Pool care articles
What is combined chlorine and how to measure it
Combined chlorine is the chlorine that has bound to what swimmers bring into the water, forming chloramines. To find it, measure total and free chlorine with a DPD test kit and subtract free from total. Health departments commonly limit it to 0.4 ppm or less in public pools, and the CDC's guidance for public pools treats a level above that as too high. Sullivan's Pool Service tests free and total chlorine on its weekly visits in Austin.
If you own a backyard pool in Austin, from Tarrytown to West Lake Hills or out toward Bee Cave, combined chlorine is one of the numbers worth tracking. It is the difference between the total chlorine and the free chlorine in the water. Measure both, subtract the free reading from the total reading, and you have it. Most city, county, and state health departments keep that figure at 0.4 ppm or less, and the CDC's guidance for public pools treats a level above that, for example more than 0.4 ppm, as too high. Sullivan's Pool Service tests free and total chlorine as part of its weekly residential visits in Austin.
Combined chlorine is the share of chlorine that has bound to waste swimmers carry into the water. Once chlorine attaches to pee, sweat, and similar body waste, it forms chemicals called chloramines, which are one kind of combined chlorine. Measuring them at home takes a reliable test kit and simple subtraction.
The Centers for Disease Control and Prevention advises residential pool and hot tub owners to use a DPD test kit, short for N,N Diethyl-1,4 Phenylenediamine Sulfate, to measure chlorine and pH. Test strips are an option too, though the CDC notes they are less accurate than a DPD kit. Whatever you use, follow the directions on the package. The CDC also wants owners testing chlorine and pH at least twice per day, and more often when a number of people are in the water.
Doing the math
The formula is simple. Measure the total chlorine in the water. Measure the free chlorine. Subtract the free number from the total number, and the result is your combined chlorine level.
One caution on readings: if the chlorine in the sampled water is more than 10 ppm, a test might partially or totally bleach out, giving a false low result or a reading of 0 ppm chlorine. Keep that in mind whenever a result looks lower than you expected.
Why the 0.4 ppm level matters
The commonly used limit across city, county, and state health departments is 0.4 ppm of combined chlorine or less, and the CDC's guidance for public pools gives more than 0.4 ppm as an example of a level that is too high. That limit, and the CDC's example, come from guidance written for public pools.
The reason for the limit is what chloramines do. The CDC notes that they can irritate skin, eyes, and the respiratory tract, including the nose, when they off gas into the air above the water, particularly indoors.
The CDC's guidance for public pools lists a distinctive chloramine odor, and eye, skin or breathing irritation, as signs to watch for.
Where combined chlorine comes from
Chlorine is added to water to kill germs, but it does not work right away, and its concentration drops as it gets used up. The CDC lists pee, poop, sweat, and dirt from swimmers' bodies among the things that reduce chlorine levels, along with sunlight. As chlorine binds to that body waste, chloramines form and the combined chlorine number climbs.
Prevention starts with keeping waste out: use the toilet before getting in, never pee or poop in the water, and rinse off first, since the CDC notes that even a quick rinse shower removes much of the body waste that helps form chloramines. Anyone with diarrhea should stay out of the water entirely.
When the number stays high
The CDC's guidance for public pools describes a procedure called superchlorination, or breakpoint chlorination, for clearing chloramines when combined chlorine is too high. It is written for public pools, with steps for closing the pool and for ventilation. For a backyard pool, readings that stay above 0.4 ppm across several tests are worth having assessed, rather than dosing from facility instructions.
Where this sits in your testing routine
Combined chlorine is one number inside the broader routine the CDC recommends for residential pools and hot tubs. Keep pH in the 7.0 to 7.8 range, which the CDC recommends because it best balances killing germs, the lifespan of the pipes, and swimmer comfort. Chlorine concentration should be at least 1 ppm in pools and at least 3 ppm in hot tubs. If you use cyanuric acid, a chlorine stabilizer, or chlorine products containing it, the CDC recommends at least 2 ppm in pools and recommends not using cyanuric acid in hot tubs at all. As pH rises, chlorine's ability to kill germs falls, especially above 8.0.
Austin tap water and a top-up
Austin Water's quarterly table lists total chloramines in the finished tap water of 2.69 mg/L at Davis, 2.52 mg/L at Ullrich and 2.60 mg/L at Handcox. Chloramines are a type of combined chlorine. Those figures are averages for tap water, not readings of your pool, so test the pool itself.
Measure total chlorine, measure free chlorine, and subtract. A result that stays above the 0.4 ppm public-pool limit across several tests is worth having assessed.
Sources
In Austin
Fill water matters here. Austin Water reported finished water leaving its three plants in the second quarter of 2026 at pH 9.65 to 9.74, total hardness of 76 to 77 mg/L as CaCO3, total alkalinity of 51 to 60 mg/L, and 2.52 to 2.69 mg/L of chloramines. Those are averages for tap water, not readings of your pool, so test the pool itself after a top-up.
Related articles
- Why does my pool smell strongly of chlorine?
- How fast does chlorine kill pool germs?
- How often should I test my pool in Austin?
Sullivan’s Pool Service maintains pools in and around Austin, including Northwest Hills, South Austin, West Lake Hills, Tarrytown, Great Hills, River Place, Crestview, East Austin, North Austin, West Campus, Hyde Park, Downtown Austin and St. Johns, and in Bee Cave, Cedar Park, Georgetown, Leander, Pflugerville and Round Rock. See every service area.
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