Guides · Updated Oct 31, 2026

Pool water chemistry basics

Pool chemistry looks complicated, but it comes down to a handful of numbers and how they interact. Get these right and you'll have clear, comfortable water that doesn't damage the pool surface or equipment. This guide explains each reading, its usual target range, and what to do when it's off.

Why balance matters

Balanced water does three jobs: it keeps the water sanitized so bacteria and algae can't grow, it's comfortable on skin and eyes, and it protects plaster, vinyl, metal heaters and pumps from corrosion or scale. Most problems — cloudy water, algae, stinging eyes, etched plaster — trace back to one or two readings drifting out of range.

Testing: how and how often

Test at least once a week, and more often in hot weather or heavy use. A drop-based (titration) test kit gives more reliable numbers than test strips, especially for chlorine and cyanuric acid. Take samples from elbow depth, away from return jets, and test promptly.

Free chlorine (FC)

Free chlorine is the active sanitizer. The right level depends on how much stabilizer is in the water. A widely used rule of thumb is to keep free chlorine at about 7.5% of the cyanuric acid level as a minimum, which works out to roughly 3–4 ppm at 40–50 ppm CYA. Without stabilizer, 1–3 ppm is typical. Chlorine is consumed by sunlight, swimmers and debris, so it needs to be added regularly — by liquid chlorine, tablets in a feeder, or a salt chlorine generator.

Combined chlorine (CC)

Combined chlorine (chloramines) forms when chlorine reacts with sweat, urine and other contaminants. It causes the "chlorine smell" people blame on too much chlorine — in reality it means there's not enough free chlorine to finish the job. Keep CC at or below about 0.5 ppm; if it's higher, shock the pool.

pH

pH measures how acidic or basic the water is. The ideal range is 7.4–7.6, with 7.2–7.8 acceptable. Below 7.2, water becomes corrosive and can sting eyes. Above 7.8, chlorine becomes much less effective and calcium scale forms more easily. Lower pH with muriatic acid or dry acid (sodium bisulfate); raise it with soda ash (sodium carbonate). Aeration from waterfalls and spillways naturally pushes pH up.

Total alkalinity (TA)

Alkalinity is the water's buffer against pH swings. A typical range is 80–120 ppm. If it's too low, pH bounces around; too high, and pH keeps drifting upward. Raise alkalinity with sodium bicarbonate (baking soda). Lower it by adding acid to drop pH, then aerating to bring pH back up without restoring the alkalinity — a slow but reliable process.

Calcium hardness (CH)

Calcium hardness matters most for plaster and other cement-based surfaces. Water that is too soft can pull calcium out of plaster and grout; water that's too hard leaves scale on tile and inside heaters. A common target is 200–400 ppm for plaster pools. Vinyl and fiberglass pools are less sensitive. Raise with calcium chloride; lowering requires partially draining and refilling with softer water.

Cyanuric acid (CYA, stabilizer)

CYA shields chlorine from sunlight, which would otherwise destroy most of it within a few hours. Outdoor chlorine pools typically run 30–50 ppm; saltwater pools often run 60–80 ppm. The catch: CYA doesn't break down or evaporate, and it builds up when you use stabilized tablets (trichlor) or shock (dichlor). As CYA rises, you need more free chlorine to get the same sanitizing power. The only practical way to reduce high CYA is to drain and refill part of the water.

Salt (saltwater pools)

A salt chlorine generator makes chlorine from dissolved salt. Most cells want about 2,700–3,400 ppm; check your manufacturer's range. A saltwater pool is still a chlorine pool, so all the readings above still apply.

Saturation index

Pros often use a saturation index (such as the Langelier Saturation Index or CSI) that combines pH, alkalinity, calcium, temperature and dissolved solids into one number. A result near zero means water is neither corrosive nor scale-forming. Many apps and calculators do this math for you.

The order of adjustments

1. Alkalinity first, because it affects pH.
2. Then pH.
3. Then calcium hardness.
4. Check CYA, then set free chlorine to match it.

Add one chemical at a time with the pump running, follow the label dose for your pool's volume, and retest after the water circulates. Never mix chemicals together, and always add chemicals to water — not water to chemicals.

Common problems at a glance

• Green or cloudy water: low free chlorine for the CYA level; shock and brush.
• Strong chlorine smell or red eyes: high combined chlorine or off pH.
• White crust on tile: high pH and/or calcium.
• Etched plaster or corroded metal: low pH, alkalinity or calcium.

Keep a record

Trends tell you more than a single reading. Logging each test makes it easy to see when CYA is creeping up or when pH climbs after a new water feature. Pool service companies using Swimming Pool On Demand record readings at every stop and share them with homeowners automatically.

Pool pros: log every reading and send photo reports with Swimming Pool On Demand.