Start here
What does your water look like right now?
Tap what you’re seeing. You’ll get the actual cause and one thing to do about it — not a list of nine products.
How this site works
We have never put a test kit in a pool. Here’s what we do instead.
Most pool sites tell you they field-test everything. Almost none of them do, and you can usually tell, because the winner is whichever product pays best.
So: we screen products against data and we show our work. Listing data, verified review counts and monthly sales volume for everything we mention, refreshed and dated. Chemistry targets from published standards, cited on the page. Our buying advice comes out of what buyers actually reported going wrong — not a rewrite of the manufacturer’s bullet points.
And when someone else is better, we send you there. If you want three seasons of real-world reagent comparisons, that lives on TroubleFreePool’s forum, and there are no affiliate links in this sentence.
Where we do recommend something, we say what it measures and what it leaves behind rather than how it feels to use — because we have not used it. Where a listing doesn’t publish a figure we need, we say the figure is missing instead of assuming a standard one. Several of the biggest-selling chlorine products on Amazon never state their available-chlorine percentage, and quietly filling that gap with an assumption is how a dosing page ends up wrong.
How we make money, plainly. Some links here go to Amazon and we earn a commission if you buy through them. It does not change your price and it does not change what we recommend — on our test-kit page the cheapest product wins two of the six possible answers, which is not what a page optimised for commission looks like. Nothing on this site is a paid placement and no brand has ever been shown any of it before publication.
Where you are changes the answer
A Phoenix pool and a Boston pool are not the same problem
Most pool advice is written as if the whole country had one climate. Season length runs from about twelve weeks to year-round, peak chlorine demand varies by more than double, and tap water goes from calcium-hungry to calcium-saturated depending on the bedrock under you.
Where's your pool?
Season length and UV load are the two biggest drivers of chlorine demand, and both are geographic.
A 20,000 gallon pool here loses roughly 3.8 ppm of chlorine a day at peak, across a 44-week season. Run that entirely on trichlor tablets and you would finish around 732 ppm of cyanuric acid from a starting point of 40 — which is why the same tablet routine behaves completely differently in gulf & southeast than it does two time zones away. Your free chlorine target would move from 4.6 ppm to 84.2 ppm over that season.
The practical consequence is that the same tablet routine produces completely different outcomes in different places. A floater habit that leaves an Ohio pool at a comfortable cyanuric acid level in September will have chlorine-locked a Phoenix pool by July, because the Phoenix pool ran twice as long at nearly double the demand. Neither owner did anything wrong; they read the same advice.
Water hardness splits the country the same way and in the opposite direction. New England’s granite gives water so soft that calcium has to be added — cal-hypo shock is quietly doing those owners a favour — while the Southwest’s tap water frequently arrives above the calcium target before anyone adds anything, making the same product the wrong choice.
We write for pool owners across the United States, so where a regional difference changes the answer, we say which region. Everything else on this site — prices in USD, products from Amazon.com, ppm and gallons rather than mg/L and litres — follows US convention throughout.
The pool year
Four moments that decide how the season goes
Almost every expensive pool problem is traceable to one of these four points, and to a reading that wasn’t taken at it.
Opening. This is the one people rush, and it is the cheapest place to get ahead. Water that has sat under a cover all winter has usually lost most of its chlorine and may have started growing something. Before you dose anything, take a full set of readings — especially cyanuric acid, because it carries over from last season untouched. CYA does not degrade over winter; whatever you finished September with is roughly what you are starting May with, and if you spent last summer on tablets that number may have moved a long way without you noticing. Establishing it now decides every chlorine decision for the next five months.
The first hot week. Demand roughly doubles when the water warms and the pool starts being used, and this is where a routine that worked in May quietly stops working. Chlorine that held for three days now lasts one. If you are on tablets, your floater empties faster and your CYA starts climbing in earnest. The move is to re-check your daily chlorine loss rather than assume the spring schedule still applies — and to start topping up with something unstabilised so the stabiliser doesn’t run away from you in August.
Mid-season, roughly July. One monthly test set here catches nearly everything before it becomes an incident. This is the point at which a tablet-fed pool has typically added enough cyanuric acid to have meaningfully changed its own chlorine target, and the owner is still dosing to the spring number. It is also when calcium creeps up in pools using cal-hypo shock. Ten minutes with a drop kit in July is worth more than any product you can buy in August.
Closing. Whatever you leave in the water is what you find in the spring, so it is worth closing balanced rather than closing tidy. Get pH and alkalinity into range before the cover goes on, because water sitting out of balance all winter attacks surfaces the whole time with nobody watching. And if the season pushed your CYA somewhere uncomfortable, the off-season is by far the easiest time to deal with it — you can replace water without ruining anyone’s weekend, and the fresh fill has months to settle before you need it clear.
Notice what is common to all four: the action is a reading, not a purchase. A pool that gets tested at those four moments almost never needs the shelf of corrective products that a pool tested twice a year eventually does.
Start here if you’re new
The questions everyone has in their first season
The whole of pool chemistry
It’s five numbers. That’s genuinely all.
Pool care looks complicated because it is sold in dozens of bottles. Underneath, there are five readings, and four of them barely move. The full levels chart adjusts these for your sanitiser and pool surface.
| Reading | Target | Test how often | What it actually does |
|---|---|---|---|
| Free chlorine (FC) | Depends on CYA | Twice a week | The only thing actually sanitising. Its target is a ratio of your CYA, not a fixed number. |
| pH | 7.2 – 7.8 | Twice a week | Swimmer comfort, and whether your chlorine works efficiently. Drifts up in most pools. |
| Total alkalinity (TA) | 60 – 120 ppm | Monthly | The buffer that stops pH bouncing. Low TA is why pH won't hold still. |
| Calcium hardness (CH) | 200 – 400 ppm | Monthly | Too low eats plaster and grout. Too high scales heaters and surfaces. |
| Cyanuric acid (CYA) | 30 – 50 (60 – 80 salt) | Monthly | Sunscreen for chlorine. Sets your FC target, and only leaves when water does. |
Two of those move daily and three barely move at all. If you test chlorine and pH twice a week and the other three once a month, you have done essentially everything pool maintenance asks of you — and you will spend far less on chemicals than someone treating symptoms they never measured.
The one that causes the trouble is the last row. Cyanuric acid is the only reading here that most owners never test, and it is the one that changes what every other number means. It protects chlorine from sunlight, which is essential — an unstabilised outdoor pool can lose most of its chlorine in an afternoon. But it also binds chlorine, so the more you have, the more total chlorine you need to keep the same amount actually working.
That is the mechanism behind almost every “my pool went green and I don’t know why” story. The owner held 3 ppm all season because that is what the bottle said, while their CYA climbed past 80 and quietly moved the real target to 9 ppm. Work out what your own target should be and the rest of this site will make more sense.
Above roughly 80 ppm your chlorine is still present but chemically asleep.
The green band is 7.2–7.8. Irritation lives on both sides of it.
Why calculators, not articles
An answer you can act on has your numbers in it
Almost every pool question has the same shape: it depends on your water volume, your current readings, and what you already put in. “How much shock do I need?” has no general answer, and an article that gives one is guessing on your behalf — which is why the advice you find is so often contradictory. Two people can both be right and recommend opposite things, because they have different pools.
So the tools here take your readings and return your number. That is also, bluntly, the only durable reason for a site like this to exist. A search engine’s AI summary can now explain what cyanuric acid is perfectly well, and it will do it faster than scrolling an article. What it cannot do is know that your pool is 18,000 gallons and sitting at CYA 95, and therefore that you are four parts per million below the minimum you need.
Every calculator on this site computes from one shared set of chemistry constants derived from first principles rather than copied between pages, so two pages here cannot quietly disagree with each other about a dose. Where a number is an estimate we say so, and where a product listing doesn’t publish the strength we need, we tell you the figure is missing rather than assuming a standard one.
By the job
Or skip the diagnosis
Which test kit do I need?
Strips vs drops, and the three readings most kits skip.
Liquid, tablets or granular?
They all deliver the same chlorine. The by-product is the decision.
How many tablets do I need?
Your count from real demand — and what it does to your CYA.
Lowering cyanuric acid
Exactly how much water to replace, and when not to bother.