The ideal free chlorine level for your pool is 1.0 to 3.0 mg/l. Use slow-dissolving 200g tablets in a floating dispenser or the skimmer. Test 2 to 3 times per week and top up when the level drops below 1.0 mg/l.
How chlorine works in a pool
Chlorine kills bacteria, viruses and algae by damaging cell walls. In water, chlorine forms hypochlorous acid (HOCl), the active form that actually disinfects.
Chlorine effectiveness is strongly dependent on pH. At pH 7.4, approximately 48% of chlorine is active. At pH 8.0 only 21%. Always check pH (7.2 to 7.6) before evaluating the chlorine level.
Free chlorine vs. combined chlorine
Free chlorine (FC): the active chlorine available for disinfection. This is what you measure and maintain. Ideal: 1.0 to 3.0 mg/l.
Combined chlorine (CC): chlorine that has already reacted with nitrogen compounds from sweat and urine, forming chloramines. These cause irritation and that unpleasant pool smell. Ideal: as low as possible, maximum 0.2 mg/l.
Did you know
That typical “pool smell” is not from chlorine itself. It is from chloramines, combined chlorine formed from dirty water. A well-maintained pool barely smells of chlorine.
How much chlorine do you need?
Rough guide for 200g chlorine tablets:
| Pool volume | Tablets per week |
|---|---|
| Up to 10,000 litres | 0.5 to 1 tablet |
| 10,000 to 25,000 litres | 1 to 2 tablets |
| 25,000 to 50,000 litres | 2 to 3 tablets |
In hot weather (above 28°C), after heavy use or with lots of sun, test more frequently and top up as needed.
Shock treatment: when and how
A shock treatment (superchlorination) is needed when:
- Combined chlorine rises above 0.2 mg/l
- Water becomes cloudy or green
- There is a strong chlorine smell
- After heavy bather load
- When opening the pool in spring
Target: raise free chlorine temporarily to 10x the combined chlorine level.
Always perform shock treatment in the evening: UV light breaks down the extra chlorine quickly. Do not swim for 24 hours after shocking.
