Free Chlorine vs. Total Chlorine (2024)

Chlorine occurs naturally as one of the basic building blocks of matter. It possesses powerful disinfecting and bleaching capabilities that we harness for a wide range of industrial and domestic purposes such as keeping our swimming pools clean. Your chosen pool installation company should walk you through exactly what you’ll need for a new or renovated pool.

This powerful sanitizer takes on different chemical forms from the time it is applied to the point at which it has done its job. In order to maintain excellent swimming pool water quality, you have to monitor both free chlorine and total chlorine.

How Does Chlorine Sanitize Pool Water?

We add chlorine to pool water to sanitize it. Once in the water, free chlorine forms hypochlorous acid which kills disease-causing microbes. When chlorine bonds with these and other organic compounds, such as sunscreen or swimmers’ body fluids, chloramines are formed. Chloramines, also known as combined chlorine, leave the pool through offgassing.

A buildup of chloramines causes the “chlorine smell” that swimmers find irritating. That smell may be an indication of poor water quality. Regularly checking free, combined and total chlorine levels will help to prevent or reveal water quality issues before problems arise.

What Is Free Chlorine?

Free chlorine is the amount of chlorine, measured in parts per million (ppm), that is available to eliminate harmful microbes and neutralize contaminants. It is not yet bound to microbes or chemicals in the water. Monitoring free chlorine is a critical part of pool maintenance in order to avoid harmful contaminant levels.

What Is Combined Chlorine?

Combined chlorine develops when free chlorine binds to contaminants during sanitization. The presence of combined chlorine indicates that contaminants are present and that they are actively being neutralized. A clean, sanitized pool contains zero combined chlorine. Consistently maintaining the proper level of free chlorine in the pool water greatly reduces the likelihood of detecting measurable combined chlorine levels.

What Is Total Chlorine?

The sum of combined chlorine and free chlorine is total chlorine (combined chlorine + free chlorine = total chlorine). Inexpensive chlorine tests typically show total chlorine only. Since clean chlorinated water contains no combined chlorine, total chlorine in a sanitized pool is the same as free chlorine.

But without a separate measurement of free chlorine, it is impossible to confirm the absence of combined chlorine. For this reason, it is important to monitor both total chlorine and free chlorine. You can then subtract the free chlorine measurement from total chlorine to confirm the level of combined chlorine (total chlorine – free Chlorine = combined chlorine).

How to Test Pool Water for Free, Combined and Total Chlorine

Choose one of three testing methods to monitor free, combined and total chlorine: test strips, a chlorine test kit or a digital chlorine sensor. Litmus style chlorine test strips change color when they contact the water, indicating the approximate total, free and combined chlorine levels. Chlorine test kits require you to add a chemical reagent to a pool water sample, causing the sample to change color based on its total, free, or combined chlorine content.

An electronic chlorine tester is the most accurate tool for monitoring chlorine levels. These devices provide accurate, precise measurements in ppm, rather than an approximated range, without the need for a color comparison.

Safe Chlorine Levels for Pool Water

Maintaining chlorine levels within the ideal range ensures that the water is safe and minimizes buildup of chloramines. Pool water test kits can help determine if levels are outside the ideal range, but swimmers might notice a chlorine smell, or complain of minor eye or skin irritation. If the levels are out of the safe range, do not allow swimmers in the pool until it is properly treated and the levels test within range.

• Free chlorine: ideal range 2.0 to 4.0 ppm, safe range 1.0 to 6.0 ppm
• Combined chlorine: ideally less than 0.2 ppm, safe below 0.5 ppm
• Total chlorine: should be equal to free chlorine most of the time