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poolcntrl

Cntrl your pool with confidence.

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How poolcntrl works

Everything else got easier. Now your pool does too.

  1. A test strip held up against the pool waterTwo fingers hold a dipped test strip in front of the water, so its pads can be read against the pool.

    Test your water

    Snap your test strip (in beta), or type the numbers in.

  2. A phone photographing the poolA phone is held over the pool with the water framed on its screen, ready to take a photo.

    Snap your pool

    Photograph the water (in beta). poolcntrl names what it sees and what to check next.

  3. The phone showing a dose beside a chlorine jugA jug of liquid chlorine sits on the pool deck. Next to it, a phone shows the amount to add, highlighted.

    Follow your steps

    Exact amounts for your pool’s size, surface and the chemicals in your garage. Jugs and cups, not chemistry.

  4. A reminder on the phone between testsA phone on the deck shows a reminder card, with the sun and a cloud overhead.

    Let it watch

    Weather, trends and reminders between tests, so the next problem is one you saw coming.

Confidence, not chemistry.

Exact math for the numbers you enter

Same readings, same answer, every time. Where a dose comes from a formula, we show you the formula.

Yours, not generic

Sized to your pool’s volume and surface, and to the chemicals already in your garage.

AI reads, chemistry decides

AI reads your strip and looks at your water, both in beta. The chemistry engine decides every amount.

How accurate are we? →

See it work.

The real engine on a sample pool. Change any number and the plan changes with it.

Your pool

Plaster · Chlorine

Water test

Your plan

Do this today

Safe to swim after 4 hours. Retest first.

Something spilled, or someone swallowed some? What to do →

Add
Add 69 fl oz of Liquid Chlorine 10%
Free chlorine (FC)Free chlorine is the chlorine that hasn't been used up yet and is still available to kill germs and algae. It's the single most important number for keeping your water safe to swim in. Glossary →
That’s about half a gallon jug. Brings free chlorine to 5 ppm. Retest in 4 hours.
Ask for: liquid chlorine, 10%, in 1-gallon jugs.
Pour slowly with the pump running, near a return jet, and let it circulate.
Best added at dusk — your stabilizer protects chlorine from sunlight, but adding it after the sun is off the water still gives it the whole night to work.
Nothing about this amount needs extra pacing. Add it as the steps above describe, and let the pump circulate before you retest.
⚠️ Strong oxidizer — wear gloves and eye protection.
What to look for on the label: Liquid chlorine
Also sold as
  • Sodium hypochlorite — the name on the ingredient panel
  • Chlorinating liquid, pool chlorine, liquid shock
  • Plain unscented household bleach is the same chemical, weaker
Strengths on the label
  • 10% and 12.5% sodium hypochlorite are the two pool-store jugs
  • 6% to 8.25% for household bleach
Sizes it comes in
  • 1-gallon jugs, usually sold in a case of two or four
  • 2.5-gallon jugs, which pool stores often stack by the door
What you can use instead
  • Unscented household bleach with no thickeners, scent or splash-less additive
  • Cal-hypo does the same job as a powder, but leaves calcium behind

Check the strength on the front and the date on the jug. Liquid chlorine weakens on the shelf, so the freshest case is worth reaching for. The app asks which strength you bought, and the amount it gives you changes with your answer — so tell it the jug you actually own, not the one it assumed.

Every amount here scales with the gallons you entered. How accurate are we? →

Scale and corrosion
Saturation index (LSI)The saturation index is scale risk in one number: whether your water will drop calcium onto surfaces as scale, or pull it out of plaster and grout. It is worked out from your pH, alkalinity, calcium and water temperature together. Glossary →

Your water is balanced.

Saturation (LSI): 0.0

Your pH, alkalinity, calcium and water temperature are in balance — the water is neither pulling calcium out of your pool nor dropping it onto surfaces.

This matters most on plaster, pebble and tile pools, and on saltwater pools — plaster has calcium the water can take, and a salt cell is usually the first thing to scale up. Vinyl and fiberglass shells are much less sensitive to it.

Why this number?
pH 7.5
Water temperature 82°F → temperature factor 0.6
Calcium hardness 300 ppm → calcium factor 2.1
Total alkalinity 90 ppm, less 12.4 ppm that is really your stabilizer (40 ppm) = 77.6 ppm → alkalinity factor 1.9
Dissolved-solids constant 12.1, the published figure for a typical pool
7.5 + 0.6 + 2.1 + 1.9 − 12.1 = 0.00
Free chlorine chart

At stabilizer (CYA) 40 ppm: Lowest safe 3 ppm · Aim for 5–7 ppm · Shock (SLAM) 16 ppm.

⚠️ Don't just watch the clock — retest before swimming. Water is safe when free chlorine is between 3 and 7 ppm, pH is 7.2–7.8, and you can see the bottom.

Re-test in about 4 hours.

Before you start

How long to run the pump after you add something →
How long the pump stays on after this goes in, and why the retest waits.

Kids, pets and pool chemicals →
Where to keep these, and who stays out of the water until the retest.

Why this amount?

Here is the whole calculation behind that amount — your pool’s volume, how far the reading has to move, and the strength of the product.

Add 69 fl oz of Liquid Chlorine 10%
18,000 gal × 3 ppm ÷ 1,000,000 ÷ 10% × 128 fl oz/gal → 69.1 fl oz, rounded to 69 fl oz

Where our numbers come from →

That dose is real — the same deterministic engine the app runs, same answer every time.

And when the numbers cannot be right — total chlorine under free chlorine, a pH no pool has — poolcntrl says so instead of dosing. Nothing gets rounded into an amount.

Pro already knows your pool, so you skip every field above — and checks your next test against the chlorine you add.

Engine ruleset mvp-seed-v1.43.0 · How this dose was worked out

Pro knows your pool.

Skip the typingPhotograph the strip. AI reads the pads into your test.
Name the problemPhotograph the water. Get a likely cause and what to check next.
Your pool, savedVolume, surface and sanitizer, saved once. Every dose is sized to your pool.
Know if it workedLog the chlorine you added. Your next test is checked against it.
Weather, before it hitsHeat, rain and storms show up as advice before they show up in your water.
See it comingA chart for every reading, so a drift shows before a problem does.
Told when to retestOn your pool’s own cadence, plus weather and filter notices.
Every test keptEvery test, dose and note kept, with what happened next.
More than one poolEvery pool and property you look after, in one account.

After your free two weeks, Pro is $5.99 a month or $49.99 a year.

The alternative is free, and it is a fair one — your pool store will read a sample on Saturday. Why pay for this instead →

Download on theApp StoreTry the calculatorSee what Pro includes →

New to this? Start here.

Plain-language guides, written for first-time owners.

How a water test worksA five-step, illustrated walkthrough from scooping a sample to getting your plan.How to add chemicalsThe safe, simple sequence for adding anything to your pool — animated step by step.What each measurement meansPlain-language explanations of every reading — what it measures and why it matters.What each chemical doesWhat every product does, how it’s added, and why the method matters.

Closing for winter? The moment you mark the pool closed the reminders stop. When the forecast warms up in spring, poolcntrl asks whether you are ready to reopen.

Explore all guides →Where our numbers come from →Closing or opening your pool →

Ready for your free trial?

Your two weeks start the moment you create the account — no credit card. After the trial it’s $49.99 a year, or the free calculator forever.

You use the trial in the iPhone app — create the account with your iPhone in reach. This site is the read-only view of your pool.

How the trial and billing work

Your 2-week free trial needs no card and starts the moment you create your poolcntrl account, on the web or in the app — and you use it in the iPhone app. Subscribing afterwards happens inside the iPhone app through your App Store account. An App Store subscription also starts with two weeks free, if your Apple ID hasn't used this offer before. After that, it renews and charges until you cancel it, any time, in Settings.

By creating an account you agree to our Terms and Privacy Policy. No spam — only product updates.

Download poolcntrl now

Take your pool’s perfect routine with you. Free to start on iPhone — Android is on the way.

Download on theApp Store
On Android?The full chemistry engine runs free in your browser — the same server-side math the iPhone app asks for, and the same answer. No app to install, no account needed.Open the free calculator →

One subscription, every device — phone at the pool, laptop at the kitchen table.