CYA Calculator — Free Chlorine Targets by Stabilizer Level

Example calculation

A pool with CYA of 50 ppm and a current free chlorine reading of 2 ppm:

At 2 ppm FC with CYA of 50 ppm, this pool is unprotected — algae can establish within 24–48 hours. FC must be raised to at least 4 ppm immediately.

CYA FC Chart — Free Chlorine Targets by Stabilizer Level

Complete FC targets for every common CYA level. Use this chart alongside your test results to know instantly whether your chlorine is safe, low, or critical.

Minimum FC = CYA × 7.5%. Target FC = CYA × 10–15%. SLAM FC = CYA × 40%. CYA above 80 ppm: consider a partial drain — see our stabilizer calculator.

Is my pool safe to swim? — FC vs CYA safety check

Use this table to instantly check whether your current FC is safe, low, or critical for your CYA level. A pool that tests "2 ppm chlorine" is safe at CYA of 20 ppm but critically under-chlorinated at CYA of 60 ppm.

✅ Safe = FC is at or above minimum threshold for that CYA level.
⚠️ Low = FC is below target but above absolute minimum — add chlorine soon.
🚫 Unsafe = FC is below the minimum threshold — do not swim, add chlorine immediately.
Use the chlorine calculator to find the exact dose needed to reach your target FC.

What minimum, target, and SLAM FC mean

Minimum FC — the floor

The minimum free chlorine level is the lowest FC at which your pool has enough active hypochlorous acid (HOCl) to prevent algae growth at your CYA level. Below this, chlorine demand from algae and bacteria exceeds chlorine supply and the pool will turn green — often within 24–48 hours in warm weather.

The minimum is approximately CYA × 7.5%. At CYA of 40 ppm, that's 3 ppm FC minimum. A reading of 2 ppm at that CYA looks like "some chlorine" but is below the protection threshold.

Target FC — where to maintain daily

The target FC range is CYA × 10–15%. This gives a comfortable buffer above the minimum so that normal chlorine consumption from sunlight, swimmers, and organics doesn't drag FC below the floor overnight or between tests. For a pool with CYA of 50 ppm, maintain FC between 5–8 ppm day-to-day.

Test at least twice a week and add chlorine whenever FC drops toward the lower end of the target range. Use our chlorine calculator for exact doses.

SLAM FC — the algae killer

The SLAM level is CYA × 40%. This is the elevated FC required to kill existing algae within hours rather than days. It must be actively maintained — not just reached once. FC drops rapidly as it kills algae, so it must be topped up every 2–4 hours during a SLAM.

Use the SLAM calculator for step-by-step guidance, liquid chlorine dose calculations, and the three completion tests.

Why CYA changes how much chlorine you need

💡 Use our stabilizer calculator to raise or lower CYA, and our chlorine calculator to dose chlorine to your FC target.

What to do when CYA is too high

CYA above 80–90 ppm creates a condition sometimes called "chlorine lock" — though the more accurate description is that the minimum FC needed to prevent algae becomes so high that it's impractical to maintain. At CYA of 100 ppm, minimum FC is 8 ppm and the SLAM target is 40 ppm. Maintaining 8 ppm FC daily in a 20,000 gallon pool is expensive and difficult.

Signs your CYA is too high:
• Pool keeps going green despite regular chlorination
• FC reads "okay" (2–4 ppm) but algae grows anyway
• You're adding more chlorine than ever but the pool won't hold it
• CYA test reads above 80 ppm

Solution: Partial drain and refill. Drain 30–50% of the pool volume and refill with fresh water. This dilutes CYA proportionally. Use our stabilizer calculator to calculate exactly how much to drain to reach your target CYA.

Switching to liquid chlorine or cal-hypo (instead of trichlor tablets) stops further CYA accumulation — neither adds CYA. This is the most important long-term fix alongside the drain.

Verify CYA accurately before and after any drain with a liquid reagent test kit — test strips frequently misread CYA and can underestimate by 20–30 ppm.