Guidance only. This guide explains the arithmetic and common targets. Every product is different: follow the label on the product you use, follow local health rules for public pools, and test before and after dosing.
Chlorine keeps pool water safe by killing bacteria and other germs and by oxidizing the sweat, sunscreen and other organic matter swimmers bring in. Understanding a few terms makes test results — and dosing — much easier.
Free, combined and total chlorine
- Free chlorine (FC) is chlorine available to sanitize: hypochlorous acid and hypochlorite ion. This is the number you manage.
- Combined chlorine (CC) is chlorine that has already reacted with ammonia and nitrogen compounds (chloramines). It is a poor sanitizer and causes the "chlorine smell" and red eyes people blame on too much chlorine.
- Total chlorine (TC) = free + combined.
A good test kit measures FC and TC separately; CC = TC − FC. A common rule is that CC should stay below about 0.5 ppm.
What ppm means
ppm (parts per million) in pool water means milligrams per litre. Raising free chlorine by 1 ppm in 10,000 litres takes 10,000 mg — 10 grams — of available chlorine. In a 10,000 US gallon pool (37,854 L) it takes 37.9 g. Everything else is dividing by the product’s strength:
product amount = litres × ppm increase ÷ 1,000 ÷ (strength ÷ 100)
| Product | Typical strength | To raise 10,000 US gal by 1 ppm |
|---|---|---|
| Liquid chlorine (sodium hypochlorite) | 12.5% | 10.2 fl oz (303 mL) |
| Plain household bleach | 6% | 21.3 fl oz (631 mL) |
| Cal-hypo (calcium hypochlorite) | 65% | 2.1 oz (58 g) |
| Dichlor granules | 56% | 2.4 oz (68 g) |
| Trichlor (tablets or granules) | 90% | 1.5 oz (42 g) |
Liquid strength is "trade percent" — grams of available chlorine per 100 mL. Solid strength is percent available chlorine by weight. The pool chlorine calculator does the arithmetic for your pool, product and target.
Typical targets
- Free chlorine: public-health guidance commonly calls for at least 1 ppm in pools (higher in hot tubs). Residential pools often run 2–4 ppm.
- pH: about 7.2–7.8. Chlorine works much better at the low end: as pH rises, less of the free chlorine is in the strong hypochlorous-acid form.
- Cyanuric acid (CYA, stabilizer): protects chlorine from sunlight in outdoor pools, but also slows it down. The more CYA, the more free chlorine you need for the same effect. A widely used guideline is to keep FC at about 7.5% of the CYA level — 2.3 ppm FC at 30 ppm CYA, 3.8 ppm at 50 ppm.
Choosing a product
- Liquid chlorine / bleach (sodium hypochlorite): adds nothing but chlorine and a little salt; raises pH slightly at first. Loses strength in storage, faster when warm.
- Cal-hypo (calcium hypochlorite): strong and stores well; adds calcium hardness, which matters in hard-water areas.
- Dichlor: dissolves quickly and is nearly pH-neutral; adds about 0.9 ppm CYA for every 1 ppm of chlorine.
- Trichlor tablets: slow-dissolving, used in floaters and feeders; acidic, and adds about 0.6 ppm CYA per 1 ppm chlorine. Long-term use steadily raises CYA.
- Salt chlorine generators make chlorine from dissolved salt; the salt calculator works out how much salt to add.
Shocking
"Shocking" means raising free chlorine well above normal to clear combined chlorine, algae or contamination. A traditional rule for breaking down chloramines (breakpoint chlorination) is to raise free chlorine by about ten times the combined-chlorine reading. Algae and stabilized pools need more — follow the product instructions, keep swimmers out until FC is back in the normal range, and keep testing.
Chlorine demand
Outdoor pools lose chlorine every day to sunlight and to swimmers. Losing 1–3 ppm a day is common for a stabilized outdoor pool in summer; an unstabilized pool in strong sun can lose most of its chlorine in a few hours. If chlorine disappears overnight with no sun and no swimmers, something is consuming it — usually early algae.
Worked example
An 18,000 US gallon pool tests at 1 ppm FC; you want 4 ppm. That is +3 ppm in 68,137 L, which needs 68,137 × 3 ÷ 1,000 = 204 g of chlorine. With 12.5% liquid chlorine: 204 ÷ 0.125 = 1,635 mL — about 55 fl oz, or a little under half a US gallon. With 65% cal-hypo: 314 g, about 11 oz by weight. Then check the label of your product, add it with the pump running, and retest after an hour.
Safety rules
- Never mix different chlorine products together, or chlorine with acid. Dangerous gases and fires can result.
- Always add chemicals to water, not water to chemicals.
- Store chemicals cool, dry, separate and out of reach of children; keep them in their original containers.
- Wear eye protection and gloves; rinse spills with plenty of water.
- Dose with the pump running and swimmers out, and wait for FC to drop to the normal range before swimming after a shock.
Get the pool volume right first — every dose depends on it. Use the pool volume calculator.