Chlorine and stabilizer calculator: what FC for my CYA?

Chlorine and cyanuric acid (CYA) bind together loosely. That bond is what shields chlorine from ultraviolet light, and it is also why stabilized chlorine works more slowly: only the unbound part, hypochlorous acid, does most of the sanitizing. The CDC’s Model Aquatic Health Code puts it plainly — because chlorine efficacy is reduced in the presence of CYA, pools that use it need higher free chlorine.

Where the numbers come from

• CDC and the Model Aquatic Health Code (official). At least 1 ppm free chlorine without CYA, at least 2 ppm with CYA, 3 ppm in spas. The 2024 code also caps CYA at 90 ppm and treats a CYA-to-chlorine ratio above 45:1 as an imminent health hazard in public pools. That works out to free chlorine of at least CYA ÷ 45, about 2.2% of CYA — a floor, not a target.
• Trouble Free Pool (hobbyist convention). The forum’s widely copied chart sets a minimum free chlorine of 7.5% of CYA (5% with a salt generator) and a shock (“SLAM”) level of 40% of CYA, citing the chlorine-CYA equilibrium research of O’Brien, Morris and Butler (1974). It is not a health code, but it is the most common way home pool owners scale chlorine to stabilizer, and it is what the minimum and shock numbers here use.
• State codes. Some states write their own scale — Michigan, for example, raises the minimum by 0.5 mg/L for each 20 ppm of CYA above 40. Public and shared pools follow the local code, whatever any chart says.

minimum FC ≈ CYA × 0.075 (salt generator: × 0.05), never below 2 ppm
shock / SLAM FC ≈ CYA × 0.40
public-pool floor = the larger of 2 ppm and CYA ÷ 45

Chlorine for each stabilizer level

The full chart, with the official and hobbyist numbers side by side, is in the pool chemistry levels chart. For salt pools specifically, see what CYA and chlorine a salt water pool needs.

How to use it day to day

1. Test CYA once a month (and a day after you add stabilizer). It doesn’t burn off; it only leaves when water splashes out or is drained.
2. Read the minimum FC for that CYA. Test chlorine daily in summer and add chlorine before it drops below the minimum — the liquid chlorine calculator turns a ppm rise into fluid ounces.
3. For algae, cloudy water or opening a green pool, raise chlorine to the shock level and hold it there until the water clears — see how much liquid chlorine to SLAM a pool.
4. If CYA climbs past about 80–90 ppm (usually from tablets or dichlor), no amount of chlorine is convenient. Drain and refill part of the water with the drain and refill calculator, and see how much water to drain to lower CYA.

Where CYA comes from

Granular stabilizer adds it directly. So does every stabilized chlorine: by stoichiometry, trichlor tablets add about 0.6 ppm of CYA for every 1 ppm of chlorine they release, and dichlor about 0.9 ppm. A pool on tablets all summer can climb 50 ppm or more — the chlorine tablet calculator shows the weekly build-up. Liquid chlorine, cal-hypo and salt generators add none.