Free Chlorine
Free chlorine is the concentration of active, unsaturated chlorine available in pool water to sanitise and disinfect.
Definition
Free chlorine is the concentration of active, unsaturated chlorine available in pool water to sanitise and disinfect.
Typical Values: Pools: 1–3 ppm; Hot tubs: 3–5 ppm; Minimum safe: 1.0 ppm
In Plain Language
Free chlorine consists of two forms: hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Hypochlorous acid is the bactericidal form — at pH 7.2, roughly 67% of free chlorine is in this effective form. Free chlorine is consumed by UV light, organic contaminants, and the act of sanitisation itself. It must be replenished regularly.
Why It Matters
Inadequate free chlorine allows bacteria and algae to establish. Excess free chlorine irritates swimmers. Both extremes are avoidable with regular testing.
Typical Values
Pools: 1–3 ppm; Hot tubs: 3–5 ppm; Minimum safe: 1.0 ppm
Last reviewed: 2026-06-01