Free vs Total vs Combined Chlorine: The Number That Actually Matters
Updated September 2026
Quick answer: Free chlorine is the active sanitizer — the number you manage to 1–3 ppm. Total chlorine includes spent, bound-up chlorine (chloramines) that irritates skin and eyes but barely sanitizes. The difference between them is combined chlorine: keep it under 0.5 ppm, and when it climbs past that, shock to break it apart. The 'strong chlorine smell' of a bad pool is actually chloramines — a sign of too little effective chlorine, not too much.
Three numbers, one relationship
The arithmetic is simple: total chlorine = free chlorine + combined chlorine. Free chlorine (FC) is chlorine still available to kill things — the only number that sanitizes. Combined chlorine (CC) is chlorine that already reacted with sweat and bather waste, forming chloramines: poor sanitizers, excellent irritants.
Cheap test strips often show only total chlorine, which is how pools fool their owners — a 3 ppm total reading can hide 2.5 ppm of useless chloramines over 0.5 ppm of actual sanitizer. A drop-based kit that reads FC and CC separately is the single most valuable upgrade in pool testing.
The smell myth, corrected
That sharp 'chlorine' smell at a badly run pool is chloramines off-gassing — the smell of chlorine that already died in service. A properly chlorinated pool with near-zero CC barely smells at all, even at 3 ppm free chlorine. Red eyes after swimming point the same direction: chloramines and bad pH, not 'too much chlorine.'
So the counterintuitive fix for a smelly, stinging pool is more chlorine, not less: a shock dose oxidizes the chloramines out (breakpoint chlorination), and the smell leaves with them. Cutting back chlorine because the pool smells is precisely backwards and is a reliable first chapter in a green-pool story.
Managing the gap
Test combined chlorine weekly by subtraction (or directly with a FAS-DPD kit). Under 0.5 ppm: all good. Over it: shock at dusk and retest next day. Chronically high CC despite shocking points at heavy bather load, poor circulation, or an indoor/covered pool where UV never helps break chloramines down.
Prevention is mostly free: rinse-off showers before swimming remove the sweat and product chloramines feed on, decent CYA keeps FC alive to do its work, and generous pump runtime keeps oxidation happening everywhere instead of only near the returns.
FAQ
What's the difference between free and total chlorine?+
Free chlorine is active sanitizer; total also counts chlorine already bound into chloramines, which sanitize poorly. Subtract them to get combined chlorine — over 0.5 ppm means it's time to shock.
Why does my pool smell like chlorine?+
That smell is chloramines — spent chlorine — not excess sanitizer. A clean, properly chlorinated pool barely smells. The fix is a shock to oxidize the chloramines, not reducing chlorine.
What should free chlorine be in a pool?+
1–3 ppm for routine operation (with CYA in the 30–50 range), 10–12 ppm during a shock, and higher for algae kills. Below 1 ppm, algae and bacteria start winning within days.