Pool water chemistry, explained.
The ideal range for every core parameter, what it actually means, and exactly how to fix it when it drifts out of balance.
All water chemistry parameters
pH measures how acidic or basic your pool water is on a 0-14 scale, with 7 as neutral. Pool water is kept slightly above neutral to stay close to the pH of the human eye and skin.
Free chlorine is the active, available sanitizer in the water that's ready to kill bacteria and algae, as distinct from chlorine that has already reacted with contaminants (see Combined Chlorine).
Combined chlorine (chloramines) is chlorine that has already reacted with sweat, oils, and other contaminants and lost most of its sanitizing power.
Total alkalinity measures the water's ability to resist pH changes, acting as a buffer that keeps pH from swinging wildly every time a chemical is added.
Calcium hardness measures the concentration of dissolved calcium in the water, which affects how the water interacts with plaster, grout, and metal surfaces.
Cyanuric acid, also called stabilizer or conditioner, shields chlorine from breaking down under UV sunlight so it lasts longer in outdoor pools.
Salt level measures the dissolved sodium chloride that a salt chlorinator converts into chlorine through electrolysis, letting a saltwater pool generate much of its own sanitizer.
Water temperature doesn't affect chemical balance directly, but it changes how fast other parameters move — warmer water typically accelerates chlorine breakdown and algae growth.