Pool chemistry is five numbers: free chlorine 1–3 ppm, pH 7.2–7.6, total alkalinity 80–120 ppm, calcium hardness 200–400 ppm and cyanuric acid 30–50 ppm. Chlorine sanitizes; the other four keep it working and stop the water attacking the pool. Adjust them in order — alkalinity, pH, calcium, CYA, then chlorine — because each one moves the ones after it.
What Pool Chemistry Actually Is
Balanced pool water comes down to five numbers. Get them in range and the water is safe, clear, and gentle on your skin, your liner, and your equipment. Let them drift and you get the whole catalogue of pool problems — cloudy water, algae, stinging eyes, corroded metal, scaled tile.
You do not need a chemistry degree. You need to know what each number does, what range it should sit in, and which order to fix them in. That is the whole job. Everything else is detail.
The one rule to remember
Chlorine sanitizes. The other four numbers exist to keep chlorine working and keep the water from attacking your pool. If chlorine is the engine, pH, alkalinity, calcium, and CYA are the oil, coolant, and fuel mix that let it run without grinding itself apart.
Free vs combined chlorine
Free chlorine (FC) is the sanitizer still available to work. Combined chlorine (CC), also called chloramines, is chlorine that has already bonded with sweat, sunscreen, and other organics — it is spent, and it is what makes eyes sting and a pool smell strongly of "chlorine." The fix is oxidation: raising FC to breakpoint (roughly 10× the CC reading) burns off the chloramines. A drop-based DPD or FAS-DPD test kit reads FC and CC separately, which is why it beats a strip when the water smells off but the FC "looks fine."
The Five Numbers That Matter
Ideal ranges below follow the recommended levels used by Trouble Free Pool / PoolMath and the APSP-11 water-balance standard. They match the targets in our calculators.
| Number | Ideal Range | What It Does |
|---|---|---|
| Free Chlorine (FC) | 1–3 ppm | The sanitizer. Kills bacteria and algae. The only number that protects swimmers. |
| pH | 7.2–7.6 | Acidity. Controls swimmer comfort, chlorine strength, and whether water corrodes or scales. |
| Total Alkalinity (TA) | 80–120 ppm | pH's shock absorber. Stops pH from bouncing around every time you add a chemical. |
| Calcium Hardness (CH) | 200–400 ppm | Dissolved calcium. Too low, the water eats plaster and metal; too high, it scales. |
| Cyanuric Acid (CYA) | 30–50 ppm | Chlorine's sunscreen. Protects FC from UV burn-off. Too high and chlorine stops working. |
Salt-chlorine-generator (SWG) pools run CYA a little higher (60–80 ppm) and add a sixth number — salt, around 2,700–3,400 ppm. Those are the only differences for a basic salt pool.
How They Affect Each Other
This is the part most beginners miss: the five numbers are not independent. Change one and you nudge the others. Three relationships explain almost every "I dosed correctly and it still went wrong" story.
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CYA controls how strong your chlorine needs to be
CYA shields chlorine from the sun, but it also weakens it. The higher your CYA, the more FC you need to get the same sanitizing power. That is why a fixed "1–3 ppm" only holds at moderate CYA — push CYA to 80+ and 3 ppm of chlorine is barely doing anything. This is the single most common reason a pool turns green while the owner swears the chlorine reading "looked fine."
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Alkalinity stabilizes pH
TA is a buffer — it absorbs acid and base so pH does not swing wildly. Too low (under ~70) and pH bounces every time you add anything. Too high (over ~120) and pH stubbornly climbs and resists correction. Always read and fix alkalinity before you chase pH, or you will be adjusting pH forever.
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pH + calcium decide whether water corrodes or scales
When pH and calcium hardness are both on the low side, water is "hungry" and pulls calcium out of plaster and metal out of your heater. When both run high, it deposits scale on tile and inside the heater. Keeping pH 7.2–7.6 and CH 200–400 keeps the water neutral — neither eating nor coating your pool. This balance is what the Langelier Saturation Index (LSI) measures.
Check All Five at Once
Enter your current readings and get an ordered, step-by-step treatment plan — what to add, how much, and in what sequence.
Open Water Balance CalculatorHow to Test Your Water
You cannot manage what you do not measure. Guessing is how pools go green. Two ways to test, in order of reliability:
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Drop test kit (best)
A reagent kit like the Taylor K-2006 measures all five numbers accurately, including the FC/CC split that strips can't read. Worth it if you own a pool for more than one season.
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Test strips (fast, rough)
Dip-and-read in seconds. Fine for a quick "is anything badly off?" check, but the color matching is imprecise — never trust a strip for shock or CYA decisions.
How often
Test FC and pH 2–3 times a week in swim season (more in a heatwave or after heavy use). Test TA, CH, and CYA every couple of weeks — they move slowly. Always test before adding any chemical, and again after it has circulated.
The Order to Adjust — Don't Skip This
Sequence matters because earlier numbers move later ones. Fix them in this order and you avoid chasing your own tail:
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1
Alkalinity (TA) first
It is the buffer. Set it to 80–120 before anything else, or pH won't hold.
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2
pH second
With TA stable, bring pH into 7.2–7.6. Adjusting alkalinity already shifts pH, so do this after.
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3
Calcium hardness third
Top up to 200–400 if low. It only goes up by adding calcium chloride; lowering it means dilution.
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4
CYA fourth
Set stabilizer to 30–50 (or 60–80 for salt pools). It dissolves slowly — wait 24–48 hours before retesting.
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5
Chlorine last
Once everything else is set — and CYA is known — dose FC to 1–3 ppm. Setting chlorine before CYA is guesswork.
Our Water Balance calculator applies exactly this order automatically and tells you the dose for each step.