If you've ever looked at the label on a bag of granulated chlorine, you'll have seen a number like "56%", "65%" or "70%". That percentage isn't decoration or a minor detail, it's the actual proportion of active chlorine the product contains, and it varies quite a bit between brands and formats.
Same weight, different active chlorine
Why not all "chlorines" are the same
What we buy as "pool chlorine" is almost never pure chlorine, it's chemical compounds (calcium hypochlorite, dichlor, trichlor, among others) that release active chlorine when dissolved in water. Each compound has a different proportion of real chlorine per kilo of product. The percentage on the label indicates exactly that: how much of that kilo is chlorine that will actually disinfect, and how much is the rest of the formula (stabilizers, binders, etc.).
Why this matters for dosing
Here's the practical part: if you were using a 56% product and switch to a 70% one without adjusting the amount, you'll be adding too much, the new product delivers more per kilo. The opposite happens too: if you go from 70% to 56% and keep adding "the usual amount", you'll fall short of active chlorine, even though the weight of product is the same.
It's one of the most common reasons a pool that "had always been fine" suddenly starts having problems, green water from a chlorine shortfall, or excess chloramines from overdosing, without anything else having changed except the brand of the bag.
Is a higher percentage "better"?
It's not a question of better or worse, but of concentration. A 70% chlorine is more concentrated, you need less of it for the same effect, which can also work out cheaper thanks to savings on transport and storage. A 56% chlorine isn't lower quality, it simply delivers less per kilo and you need more of it to reach the same result. What matters isn't which one to choose, but knowing exactly which one you're using at any given time.
Frequently asked questions
Why are there chlorine bags with different percentages?
Because not all the compounds used as pool chlorine contain the same proportion of genuinely active chlorine per kilo of product.
Can switching chlorine brands throw off my maintenance?
Yes, if you keep adding "the usual amount" without checking the percentage of the new product.
Is 70% chlorine always better than 56%?
Not necessarily better, but more concentrated: you need less of it for the same effect.