Perfect Range 1 - 3 PPM (pools), 3 - 5 PPM (spas)

Free Chlorine is the Chlorine that's available for sanitizing. It's also referred to as "Residual Chlorine". This is the active sanitizer that needs to be present at all times in chlorine pools and spas. Free Chlorine reacts instantly with bather wastes and contaminants. Free Chlorine that combines with waste and contaminants is referred to as "Combined Chlorine".

Once it's turned from Free Chlorine to Combined Chlorine, it no longer has sanitizing capabilities and has to be replenished. It's best to use an automatic chlorinator to deliver a constant small amount of Free Chlorine.
The Perfect range for Free Chlorine in pools is 1 to 3 PPM (Parts Per Million). For spas, 3 to 5 PPM.