What is free chlorine in a pool?
Free chlorine is the form of chlorine still available to sanitize. Free chlorine is hypochlorous acid (HOCl) plus hypochlorite ion (OCl⁻). Free chlorine is the only sanitizer that actually kills pathogens. Pool water needs 1–4 ppm free chlorine at all times. Pool water with 0 ppm free chlorine grows bacteria within 4 hours.
According to CDC pool operation guidance, HOCl is the active killer. The ion form OCl⁻ is 80–100× weaker than HOCl. Research published in the Water Quality & Health Council bulletins shows that free chlorine at 1 ppm kills E. coli in under 1 minute when pH sits at 7.4.
How is free chlorine measured?
The test is DPD-1 reagent or test strips. The result is read in parts per million. The DPD test gives free chlorine and total chlorine; the difference is combined chlorine. Test strips give a rough range only. Research from the Pool & Hot Tub Alliance shows that strip readings are accurate within ±1 ppm at best.
| Reading | Status | Action |
|---|---|---|
| 0 ppm | Unsanitary | Shock immediately |
| 0.5 ppm | Below minimum | Top up with chlorine |
| 1–4 ppm | Ideal band | Maintain |
| 5–7 ppm | High but safe | Wait for natural drop |
| >10 ppm | Over-shocked | Do not swim |
What raises and lowers free chlorine?
- Sunlight (UV) destroys 1–3 ppm per day in summer; CYA shields it.
- Bather load consumes 0.2 ppm per swimmer-hour in a 20,000 gal pool.
- Rain runoff adds organic load that consumes free chlorine.
- High pH (above 7.8) shifts the HOCl/OCl⁻ balance toward weak OCl⁻.
- Adding chlorine products raises free chlorine directly.
Why does CYA change the free chlorine target?
Cyanuric acid binds free chlorine. The bond reduces sanitizing power. The CDC publishes a 7.5% free-chlorine-to-CYA ratio as a daily minimum. At 30 ppm CYA the minimum is 2.25 ppm. At 50 ppm CYA the minimum is 3.75 ppm. Data shows that pools running below this ratio grow algae within 5–10 days. Use the chlorine calculator to land the dose, and the CYA calculator to confirm stabilizer is in range.