Why it matters
Below the minimum, the pool is no longer sanitary. Above the maximum, swimmers experience skin and eye irritation.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Chlorine pool | 1 to 4 ppm | — |
| Salt chlorine pool | 1 to 4 ppm | — |
| Spa (chlorine) | 3 to 5 ppm | — |
| After shock | below 5 ppm before swimming | — |
How to test
- 1
FAS-DPD drop test for accuracy
Taylor K-2006 is the standard. Reads up to 50 ppm in 0.2 ppm increments.
- 2
Test strips for daily checks
Acceptable for routine. Drop kit weekly to verify.
- 3
Test in the morning before sun burns off chlorine
Afternoon readings will look low.
How to correct low free chlorine
Add liquid chlorine or cal hypo
Fastest way to raise FC. 1 gallon of 10 percent liquid chlorine per 10,000 gallons raises FC by ~10 ppm.
Salt pool: increase chlorinator output
Bump 10 percent at a time. Verify cell is clean and salt level is correct.
Check CYA
High CYA (over 50) suppresses chlorine effectiveness. May need to drain and refill.
How to correct high free chlorine
Stop adding chlorine, let sunlight reduce it
FC drops 1 to 3 ppm per sunny day depending on CYA.
Use sodium thiosulfate to neutralise quickly
Only if you need to swim immediately. Throws off other chemistry.
Common causes
- •Heavy use, rain, or hot weather burns chlorine fast
- •Algae bloom consumes chlorine
- •Salt cell dirty or undersized
- •Sunlight degrades chlorine without CYA buffer
FAQs
What is the FC/CYA ratio?
Free chlorine should be at least 7.5 percent of CYA. So 30 ppm CYA needs minimum 2.25 ppm FC. This is the Trouble Free Pool standard.