Your test kit gives two chlorine numbers, and they are not the same. The difference between them tells you more about your water than either number on its own.
The short version: free chlorine is the chlorine still available to kill germs. Total chlorine is free chlorine plus combined chlorine, the chlorine that has already reacted with sweat, dirt and pee. Combined chlorine = total chlorine minus free chlorine (CDC). Most health departments keep combined chlorine at 0.4 ppm or less; TroubleFreePool acts above 0.5 ppm.
Part of: Pool Chlorine: Types, Levels and How to Keep Them Steady, our overview of every chlorine question.
The three numbers
| Reading | What it is |
|---|---|
| Free chlorine (FC) | Chlorine still available to kill germs. The CDC minimums (1 ppm, or 2 ppm with stabilizer) are free chlorine. |
| Combined chlorine (CC) | Chlorine that has combined with sweat, dirt and pee into chloramines (CDC). |
| Total chlorine (TC) | Free plus combined. |
How to calculate combined chlorine
The CDC's steps: measure total chlorine, measure free chlorine, then subtract: combined chlorine = total chlorine − free chlorine.
Example (our numbers): total chlorine 3.5 ppm, free chlorine 3.0 ppm, so combined chlorine is 0.5 ppm.
What a high combined chlorine reading means
- Above 0.4 ppm: most city, county and state health departments limit combined chlorine to 0.4 ppm or less (CDC, for public pools).
- Above 0.5 ppm: TroubleFreePool says something in the water is being oxidized and the pool should be evaluated; its fix is the SLAM process.
- The smell: the CDC says that if you smell "chlorine" at a pool, you are probably smelling chloramines. See why does my pool smell like chlorine?
How to bring combined chlorine down
The CDC's advice for pool operators is to superchlorinate: raise free chlorine to 10 times the combined chlorine level, with good ventilation. TroubleFreePool's SLAM process does the same job by holding free chlorine at a level based on your stabilizer. Step by step: how to shock a pool.
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Test kit A test kit that reads free and total chlorineFree chlorine and total chlorine are two separate readings. The CDC calls a DPD test kit the most accurate option for home pools; test strips are less accurate. Follow the kit's own instructions.
FAQ
Can free chlorine be higher than total chlorine?
Not in the water: total includes free. A test that shows free higher than total usually comes from test error or reading the two tests at different moments (our note); retest.
Are test strips good enough?
The CDC calls a DPD test kit the most accurate for home pools and says test strips are less accurate.
Sources
- CDC: Chloramines and pool operation (written for pool operators) (combined chlorine formula, 0.4 ppm, 10 times rule, ventilation)
- CDC: Preventing eye irritation from pool chemicals (chloramines and the chlorine smell)
- CDC: Home pool and hot tub water treatment and testing (free chlorine minimums, DPD kits vs strips)
- TroubleFreePool: ABCs of pool water chemistry (combined chlorine above 0.5 ppm)
- TroubleFreePool: SLAM process (SLAM triggers)
Sources read on November 4, 2026. Product labels change: the label on the product you buy always wins.
