The Short, Short Version
Free chlorine is the amount of chlorine available to sanitize contaminants. Combined Chlorine is chlorine that has combined with contaminants. Total chlorine is the sum of the two.
When it comes to caring for your swimming pool and keeping your pool water balanced, no chemical is more important than chlorine.
Chlorine is the chemical compound that works to kill contaminants in your pool water. Contaminants include things like bacteria, perspiration, urine, and other microorganisms that could otherwise cause harm. Pool water that is not properly sanitized with chlorine could foster the growth of bacteria that leads to things like ear infections and Legionnaires disease. There are three types of chlorine: Free Chlorine (FC), Combined Chlorine (CC), and Total Chlorine (TC).
In this article we will discuss what each type of chlorine is and how you should interpret them when caring for your pool. After reading this article, you may want to check out our articles on How to Test, Raise, and Lower Pool Chlorine and Types of Swimming Pool Chlorine Sanitizer.
What is Free Chlorine?
Free Chlorine is the amount of chlorine that has not yet combined with chlorinated water to sanitize contaminants. In effect, free chlorine is the amount of chlorine that is free to kill harmful microorganisms in your swimming pool’s water. Free chlorine is the most important type of chlorine to monitor because it is responsible for actively sanitizing your pool.
The Association of Pool and Spa Professionals recommends keeping free chlorine levels between 2.0 and 4.0 parts per mil (ppm). The easiest way to check your chlorine levels is with test strips. However, ideal free chlorine levels are dependent on cyanuric acid levels. We recommend using the pool calculator to manage your pool’s entire chemistry. When your free chlorine levels get too low, we recommend using calcium hypochlorite to raise them back up.
PRO TIP: We recommend testing free chlorine every day and adding chemicals as you need to keep your free chlorine in recommended ranges. We also recommend adding your chemicals at night. UV rays from sunlight cause chlorine compounds to break down faster. Adding chemicals at night is a great way to save money on chlorine costs.
What is Combined Chlorine?
Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. Combined chlorine causes the smell many people associate with chlorine pools. If CC is above 0.5, you should shock your pool. Combined chlorine indicates that there is something in the water free chlorine is in the process of breaking down. In an outdoor pool, combined chlorine will normally stay near zero so long as you maintain an appropriate free chlorine level and the pool gets some direct sunlight.
Potassium monopersulfate – a common non-chlorine shock – will often show up on tests as combined chlorine. If you use Potassium monopersulfate as a sanitizing agent, we recommend using the Taylor K-1518 – Drop Test, Chlorine FAS-DPD/Monopersulfate testing kit. This is a specially designed testing kit that distinguishes Potassium monoersulfate from true combined chlorine.
PRO TIP: The pool calculator does not include a test for combined chlorine because no action should be taken if combined chlorine is sufficiently low (0.2 ppm). If combined chlorine raises to 0.5 ppm or higher, you should shock your pool. To determine how much shock to add, simply enter the suggested shock value in the Free Chlorine “Goal” input and select the shock chemical you aim to use (e.g. bleach with a concentration of 15%).
What is Total Chlorine?
Total chlorine is the sum of free chlorine and combined chlorine. Inexpensive chlorine tests, such as the common OTO test, which shows total chlorine as different shades of yellow, measure true chlorine because it is easier to test for than free chlorine and combined chlorine. In normal operation total chlorine can be used as if it was free chlorine because combined chlorine is usually zero. However when you have algae or some other problems, combined chlorine levels can be significant and total chlorine becomes useless.
PRO TIP: Save money on testing equipment by using a low-cost testing kit or test strips that measure total chlorine. Under normal conditions, you may simply assume total chlorine is the same as free chlorine. If you notice any algae, or experience a lingering chlorine smell, you may want to shock your pool.