Free Chlorine (FC)
What is Free Chlorine (FC)?
Free chlorine is the portion of total chlorine in pool or spa water that has not reacted with ammonia or other nitrogen compounds and is available to sanitize. It exists as two chemical species in solution: hypochlorous acid (HOCl) and hypochlorite ion (OCl-), with HOCl being the stronger disinfectant of the two. The 2024 CDC Model Aquatic Health Code (5th edition, section 2.0 Glossary of Terms) defines free available chlorine (FAC) as the portion of the total available chlorine that is not combined chlorine or cyanurate-bound available chlorine and is primarily present as HOCl or OCl-. The MAHC adds that DPD-FC equals FAC plus cyanurate-bound chlorine (CBC), so the terms DPD-FC and FAC are interchangeable only in the absence of cyanuric acid. A DPD (N,N-diethyl-p-phenylenediamine) test kit reads free chlorine directly; the gap between the total chlorine reading and the free chlorine reading is the combined figure. Free chlorine drives the sanitation: a pool can show an adequate total chlorine number while most of that chlorine is bound up as chloramines, leaving too little free chlorine to kill pathogens.
Free chlorine is the reading that tells an operator whether the water can actually disinfect right now, as opposed to total chlorine, which includes spent sanitizer that has already reacted. The two species that make up free chlorine, hypochlorous acid (HOCl) and hypochlorite ion (OCl-), are in a pH-dependent equilibrium. The MAHC notes that the pH of the water determines the relative amounts of HOCl and OCl-, and the CDC puts it plainly: as pH goes up, the ability of chlorine to kill germs decreases. That pH sensitivity is why the pool water chemistry guide pairs free chlorine targets with a pH window rather than treating them in isolation. Total alkalinity is what holds that pH window steady: it buffers pH against swings after every chemical add, and when alkalinity drops, keeping pH in the range where free chlorine is effective becomes a daily errand.
The arithmetic that separates free from combined chlorine is straightforward: total chlorine (TC) equals free chlorine (FC) plus combined chlorine (CC), so CC = TC minus FC. A DPD test kit reads both values, and the combined figure is the subtraction. MAHC section 2.0 defines combined available chlorine as the fraction present as chloramines and notes it is “less reactive than free available chlorine,” which is why a route stop with a high total chlorine number but low free chlorine still has a sanitation problem.
When cyanuric acid (CYA) is present in an outdoor pool, it binds with HOCl and reduces effective sanitizing strength, so operators track the ratio of free chlorine to CYA rather than free chlorine alone. The FC/CYA ratio calculator computes that ratio. Adjusting the free chlorine level itself is a dose question: the pool chlorine dose calculator handles the product-strength math.