Pool Chemistry 101: The Five Numbers That Actually Matter

Most pool problems don't start as algae or cloudy water. They start weeks earlier, as one of five numbers quietly drifting out of range while nothing visible happens yet. By the time you can see the problem, you're usually looking at a multi-day fix instead of a five-minute one.

Here's what each number actually does, in the order it's worth understanding them.

Free Chlorine (FC) — the number doing the work

Free chlorine is the chlorine that's actually available to kill bacteria and oxidize organic matter right now — sweat, sunscreen, leaves, whatever fell in. It's consumed as it works, which is why it needs topping up regularly rather than dosed once and forgotten.

Target: 1–4 ppm for most residential pools, though the real minimum depends on your stabilizer level (more on that below — this is the one number that doesn't have a fixed target).

Too low, and you're not sanitizing — algae and bacteria get a foothold within a day or two of hot weather. Too high isn't usually dangerous, but it's wasted chlorine and can irritate skin and eyes.

pH — how comfortable and how corrosive

pH measures how acidic or basic the water is, on a 0–14 scale. It affects two things directly: how well your chlorine actually works, and whether your water is comfortable to swim in and gentle on your equipment.

Target: 7.2–7.6, with 7.4 as the sweet spot — it's close to the pH of human tears, which is why well-balanced pools don't sting your eyes.

Low pH (acidic) corrodes metal fittings and etches plaster. High pH (basic) causes cloudy water, scale buildup, and — critically — makes your chlorine far less effective at actually sanitizing, even if the FC number itself looks fine.

Total Alkalinity (TA) — the buffer, not the target

This is the number people skip past because it doesn't affect swimmers directly. But TA is what keeps pH from swinging wildly every time you add another chemical or a thunderstorm rolls through. Think of it as a shock absorber for pH.

Target: 80–120 ppm for most pools (lower, around 60–90 ppm, if you're running a salt chlorine generator).

Low TA means pH will bounce around unpredictably no matter how carefully you dose it — you'll be chasing it constantly. High TA makes pH difficult to lower and tends to push it upward on its own over time.

Calcium Hardness (CH) — for the pool, not for you

Calcium hardness is dissolved calcium in the water. It doesn't do much to swimmers, but it matters a lot to your pool's surface and equipment.

Target: 200–400 ppm, with plaster and gunite pools wanting the higher end of that range (225–400 ppm) and vinyl or fiberglass pools able to run lower (175–250 ppm) since they don't have a mineral surface to protect.

Too low, and the water becomes "aggressive" — it starts pulling calcium out of your plaster or grout to make up the difference, which is how surfaces get pitted and etched over years. Too high causes cloudy water and scale on tile, heaters, and pump seals.

Cyanuric Acid (CYA) — chlorine's sunscreen

Cyanuric acid, also called stabilizer or conditioner, protects chlorine from being burned off by UV light. Without it, direct sunlight can destroy most of your free chlorine in a couple of hours — which is why unstabilized outdoor pools need constant, expensive redosing.

Target: 30–50 ppm for a standard chlorine pool, 60–80 ppm if you're on a salt system (salt cells produce chlorine steadily, so a higher CYA buffer helps it last).

Here's the part most people miss: your minimum safe free chlorine level actually rises as CYA rises. At 0 ppm CYA, 1–2 ppm FC is enough. At 40 ppm CYA, you need closer to 4–5 ppm FC to sanitize just as effectively — the stabilizer that protects chlorine from sunlight also dilutes how hard it can work at any given concentration. This is the single most common reason a pool reads "2 ppm chlorine, should be fine" and still goes green.

Unlike the other four numbers, there's no chemical that lowers CYA. If it climbs too high — which happens gradually with trichlor tabs, since every dose adds a little — the only fix is diluting the water with a partial drain and refill.

Why this only matters as a system

None of these numbers exist in isolation. High pH makes chlorine less effective even at a "good" FC reading. Low TA makes pH impossible to hold steady. High CYA quietly raises the FC you actually need. Testing one number and ignoring the rest is how a pool that "should be fine" turns green anyway.

The fastest way to see how they interact for your specific pool — your volume, your surface, your sanitizer — is to run an actual test and let something do the cross-referencing for you instead of eyeballing five numbers against a strip chart.