Your Total Chlorine Reads Higher Than Your Free. Here Is Why
The definitions are the easy part of this question. Free chlorine sanitises, combined chlorine is spent, total chlorine is the two added together. The part worth your afternoon is what to do when those last two numbers are not the same, because that gap is the reading that decides whether your pool needs anything at all.
Three numbers, and only one of them sanitises
Free chlorine is the part still available to do the work. Combined chlorine is the part that has already reacted with what came into the pool, sweat, sunscreen, leaves, and has nothing left to give. Total chlorine is simply the two of them added together.
Which means the arithmetic runs one way only:
Combined chlorine = total chlorine minus free chlorine.
Nothing else on this page matters as much as that subtraction. A pool reading total chlorine of 3.0 tells you nothing on its own. The same 3.0 could be a healthy pool with a strong free reading and nothing spent, or a pool with almost no working sanitiser and a backlog of used-up chlorine sitting in the water.
Your test kit may not be showing you this
This is where the question usually goes wrong in practice rather than in theory. A lot of strips report one chlorine value, and the value they report is total. That number looks reassuring in exactly the situation where it should not, because a pool that has burned through its free chlorine still carries the combined chlorine it produced doing so, and the total stays up while the sanitiser is gone.
So the first fix is not chemical. It is getting a kit that reports free and total as separate readings, and then doing the subtraction yourself every time rather than glancing at one number.
What the gap is telling you
When free and total come out the same, there is no combined chlorine and there is nothing to act on. As the two readings separate, more of what is in your water is spent product rather than sanitiser.
That gap is also the source of the smell. A pool that reeks of chlorine is not a pool with too much of it; it is a pool carrying a load of combined chlorine, and the people who back off adding chlorine because of the smell are reading the water backwards.
Closing the gap costs free chlorine, and here is what that costs
Our pool chlorine calculator is built on the quantity each product needs to raise free chlorine by 1 ppm in 10,000 gallons, then scales it to the volume you enter. For a 20,000 gallon pool, each 1 ppm asks for:
| Product | Amount per 1 ppm |
|---|---|
| Liquid chlorine, 12.5% | 20.4 fl oz |
| Household bleach, 10% | 25.6 fl oz |
| Cal-hypo granules, 67% | 4.0 oz by weight |
| Dichlor granules, 56% | 4.8 oz by weight |
Take that pool reading 1.5 ppm free and 2.6 ppm total. The gap is 1.1 ppm of combined chlorine, and none of it is sanitising anything. Say you settle on 6.0 ppm as the free chlorine target: that is 4.5 ppm to add, which the calculator puts at 91.8 fl oz of the liquid, about 0.72 gallons.
Where that target comes from is the one thing this page will not decide for you, because it depends on your stabilizer reading and on the label of the product you are holding. Cyanuric acid protects chlorine from sunlight and holds some of it back at the same time, so a stabilized pool and an unstabilized one want different numbers out of the same jug. If you have never tested yours, start with what cyanuric acid actually does.
All of it rests on knowing your volume. Get that wrong and every dose you have ever added was wrong in the same proportion, so if yours is a guess rather than a measurement, the pool volume calculator settles it from your dimensions.
When the gap keeps reopening
A gap that returns within days is not a dosing failure, it is something feeding it.
- Bather load. Combined chlorine is made from what people bring in. A busy weekend produces it faster than any weekly routine clears it.
- Sunlight on an unstabilized pool. Free chlorine burns off, the combined chlorine it already made stays behind, and the two readings drift apart on their own.
- Circulation and filtration. Water that is not moving is not being treated, and a pool running a short pump schedule in high season is dosing one part of itself.
- Old product. Chlorine loses strength sitting on a shelf, so an aged jug delivers less than the label claims and the dose you calculated quietly falls short.
If the water has also gone hazy while you have been working on this, that is a filtration and circulation question rather than a chemistry one, and clearing cloudy pool water is the page for it.
Test free and total separately, subtract, and treat the difference as the actual reading. It is the number that tells you whether the pool needs anything, and on a one-value strip it is the number you never get to see.
Frequently asked questions
How do I work out combined chlorine?
You subtract. Combined chlorine is total chlorine minus free chlorine, and almost no test kit prints it for you as a third result. Read free, read total, take one from the other. If your kit only gives you a single chlorine number, you cannot do this at all, and that is the case for a lot of the cheaper strips.
Why does my pool smell strongly of chlorine when the reading looks fine?
That smell is the combined chlorine rather than the free chlorine, which is why it gets worse as the sanitiser gets weaker. People read the smell as a sign of too much chlorine and hold off adding any, which is the opposite of what the water is asking for. If it smells sharp and the eyes of everyone in it are stinging, test free and total separately before you decide the pool has had enough.
Does a non-chlorine shock close the gap?
It is designed to oxidise the combined chlorine without adding free chlorine, so it is a different tool from the one our calculator doses. It will not raise a free chlorine reading that is low, so it is no help in a pool whose real problem is that it ran out of sanitiser. Read the label of the product in your hand, including what it says about retesting and about when people can get back in.
Can I swim while free and total are still apart?
Swimming is what widens the gap in the first place, since combined chlorine is made from what swimmers bring into the water. A pool that is only slightly apart and holding a normal free chlorine reading is an ordinary pool. One that is well apart with a weak free reading has very little working sanitiser in it, which is a reason to fix the chemistry before the next swim rather than after it.