Pool Chlorine Calculator

Understanding pool chlorines in Australia

How pool chlorine dosing arithmetic works

The underlying calculation is simple enough to do by hand, and it is worth understanding rather than trusting any calculator blindly. Raising free chlorine by 1 part per million in 10,000 litres of water requires about 10 grams of pure available chlorine. Because no product is pure, you divide by the product's available chlorine percentage. Granular calcium hypochlorite at roughly 65 per cent available chlorine therefore needs about 15 grams per 10,000 litres per 1 ppm. Granular dichlor at roughly 56 per cent needs about 18 grams. Liquid chlorine at roughly 12.5 per cent available chlorine needs about 80 millilitres. Multiply that figure by your pool volume in units of 10,000 litres, and then by the gap between your current reading and your target reading. As a worked example, a 50,000 litre pool reading 0 ppm and targeting 3 ppm needs 15 grams multiplied by 5, multiplied by 3, which is 225 grams of 65 per cent granular chlorine. The calculator above follows this arithmetic and deliberately rounds down rather than up, because adding more is easy and removing chlorine is not.

Safety rules that are not negotiable

Pool chemicals are genuinely hazardous and the accidents are avoidable. Never mix pool chemicals together, in a container, in a bucket, or in quick succession in the same spot in the pool. Combining chlorine with acid produces chlorine gas, and combining different chlorine types can react violently. Always add the chemical to water rather than water to the chemical, because the reverse can cause a violent reaction and splash back. Dissolve granular products in a bucket of pool water first and pour the solution into the pool while the pump is running, spreading it around the perimeter rather than dumping it in one place. Wear eye protection and gloves. Store chemicals in their original labelled containers, in a cool dry place, well separated from each other and out of reach of children. And read the product label every time, because concentrations differ between brands, between product lines and occasionally between batches, and the label always overrides a general calculator.

Test first, then dose, then retest

The most common cause of a pool going wrong is dosing without testing. A test kit or strip that has been sitting in the sun for two summers is not giving you real numbers, and reagents have expiry dates for a reason. Test free chlorine, pH and total alkalinity at minimum, weekly through summer and fortnightly in cooler months. Then dose in stages. Add roughly the calculated amount, run the filter for at least four to six hours so the water genuinely turns over, and retest before considering another dose. Adding a second dose because the first has not shown up on a test taken twenty minutes later is how pools end up massively over chlorinated. If a reading is wildly different from what you expect, take the water to a pool shop for a full analysis rather than chasing it with product.

Why pH and stabiliser change everything

Chlorine does not work in isolation. Free chlorine exists in water in two forms, and the proportion that is the strongly sanitising form depends heavily on pH. At a pH around 7.2 to 7.6 a good share of your chlorine is doing useful work. By the time pH reaches 8.0 the majority of it is in the weaker form and the sanitiser you paid for is largely inactive. This is why correcting pH first and dosing chlorine second is the correct order of operations, and why a pool that seems to need endless chlorine very often has a pH problem instead. Cyanuric acid, sold as stabiliser or conditioner, is the other half of the story. It shields chlorine from ultraviolet degradation, which in Australian summer sun is the difference between chlorine lasting days and lasting hours. Too little and you burn through product. Too much and the chlorine becomes over protected and sluggish, which is a common problem in pools dosed for years with stabilised chlorine products such as dichlor and trichlor that add cyanuric acid every time they are used.

Choosing between granular and liquid

The three common residential products behave differently. Granular calcium hypochlorite is the workhorse, around 65 per cent available chlorine, stores well and is cost effective per dose, but it adds calcium to the water which matters over years in a pool that already has high calcium hardness. Granular dichlor is around 56 per cent available chlorine, dissolves quickly, is pH neutral and contains stabiliser, which is convenient in a new pool and a problem in an old one where cyanuric acid has already accumulated. Liquid chlorine, usually sodium hypochlorite at around 12.5 per cent, adds no calcium and no stabiliser and acts fast, which makes it the preferred shock product for many pool professionals, but it is heavy to handle, raises pH, and degrades on the shelf noticeably within weeks in heat. Salt chlorinated pools generate their own chlorine and are dosed differently again, by adjusting cell output and salt level rather than by adding product, though they still need shocking occasionally.

What to do when the numbers do not add up

If you dose correctly and the free chlorine reading does not move, something is consuming it. The usual suspects are a heavy organic load after a storm or heavy use, algae that has established below visible levels, high stabiliser locking the chlorine up, or a large amount of combined chlorine, meaning chlorine that has already reacted with contaminants and is no longer available. A total chlorine reading well above the free chlorine reading points to that last case, and it is generally addressed with a proper shock dose rather than by topping up incrementally. Phosphates from decaying leaves and some fertilisers feed algae and quietly increase chlorine demand. Rather than repeatedly doubling doses, get a full water analysis. Chasing a stubborn reading with escalating amounts of chlorine is expensive, hard on the pool surface and equipment, and rarely solves the underlying problem.

Keeping a pool stable through an Australian summer

The practical routine that keeps a residential pool healthy is unglamorous. Test weekly through the warm months. Keep free chlorine in the 1 to 3 ppm band for a residential pool, pH between roughly 7.2 and 7.6, and total alkalinity in the band your pool shop recommends for your surface type. Run the filter long enough each day to turn the water over properly, which in summer generally means considerably longer than most people run it. Brush and vacuum regularly, because chlorine cannot reach organic matter sitting in a corner. Shock the pool after heavy rain, after a party, and after any period of hot weather with heavy use. And keep a written log of readings and doses, because a pattern across weeks tells you far more about what your pool actually needs than any single test does.