Free chlorine (FC) is the measure of active, available disinfectant in pool water. It kills bacteria, viruses, and algae on contact. However, its killing power is highly pH-dependent, which is why chlorine and pH must be managed together.
Key Facts
- Target 1–3 ppm free chlorine for pools; 3–5 ppm for hot tubs.
- At pH 7.2, about 67% of free chlorine is in its most effective form (HOCl). At pH 8.0, this drops to 22%.
- Free chlorine is consumed continuously by sunlight, bathers, and organic matter.
- CYA (stabiliser) slows UV depletion but also reduces the active fraction of free chlorine.
How Free Chlorine Sanitizes
When chlorine is added to pool water, it reacts with water to form hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Hypochlorous acid is the active disinfectant — it penetrates the cell walls of bacteria and viruses and destroys them. Hypochlorite ion is much weaker. The ratio of HOCl to OCl- is determined by pH: lower pH produces more HOCl; higher pH shifts the balance toward less effective OCl-. This is why identical chlorine readings have very different sanitising power at different pH levels.
Maintaining the Right Level
Pool free chlorine should stay between 1 and 3 ppm at all times. Below 1 ppm, sanitisation is inadequate and algae can begin to establish. Above 5 ppm, the water is uncomfortable for swimmers and may bleach swimwear or pool surfaces. In outdoor pools exposed to direct sunlight, without cyanuric acid, chlorine can deplete to zero within two to three hours. A target of 2–3 ppm provides a safety buffer against spikes in organic load or periods of higher-than-normal UV.
Free Chlorine vs. Combined Chlorine
Total chlorine equals free chlorine plus combined chlorine. Combined chlorine (chloramines) forms when free chlorine reacts with nitrogen-containing compounds from bathers — sweat, urine, body oils. Chloramines are much weaker disinfectants than free chlorine and are responsible for the "pool smell" and eye irritation most people associate with chlorine. If your combined chlorine exceeds 0.5 ppm, shock treatment is needed to destroy it. Always test specifically for free chlorine (using a DPD kit) rather than total chlorine.
Examples
Two pools both read 2 ppm free chlorine. Pool A has pH 7.2; Pool B has pH 8.0. Pool A has approximately 1.34 ppm of active HOCl. Pool B has only 0.44 ppm of active HOCl — less than one-third of Pool A's effective sanitiser level, despite the same total reading. Lowering Pool B's pH to 7.4 increases effective chlorine without adding a single drop of chlorine product.
Common Mistakes to Avoid
- Keeping pH above 7.8 while wondering why chlorine keeps depleting quickly.
- Using an OTO test that measures total chlorine and treating that result as free chlorine.
- Adding chlorine during the day in an outdoor pool without CYA — most of it will be destroyed by UV before the pump can circulate it.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- CDC — Healthy Swimming Guidelines