What Is the Difference Between Free Chlorine and Total Chlorine?

Free chlorine is the chlorine still available to sanitize. Total chlorine is everything: free chlorine plus combined chlorine, which is chlorine that has already reacted with contaminants and is largely spent.

The number that actually matters day to day is free chlorine, and it should read 2 to 4 ppm. Total chlorine on its own tells you very little.

The three readings and what they mean

Free chlorine (FC) is your working sanitizer. This is the number you dose to, the number that keeps algae out, and the only one worth reporting if you only report one.

Combined chlorine (CC), also called chloramines, is chlorine that has bound to sweat, urine, sunscreen, oils and other nitrogen compounds. It sanitizes very poorly, smells sharp, and irritates eyes and skin.

Total chlorine (TC) is simply FC plus CC.

The math you actually use:

Combined chlorine = total chlorine minus free chlorine

Reading the gap

• FC 3.0, TC 3.0, CC 0.0 is ideal. All your chlorine is working.
• FC 3.0, TC 3.3, CC 0.3 is acceptable. Keep an eye on it.
• FC 3.0, TC 3.9, CC 0.9 means shock the pool. Nearly a third of your chlorine is spent and the water will smell and sting.
• FC 0.2, TC 2.5, CC 2.3 is a pool in trouble. It looks like it has chlorine but has almost no sanitizer at all.

That last case is exactly why total chlorine alone misleads people. A meter reading 2.5 ppm total sounds fine and the pool is effectively unprotected.

How to test for both

You need a test that separates the two. Options that work:

• DPD drop kit. DPD reagent 1 gives free chlorine. Adding DPD reagent 3 develops the rest, which reads total chlorine. Subtract for combined.
• FAS-DPD titration kit. More precise, especially at high chlorine levels, and the standard for anyone serious about their water.
• Digital photometers. Fast and accurate if you keep the reagents fresh.

Most cheap test strips report total chlorine only, or report free chlorine with poor resolution. They are fine for a quick glance and inadequate for diagnosing a problem.

Test in the shade, take the sample elbow-deep away from returns and skimmers, and rinse the vial with pool water before filling.

What to do when combined chlorine is high

Chloramines do not clear on their own. They need breakpoint chlorination: raise free chlorine to roughly 10 times the combined chlorine reading.

If CC is 0.8 ppm, you need to add about 8 ppm of free chlorine, which is roughly 0.8 gallons of 12.5 percent liquid chlorine per 10,000 gallons.

Shock after dark, run the pump 8 or more hours, and retest in the morning. Stopping partway through creates more chloramines than it removes.

Keys conditions that widen the gap

Year-round swimming means year-round bather load, and combined chlorine tracks bather load closely. Sunscreen goes on every day of the year here, which puts a steady organic input into pools that never gets a seasonal break.

Our intense UV also destroys free chlorine faster than it destroys chloramines, so a Key West pool can drift toward a bad free-to-total ratio in a single hot afternoon if the stabilizer level is not right. Vacation rentals with irregular occupancy see the widest swings, going from empty to full and back within a week.

Test free and total together every time, because the difference between them is the earliest warning you get that a pool is falling behind.