Free Chlorine vs Total Chlorine: What the Numbers Mean

The three numbers and what they mean

Every chlorine test really gives you three numbers, even if the kit only prints two:

Free chlorine (FC) is the active sanitizer. This is the chlorine available to kill germs and algae. Keep it at 1 to 3 ppm at all times, and never swim above 5 ppm after a shock dose.
Total chlorine (TC) is everything the test detects: free plus used-up chlorine together.
Combined chlorine (CC) is the used-up remainder: TC minus FC. It forms when chlorine binds to sweat, sunscreen, urine, and other contaminants. Combined chlorine is what gives pools the sharp "chlorine smell" and irritates eyes, and it does almost no sanitizing. Keep it under 0.2 ppm ideally, and never let it sit above 0.5 ppm.

The formula: combined = total minus free

If your test reads total chlorine 2.5 ppm and free chlorine 1.5 ppm, your combined chlorine is 1.0 ppm (2.5 minus 1.5). That is double the 0.5 ppm trouble threshold, so this pool needs a shock. If the readings are total 1.8 and free 1.6, combined is only 0.2 ppm and the water is fine; just top up free chlorine normally.

Breakpoint: how much shock to destroy combined chlorine

The fix for combined chlorine is breakpoint chlorination: raise free chlorine to 10 times the combined chlorine level, all at once, in the evening. That high dose oxidizes the combined compounds completely instead of just converting one smelly compound into another.

Worked example 1: 15,000 gallons, combined chlorine 1.0 ppm. Breakpoint target is 10 ppm of free chlorine (1.0 x 10). Using 10% liquid chlorine, and the rule that 1 gallon raises 10,000 gallons by 12 ppm:

10 ppm x (15,000 / 10,000) / 12 = about 1.25 gallons of 10% liquid chlorine.

Worked example 2: 10,000 gallons, combined chlorine 0.5 ppm. Breakpoint target is 5 ppm:

5 ppm / 12 = about 0.42 gallons, or roughly 53 fluid ounces, of 10% liquid chlorine.

After shocking, run the pump overnight and retest in the morning. If combined chlorine is still above 0.5, the breakpoint was not reached and you repeat the process.

Why combined chlorine climbs

Heavy bather loads, hot weather, rain, and lots of sunscreen all feed combined chlorine. Pools sanitized with trichlor tablets can develop a slow buildup too, because the tablets keep adding chlorine while adding cyanuric acid that locks some of it up. Weekly shocking during swim season prevents the buildup: it is easier to oxidize 0.2 ppm of combined chlorine than to breakpoint 1.5 ppm after a month of neglect.

Shock choices and when not to shock

Liquid chlorine is the most predictable shock: no added stabilizer, no calcium. Dichlor and cal-hypo granules also work, but dichlor adds cyanuric acid (a problem if yours is already high) and cal-hypo adds calcium (a problem if your water is already hard). Do not shock when combined chlorine is fine; shocking a healthy pool wastes chemical and pushes free chlorine above the 5 ppm swim ceiling. And always shock in the evening, since sunlight burns off the dose before it can do its work.