How chlorine dosing works
Pool chlorine levels are measured in parts per million (ppm), which is the same as milligrams per liter. To raise free chlorine by a given amount, you need a precise mass of available chlorine for your water volume, then divide by the strength of your product.
- Chlorine needed (g) = pool volume (L) × increase (ppm) ÷ 1,000
- Product needed = chlorine needed ÷ available chlorine percentage
For liquids, the strength is given as trade percent, roughly grams of chlorine per 100 mL, so the same formula gives milliliters. For dry products, it gives grams.
Worked example
A 15,000-gallon pool (56,800 liters) has 1 ppm of free chlorine, and you want 3 ppm using 10% liquid chlorine.
- Increase: 3 − 1 = 2 ppm.
- Chlorine needed: 56,800 × 2 ÷ 1,000 = 113.6 g.
- Liquid chlorine: 113.6 ÷ 0.10 = 1,136 mL, or 38.4 fl oz (about 0.30 gallon).
Product strengths
| Product | Available chlorine | Notes |
|---|---|---|
| Liquid chlorine (sodium hypochlorite) | 10 to 12.5% | No residue; raises pH slightly at first |
| Household bleach (unscented) | 6 to 8% | Same chemical, weaker |
| Cal-hypo shock (calcium hypochlorite) | 65 to 73% | Adds calcium hardness |
| Dichlor granules | 56 to 62% | Adds cyanuric acid |
| Trichlor tablets | 90% | Slow-dissolving; adds CYA and lowers pH |
Liquid chlorine loses strength over time, faster when warm, so an old jug may be weaker than its label.
Chlorine and stabilizer
Cyanuric acid (CYA), or stabilizer, protects chlorine from sunlight but also slows its sanitizing action. The more CYA in the water, the more free chlorine you need:
- Minimum free chlorine: about 7.5% of CYA, so 2.3 ppm with 30 ppm of CYA.
- Shock level: about 40% of CYA to clear algae, so 12 ppm with 30 ppm of CYA.
Most outdoor pools keep CYA between 30 and 50 ppm. Tablets and dichlor add CYA with every dose, so it builds up over the season; only partial draining lowers it.
Safe handling
- Never mix chlorine products with each other or with acid. Dangerous gases can form.
- Add chemicals to water, never water to chemicals, and pre-dissolve granules in a bucket if the label says so.
- Add chlorine in the evening with the pump running, and keep swimmers out until levels return to the normal range.
- Test the water with a reliable kit (a FAS-DPD test is the most accurate for free chlorine) before and after dosing.