Free chlorine is the portion of chlorine in water that is still available to kill germs, while total chlorine is the sum of free chlorine plus combined chlorine (chlorine that has already reacted with contaminants and lost most of its sanitizing power). The simple formula: Total Chlorine = Free Chlorine + Combined Chlorine. Understanding this distinction matters because a high total chlorine reading can mask the fact that very little active disinfectant remains in the water.
How Free Chlorine Works
When you add chlorine to water, whether from a tablet, liquid bleach, or a salt chlorine generator, it dissolves into two chemical forms: hypochlorous acid and hypochlorite ion. Both count as free chlorine, but hypochlorous acid is the real workhorse. It’s roughly 80 to 100 times more effective at killing bacteria than hypochlorite ion.
The balance between these two forms depends almost entirely on pH. At a neutral pH of around 6.0 to 7.5, hypochlorous acid dominates, giving you strong disinfection. As pH climbs above 7.5, the balance shifts toward the weaker hypochlorite ion. This is why pH control matters just as much as the chlorine level itself. A pool or water system with plenty of free chlorine but a pH of 8.0 is disinfecting far less effectively than the numbers might suggest.
What Combined Chlorine Is
Combined chlorine forms when free chlorine reacts with ammonia and nitrogen-containing organic compounds in the water. These reaction products are called chloramines, primarily monochloramine, dichloramine, and trichloramine. In pools, the ammonia and nitrogen come from sweat, urine, body oils, and other organic matter that swimmers introduce. In drinking water, ammonia can be present naturally in the source water or added intentionally during treatment.
Chloramines still have some disinfecting ability, but it’s dramatically weaker than free chlorine. More importantly, they’re responsible for the problems people typically blame on “too much chlorine.” That harsh chemical smell at an indoor pool? That’s not free chlorine. It’s chloramines, particularly trichloramine, off-gassing from the water’s surface. Because chloramine gas is heavier than air, it hovers just above the waterline where swimmers breathe, causing eye irritation, skin rashes, and respiratory discomfort. The CDC specifically identifies chloramine off-gassing as a primary cause of irritation in swimmers and even spectators at indoor pools.
A strong “chlorine smell” is actually a sign that the water needs more free chlorine, not less.
How to Calculate Combined Chlorine
You can’t measure combined chlorine directly with a standard test kit. Instead, you test free chlorine first, then test total chlorine, and subtract:
Combined Chlorine = Total Chlorine − Free Chlorine
Most test kits and strips use a chemical indicator called DPD. When DPD contacts free chlorine, it turns magenta. To get total chlorine, a second reagent (potassium iodide) is added, which causes any chloramines present to also react with the DPD, deepening the color. The difference between the two readings gives you the combined chlorine level.
If your free and total chlorine readings are the same, your combined chlorine is zero, which is ideal. Any gap between the two numbers tells you how much chlorine has been “used up” by contaminants.
Recommended Levels for Pools and Drinking Water
The CDC recommends a free chlorine concentration of at least 1 ppm (parts per million) in swimming pools and at least 3 ppm in hot tubs, with pH maintained between 7.0 and 7.8. If you use a stabilizer like cyanuric acid (common in outdoor pools), the minimum free chlorine rises to 2 ppm because cyanuric acid slows chlorine’s killing speed.
For drinking water, the EPA sets a maximum residual disinfectant level of 4 ppm for chlorine. Most tap water contains well under that, typically 0.2 to 2.0 ppm of free chlorine when it leaves the treatment plant.
Combined chlorine should ideally be zero. The Pool Water Treatment Advisory Group recommends it never exceed half the free chlorine level, and even then, it should stay below 1 ppm regardless of how high free chlorine is. If your combined chlorine consistently rises alongside your free chlorine rather than dropping, something in the water is generating contaminants faster than chlorine can handle them.
Removing Combined Chlorine
The standard fix for high combined chlorine is breakpoint chlorination, commonly called “shocking” the pool. This means adding enough free chlorine to overwhelm and chemically destroy the chloramines. The required dose is roughly 10 times the measured combined chlorine level. So if your combined chlorine reads 0.5 ppm, you’d need to raise the free chlorine by about 5 ppm above its current level to hit breakpoint.
This works because at a high enough concentration, free chlorine breaks the nitrogen-chlorine bonds in chloramines, converting the nitrogen to harmless gas that escapes into the atmosphere. Below that threshold, adding small amounts of chlorine can actually increase combined chlorine by creating more partial-reaction products.
Non-chlorine shock products, typically based on potassium monopersulfate, offer an alternative. These oxidizers destroy ammonia and nitrogen-containing waste without adding chlorine to the water, which can be useful for indoor pools where chloramine off-gassing is a concern. However, non-chlorine oxidizers don’t kill bacteria or algae, so they complement rather than replace chlorine-based treatment.
Why Total Chlorine Alone Is Misleading
If you only test total chlorine, a reading of 3 ppm looks perfectly healthy. But that number could mean 3 ppm of active, germ-killing free chlorine and zero combined chlorine, or it could mean 0.5 ppm of free chlorine and 2.5 ppm of irritating, mostly ineffective chloramines. The water in the second scenario smells worse, feels harsher on skin, and is doing a fraction of the disinfection work.
This is why pool codes in most states require testing free chlorine specifically, not just total chlorine. For routine home pool or hot tub maintenance, always check free chlorine as your primary number, and periodically compare it against total chlorine to catch combined chlorine buildup before it becomes a problem.