How much chlorine to add to a pool
The right question isn't "how much chlorine", but "by how much do I need to raise free chlorine". The answer depends on three things: the pool volume, the product you use and the stabiliser already in the water.
1. How much free chlorine to keep
The stabiliser (cyanuric acid) protects chlorine from the sun but slows it down. The more there is, the more chlorine you need. Labels say "1–1.5", but that only applies to water with no stabiliser.
| Stabiliser | Free chlorine to keep |
|---|---|
| 0 | 1–3 |
| 30 | 4–6 |
| 50 | 6–8 |
If the stabiliser goes above 90, chlorine can no longer do its job. In that case the fix is to replace some of the water, not to add more chlorine.
2. How much product to raise chlorine by 1 point
These values are for 10 m³ (10,000 litres) and come from the percentage of active chlorine:
| Product | For +1 free chlorine in 10 m³ | Side effect |
|---|---|---|
| Fast granular chlorine (dichlor 56%) | 18 g | adds stabiliser |
| Slow tablets (trichlor 90%) | 11 g | adds stabiliser, lowers pH |
| Liquid chlorine in a drum (15%) | 67 mL | raises pH a little |
| Calcium shock (65%) | 15 g | raises hardness |
For your pool: dose = table value × points to raise × (litres ÷ 10,000).
Example: 15 m³ pool, free chlorine at 2, stabiliser 30, you want to reach 5. You need to raise it by 3 points. With dichlor: 18 × 3 × 1.5 ≈ 80 g. With liquid chlorine: 67 × 3 × 1.5 ≈ 300 mL.
The chlorine calculator does this for you.
3. Tablets or granules?
- Tablets (trichlor): convenient, one per 25–30 m³ a week in a floating dispenser. But every tablet raises the stabiliser, and after two months it can be too high. Measure it once a month.
- Fast granules (dichlor): works straight away, dissolve it in a bucket first. It adds stabiliser too.
- Liquid chlorine: adds no stabiliser, the best for shocking. It loses strength with heat and time: a drum opened months ago can be worth half.
- Calcium chlorine: unstabilised, excellent for shocking. It raises hardness: fine now and then, not every day.
4. When to add it
In the evening. During the day the sun destroys half of it in a few hours. Always keep the pump running while you dose.
The most common mistakes
- Copying the dose from the label without measuring. Labels give average values and sometimes contradict themselves.
- "It smells of chlorine, so there's too much." It's the opposite: that smell is combined chlorine, and you need a shock.
- Using only tablets all summer. The stabiliser builds up and in July the water turns green "with chlorine in range".
Safety
Never mix different products. The product goes into the water, never the other way round. Trichlor tablets and calcium chlorine never in the same container. Swim only with free chlorine below 10.
Let Serenpool do the maths
Choose your pool and your product: Serenpool works out the exact grams or millilitres, takes the stabiliser into account and reminds you when to retest.
Doses are calculated from the stated active chlorine percentages. Always check the label and retest after every addition.