Illustration.
Calculate a dose
Safety. Follow the product label. Add chemicals to water, never water to chemicals. Never mix chlorine and acid. Keep children and pets away and never add chemicals while anyone is in the pool. Wait at least 30 minutes, or longer when the product label requires it; retest and meet its reopening conditions before anyone swims.
How the sum works
1 mg/L is 1 gram per 1,000 litres. So to lift free chlorine by 2 mg/L in a 50,000 L pool you need 2 × 50 = 100 g of chlorine. Liquid pool chlorine is usually sold at about 12.5% available chlorine, which is about 125 g per litre, so 100 g is about 0.8 L. Granular calcium hypochlorite is commonly about 65%, so 100 g of chlorine is about 154 g of granules. Check the strength on your product's label and type it in.
What to aim for
HealthyWA (the WA Department of Health) says an outdoor pool with stabiliser should hold free chlorine of at least 2 mg/L, or 3 mg/L when the water is above 26°C, with pH between 7.2 and 7.8 (7.5 ideal) and stabiliser between 30 and 50 mg/L. Indoor and unstabilised pools need at least 1 mg/L (2 above 26°C).
Add liquid chlorine in the evening with the pump running. Sunlight breaks chlorine down fast, so a dose at dusk lasts longer.
Choosing a chlorine product
Liquid chlorine (sodium hypochlorite) is quick, adds no stabiliser or calcium, and suits regular dosing and shocking, but it raises pH a little and loses strength in storage, especially in heat.
Granular calcium hypochlorite is strong and stores well but adds calcium, which can raise hardness and cause scale over time.
Stabilised chlorine (dichlor granules and trichlor tablets) adds stabiliser with every dose, convenient for outdoor pools at first but able to build stabiliser too high over a season.
Salt chlorinators make chlorine from salt in the water; you set the output rather than dosing by hand.
Dosing safely
Add one chemical at a time, with the pump running, and wait for it to mix before adding another. Never mix chemicals in the same bucket, and never add water to acid. Store chlorine and acid apart, in a cool, ventilated place out of reach of children. Wear gloves and eye protection, and keep swimmers out until levels are back in range.
When to retest
Retest free chlorine and pH a few hours after a dose, and again the next day, to see whether the pool is holding its chlorine. A pool that loses chlorine overnight may have algae, a high organic load or low stabiliser.
A worked example
For a 40,000 litre pool reading 1 mg/L free chlorine, lifting it to 3 mg/L needs 2 x 40 = 80 g of available chlorine. At about 12.5 per cent strength, that is roughly 640 mL of fresh liquid chlorine. Older liquid chlorine is weaker, so you may need a little more.
Our short answer
Know your litres, test first, dose in stages, add chlorine at dusk, and retest after a few hours.
Storing chemicals
Keep chemicals in their original containers, upright, cool and dry, away from children, with chlorine and acid stored separately.
Shock doses
For a shock dose to treat algae, use the higher target your pool shop or the label gives.
Test kits
Use a kit that reads free chlorine, not only total chlorine, for accurate dosing.
Common questions
How much liquid chlorine do I add to my pool?
About 80 mL of 12.5% liquid chlorine raises 10,000 litres by 1 mg/L. Multiply by your litres (in tens of thousands) and by how many mg/L you need to rise.
How much chlorine for a 50,000 litre pool?
To lift free chlorine by 1 mg/L, about 0.4 L of 12.5% liquid chlorine or about 77 g of 65% granular chlorine.
Can I swim after adding chlorine?
Health guidance says to wait at least 30 minutes for chemicals to dissolve and never to add chemicals while anyone is in the water. Retest before swimming.