Pool Chlorine Calculator
A 65% product is not 100% — dose the active, not the tub.
Work out Pool Chlorine. A 65% product is not 100% — dose the active, not the tub. Every rate is an input, not an assertion.
From the label — it is rarely 100%
Leave at 0 for a granular product
Product to add
153.8 g
To raise 2 ppm across 50,000 litres
Active strength is the step people skip. A granular product at 65% available chlorine delivers 65 g of active per 100 g, so dosing as though it were pure under-treats by more than a third. Add chemicals to water rather than water to chemicals, add them with the pump running, and never mix two products in the same container — some combinations react violently. Test after circulating rather than immediately.
How the Pool Chlorine Calculator works
How much product to add to move a pool from its current reading to a target, accounting for the active strength of what you are using. Dosing as though a granular product were pure chlorine under-treats by a third.
Also known as: how much chlorine to add to my pool · pool shock dose calculator · ppm chlorine dosing calculator · granular chlorine dose by pool size
Active strength, and the third that never arrives
One part per million is one milligram per litre, so the active chlorine needed is simply ppm × litres ÷ 1000. That part is straightforward. The step people skip is dividing by the product's active fraction.
A granular product at 65% available chlorine delivers 65 g of active per 100 g of product. Dosing 100 g where 100 g of active was needed under-treats by more than a third, and the next test then reads low for reasons that look mysterious.
Different products differ enormously — dichlor, trichlor, calcium hypochlorite and liquid sodium hypochlorite all carry different active fractions and different side effects on pH and stabiliser. Reading the percentage off the tub is not optional.
Free, combined and why chlorine smells
Free chlorine is available to sanitise. Combined chlorine is already bound to nitrogen compounds from sweat and urine, forming chloramines, and is largely spent. Total chlorine is the sum of the two.
The strong chlorine smell of a poorly maintained pool is chloramines, not chlorine. It signals too little free chlorine rather than too much, which is exactly backwards from the intuitive reading — and adding more chlorine is the correct response.
Breakpoint chlorination is raising free chlorine high enough to destroy the chloramines rather than merely adding to them. Under-shooting it makes the smell and the eye irritation worse, which is why partial shocking is worse than none.
Stabiliser, sunlight and pH
Unstabilised chlorine is destroyed by UV within hours. Cyanuric acid binds it loosely and slows that dramatically, which is why outdoor pools use it. Too much, though, binds the chlorine so tightly that its sanitising power falls sharply at the same measured level.
That is why there is no single correct free chlorine target — the right level depends on the stabiliser concentration. Published ratios exist for exactly this reason, and a pool with high cyanuric acid needs proportionally more chlorine to achieve the same effect.
pH compounds it. The same free chlorine sanitises far less effectively at pH 8 than at 7.4, because the proportion present as hypochlorous acid falls as pH rises. Balancing pH before chasing a chlorine reading is why test kits list it first.
Where to go next
The Pool Chlorine question rarely arrives on its own. These are the ones that usually come with it:
- Pool Volume Calculator — Measure at the deep end and you overdose everything.
- Hot Tub Chemical Calculator — Small volume means the margin for error is tiny.
- Fish Stocking Calculator — Oxygen enters at the surface, not through the volume.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate a chlorine dose?
One ppm is one milligram per litre, so grams of active = ppm × litres ÷ 1000. Then divide by the product's active fraction — a 65% product needs 154 g to deliver 100 g of active.
What is free chlorine versus total chlorine?
Free chlorine is available to sanitise; combined chlorine is already bound to contaminants and is largely spent. Total is the sum, and a large gap between them is what shocking is meant to clear.
What level should free chlorine be?
Target ranges vary by authority and by whether the pool is stabilised, so use the figure your own testing guidance gives. What is consistent is that it depends on cyanuric acid level, which is why a single universal number does not exist.
Why does my chlorine disappear so fast?
Sunlight is the usual answer — UV destroys unstabilised chlorine within hours. Cyanuric acid slows it considerably, and too much of it makes the chlorine ineffective instead.
Can I mix pool chemicals?
Never in the same container. Some combinations react violently, and calcium hypochlorite with trichlor is a known fire risk. Add each to the water separately, with the pump running.
When should I test after dosing?
After a full turnover, so the chemical has circulated. Testing immediately reads the water near the return rather than the pool.
What is cyanuric acid and why does it matter?
A stabiliser that protects chlorine from UV. Too little and chlorine burns off in hours; too much and it binds the chlorine so tightly that it stops sanitising effectively.
What is breakpoint chlorination?
Raising free chlorine high enough to destroy combined chloramines rather than just adding to them. Under-shocking makes the smell worse, because chloramines are what smells of chlorine.
Why does my pool smell strongly of chlorine?
Chloramines, not chlorine — the smell means the chlorine is spent and bound to contaminants. It signals too little free chlorine rather than too much.
Should I dose in the morning or evening?
Evening, so the chlorine has all night to work before UV starts destroying it. Dosing at midday wastes a meaningful share of it.
What does pH do to chlorine?
High pH sharply reduces its effectiveness — the same free chlorine reading sanitises far less at pH 8 than at 7.4. Balancing pH first is why test kits list it before chlorine.
Can I swim right after dosing?
Wait until the chemical has fully circulated and the level has been retested — typically after a turnover. Swimming with undissolved granules present risks skin and eye burns.
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Measure at the deep end and you overdose everything.
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Small volume means the margin for error is tiny.
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Open