Pool Chemistry 101
Every pool problem you will ever have shows up in seven test numbers. Learn what each one means once, and pool care stops being mysterious forever. No chemistry degree required — that's the whole point of this page.
The one relationship that explains everything
If you remember a single thing: chlorine and stabilizer are a pair. Stabilizer (CYA) is sunscreen for chlorine — without it the sun destroys chlorine in hours, but the more of it you have, the more chlorine you need for the same killing power. The working rule is free chlorine around 7.5% of your CYA. CYA at 40? You want chlorine near 3. CYA drifted to 80? Now 3 ppm is quietly too little, and that's how pools with "fine" chlorine readings turn green. Most beginner pool problems are this relationship going unnoticed.
1. Free chlorine (FC) — the worker
The sanitizer that's actually available to kill things. Target:7.5% of CYA (typically 3–5 ppm). Too low and algae gets a foothold within days; "too high" after shocking just needs a waiting period, not a countermeasure. Dose it with the chlorine calculator, and when you need to know if you can swim, the answer is at Can I Swim Yet.
2. pH — the comfort number
How acidic or basic the water is. Target: 7.2–7.8. High pH makes chlorine sluggish and water scale-forming; low pH stings eyes and corrodes equipment. Most pools drift UP (salt pools especially) — the muriatic acid calculator brings it down, and the pH increaser handles the rarer other direction.
3. Total alkalinity (TA) — the shock absorber
Alkalinity doesn't do anything directly — it keeps pH from bouncing on every rainstorm and swimmer. Target:80–120 ppm. Low TA means pH whiplash; high TA means pH constantly climbing. Baking soda raises it — the alkalinity calculator does the ounces.
4. CYA / stabilizer — the sunscreen
Target: 30–50 ppm (60–80 for salt pools). The trap: stabilized chlorine pucks add CYA with every puck, forever. A season of tabs can push CYA past 100, where no normal chlorine level keeps up — and the only real fix is draining some water. The CYA calculator doses it up; the drain planner handles the too-high case.
5. Calcium hardness (CH) — the finish protector
Target: 250–450 ppm for plaster pools (vinyl and fiberglass are relaxed about it). Too low and the water eats calcium out of the plaster; too high and it deposits scale. The calcium calculator raises it; high CH, like high CYA, is a dilution problem.
6. Combined chlorine (CC) — the smell of trouble
Used-up chlorine bound to contaminants. Target:under 0.5 ppm. That "pool smell" and eye sting people blame on too much chlorine is actually CC — chlorine that needs replacing, not reducing. Above 0.5, it's shock calculator time.
7. Salt — only if you have a salt pool
Target: whatever your salt water generator's manual says, typically 2,700–3,400 ppm. Salt doesn't sanitize — the cell converts it to chlorine, so all six numbers above still apply. The salt calculator doses top-ups and the SWG % calculator sets the cell's output.
Putting it together
Test FC and pH two to three times a week, the rest monthly — the testing schedule builder makes the routine, and the Water Test Report Card grades a full set of readings and orders the fixes for you. Every dose in every tool scales from your pool's gallons — measure once with the volume calculator and put it on the printable cheat sheet. If the water's already green, skip the theory and go straight to the green pool calculator — the cure is chlorine, patience, and this page explains why it works when you're back.