Free Chlorine (FC)
Free chlorine (FC) is the chlorine in your pool water that's actively available to kill bacteria, algae, and other contaminants. It's the single most important pool chemistry reading — the number that determines whether your water is safe to swim in.
Target ranges
Here's where most pool advice gets it wrong: there is no single FC target. Your minimum FC depends entirely on your cyanuric acid (CYA) level. A "good" FC reading can be dangerously low if CYA is high.
| CYA level | Minimum FC | Target range |
|---|---|---|
| 0 ppm (no CYA) | 1 ppm | 1–3 ppm |
| 30 ppm | 2 ppm | 3–5 ppm |
| 40 ppm | 3 ppm | 4–6 ppm |
| 50 ppm | 4 ppm | 4–6 ppm |
| 60 ppm | 5 ppm | 5–7 ppm |
| 70–80 ppm (SWG) | 6 ppm | 5–8 ppm |
| 100 ppm | 8 ppm | 7–10 ppm |
The 7.5% rule
Minimum FC should be at least 7.5% of your CYA level. At CYA 60, that's 4.5 ppm. Below this threshold, chlorine can't keep up with algae and bacteria — even though the FC reading "looks fine" by old generic charts. See the full FC/CYA chart.
What free chlorine does
Free chlorine sanitizes by oxidizing contaminants — bacteria, viruses, algae, sweat, urine, skin oils. The oxidation reaction destroys microorganisms and converts organic material into harmless byproducts.
"Free" means the chlorine hasn't yet reacted with anything. Once it reacts with a contaminant containing nitrogen, it becomes combined chlorine (a chloramine) — no longer active. Free chlorine is the only chlorine that's actually working.
The two forms of free chlorine
Free chlorine exists in two forms that exchange depending on pH:
- Hypochlorous acid (HOCl) — the active killer. Strong oxidizer, destroys microorganisms efficiently.
- Hypochlorite ion (OCl−) — much weaker. Only about 1/80 as effective at sanitizing.
The percentage of FC in the active HOCl form depends on pH. At pH 7.0, about 73% of FC is HOCl. At pH 8.0, only 22% is. This is why high pH wastes chlorine — the FC reading might look fine but most of it is in the weak OCl− form.
How free chlorine drops
FC doesn't stay at a fixed level — it's consumed continuously by:
- UV from sunlight — destroys unprotected FC rapidly. A pool with no CYA can lose 90%+ in a few hours of direct sun. CYA shields FC from UV; see CYA explained.
- Reacting with contaminants — sweat, sunscreen, leaves, organics. Every reaction consumes one FC molecule.
- Algae and bacteria — even an early bloom you can't see consumes FC fast.
- Warm water — bacteria multiply faster, chlorine reactions speed up.
- Chloramine buildup — when FC drops too low, contaminants form chloramines instead of being destroyed, locking up your available chlorine in non-active form.
How to maintain free chlorine
Three habits keep FC stable:
- Set your real target based on CYA. Generic "2-4 ppm" advice fails for stabilized pools. Use the FC/CYA chart.
- Test 2-3 times per week minimum. Daily during heavy use. FC can swing fast between tests.
- Add chlorine before FC drops below minimum, not after. Once you fall below the threshold, algae starts winning the chemistry race.
For dosing methods, see how to raise pool chlorine safely. For diagnosing rapid FC loss, see why won't chlorine stay in my pool.
Free chlorine vs total chlorine
Test kits show two numbers: free chlorine (FC) and total chlorine (TC). TC includes both free and combined (used-up) chlorine. The gap between them tells you something important:
- TC = FC — perfect. No chloramines, all chlorine is active.
- TC slightly above FC — normal. Small amount of CC building up.
- TC much higher than FC — high combined chlorine. Time to shock.
See free chlorine vs total chlorine for the full breakdown.
Want to...
Never miss your real FC target
PoolChem Tracker calculates your minimum effective FC based on your actual CYA level — and warns you the moment your chlorine drops below the algae threshold.
