Pool Chemistry for Beginners: The 5 Numbers That Matter

Open a pool test strip bottle and you'll see five or six little pads, each one measuring something with a name like "cyanuric acid" or "total alkalinity," with no explanation of why any of it matters. It's a lot to take in on day one, especially if the previous owner or a big-box store clerk handed you a jug of three different chemicals and wished you luck. The good news is that almost everything about how your pool looks, feels, and behaves comes down to just five numbers. Learn what each one does and roughly where it should sit, and the rest of pool chemistry starts to make sense on its own.

The 5 numbers, at a glance

pH and free chlorine: the two that move day to day

pH measures how acidic or basic the water is, on the same 0 to 14 scale you probably remember from a science class, where 7 is neutral. Pool water sits a little above neutral, in the 7.4 to 7.6 range, which is often described as close to the pH of human eyes and mucous membranes, part of why water in that range feels comfortable to swim in rather than stinging or drying out your skin. pH also affects how well your sanitizer works: chlorine is far more effective at killing bacteria and algae when pH is in range, and noticeably weaker once it drifts high. Free chlorine (FC) is the number that actually does the sanitizing, the portion of chlorine still available to react with contaminants. It's different from combined chlorine, which is chlorine that has already reacted with sweat, sunscreen, and other organic material and lost most of its sanitizing power. Combined chlorine is also the source of that sharp "chlorine smell" people associate with too much chlorine, when it usually means the opposite: not enough free chlorine to keep up with what's being thrown at the water. The ideal combined chlorine reading is 0 ppm, with 0 to 0.5 ppm considered acceptable; if yours is climbing, the fix is typically more chlorine (a shock dose), not less.

Total alkalinity, calcium hardness, and cyanuric acid: the numbers that move slowly

Total alkalinity (TA) doesn't affect swimmers directly, its job is to keep pH from swinging around every time something is added to the water, whether that's chlorine, rain, or a heavy day of swimmers. Think of it as a buffer, or a shock absorber for pH. When TA is too low, pH can bounce up and down with small additions, making it hard to keep anything stable. When TA is too high, pH tends to creep upward on its own and becomes stubborn to bring back down. Because TA and pH are easy to confuse (they're both tested with drops or a strip pad, and both described in similar-looking ranges), it helps to remember they measure different things: pH is where the water is right now, TA is how much it resists moving away from there. Get TA into range first, and pH usually becomes much easier to hold steady.

• Calcium hardness (CH) measures dissolved calcium and mostly protects the pool itself rather than the swimmers. Too low, and the water becomes corrosive, pulling calcium out of plaster and pitting metal equipment. Too high, and calcium starts coming back out of solution as scale, showing up as cloudy water or a rough, chalky texture on plaster and tile. There's no chemical that directly lowers calcium hardness; if it's too high, the standard fix is a partial drain and refill with lower-hardness water. Vinyl and fiberglass pools have a bit more flexibility here since there's no plaster to protect, though manufacturers often still recommend staying in range to protect metal fittings and heat exchangers.
• Cyanuric acid (CYA), also called stabilizer or conditioner, acts like sunscreen for chlorine, shielding free chlorine from being burned off by UV rays. Outdoor pools need some CYA in the water or they'll burn through chlorine within a few hours on a sunny day. Too little, and you'll find yourself adding chlorine constantly. Too much, and many pool professionals believe it reduces how effectively that chlorine can sanitize, even when the free chlorine number on your test strip looks fine, part of why some testers use a target FC-to-CYA ratio rather than a flat FC number. Like calcium hardness, there's no chemical that lowers CYA, only dilution, so it's worth adding stabilizer gradually and testing before adding more.