a lot of active chlorine. 3 ppm FC with 80 ppm CYA is barely enough to keep algae away.

Trouble Free Pool summarises it this way: CYA does not reduce chlorine's effectiveness, it redistributes it. If you keep FC as a roughly constant percentage of CYA, the active chlorine level stays roughly constant too. This is the whole idea behind the FC/CYA chart, originally worked out by the chemist known on the forum as "Chem Geek" (Richard Falk) and refined from forum members' experience.

The chart and the shortcut percentages

CYA, ppm	Minimum FC	Target FC	SLAM FC
20	2	3–5	8
30	2	4–6	12
40	3	5–7	16
50	4	6–8	20
60	5	7–9	24
70	5	8–10	28
80	6	9–11	31
90	7	10–12	35
100	8	11–13	39

Values from the Trouble Free Pool chart for pools chlorinated with liquid chlorine or tablets. If you don't have the table handy, the approximations are:

• Minimum FC ≈ 7.5 % of CYA. Below this, algae can start to grow.
• Target FC ≈ 11.5 % of CYA (the chart's range runs a bit above that).
• SLAM FC ≈ 40 % of CYA. The level you hold while clearing algae or cloudy water.

Example: CYA 50. Minimum 3.75 → 4 ppm, target around 6–8 ppm, SLAM 20 ppm.

Salt water pools

A salt water chlorine generator (SWG) produces chlorine continuously through the day instead of in one daily dose, so FC never swings as low. TFP notes that SWG pools can be maintained at a minimum of about 5 % of CYA instead of 7.5 %, and recommends higher CYA for them, around 60–90 ppm, which reduces how hard the cell has to work.

A worked example

Say your outdoor pool on liquid chlorine tests at FC 3 and CYA 60. A label that targets 1–3 ppm says you are fine. The chart says the minimum at CYA 60 is 5 ppm and the target is 7–9 ppm, so you are below the level where algae is held back. The fix is not more stabilizer and not a weekly "shock" bag; it is raising FC to about 8 ppm and dosing daily so it never falls below 5 before the next test.

Now the opposite case: the same pool with CYA 0 after a refill. FC 3 is plenty of active chlorine, but the sun can burn most of it off within a day. Adding stabilizer to about 30–40 ppm keeps chlorine from vanishing and lowers your daily dose, as long as you then raise FC to match the new CYA.

How much CYA should you have?

Trouble Free Pool's recommendations:

• Indoor pools: 20–30 ppm (little sun to protect against).
• Outdoor pools on liquid chlorine: 30–60 ppm. More CYA means you need more FC every day.
• Salt water pools: 60–90 ppm.

For hot tubs the CDC recommends not using cyanuric acid or stabilized chlorine products at all.

The tablet trap

CYA does not evaporate, break down in normal use or get filtered out. It only leaves with water: splash-out, backwash, draining. Meanwhile every stabilized chlorine product adds more:

• Trichlor tablets add about 0.6 ppm CYA for every 1 ppm of FC.
• Dichlor granules add about 0.9 ppm CYA for every 1 ppm of FC.

Feed a pool 3 ppm of chlorine a day from tablets for 100 days and you have added roughly 180 ppm of CYA. Long before that, the minimum FC on the chart has climbed past what the tablets can deliver, and the pool turns green "even though there is chlorine in it". This is one of the most common stories in r/pools. The fix is dilution (partial drain and refill) and switching to unstabilized chlorine such as liquid chlorine or cal-hypo for daily dosing.

Common mistakes

• Dosing to the label, not the chart. Labels often target 1–3 ppm, which assumes little or no CYA.
• Never testing CYA. Strips are rough at this; a turbidity test from a drop kit is better. TFP suggests reading in steps of 10 and rounding up when in doubt.
• "Shocking" once instead of SLAMming. One big dose that drops below SLAM by the next day does not clear algae. SLAM means holding the level until the water is clear and the chlorine stops dropping overnight.
• Adding stabilizer too fast. Granular CYA dissolves slowly, often taking days to show up fully in a test. Re-test after about a week before adding more.

Turning the chart into doses

Once you know where FC should sit, the dose is the difference times your volume, adjusted for product strength; see how much chlorine to add for the formula. PoolDose uses this FC/CYA chart for its chlorine targets: enter FC and CYA from your test and it shows the minimum, the target and the SLAM level for your pool, plus the amount of your own chlorine product needed to get there. It is a calculator; your test kit and the product label remain the authority.