Pool School
New to pool care, or just tired of nodding along to terms you don't quite know? This is your plain-English glossary of the water chemistry and maintenance words that actually matter — each with a quick definition and, where it helps, a tool to put it into practice.
Water Chemistry Basics
These are the core readings you'll test for. Keeping them in range is what keeps your water clear, safe, and comfortable.
- ppm (parts per million) (ppm)
- The unit used for most pool chemical readings. 1 ppm means one part of a chemical per million parts of water — a tiny but meaningful amount. Chlorine, alkalinity, calcium, and stabilizer are all measured in ppm.
- pH
- A measure of how acidic or basic your water is, on a scale of 0 to 14. Pool water should sit between 7.2 and 7.6. Too low corrodes equipment and stings eyes; too high causes cloudy water and scale and makes chlorine less effective.
- Free Chlorine (FC)
- The chlorine still available to sanitize your water — the number that matters most day to day. Keep it around 2 to 4 ppm. Work out a dose with the Chlorine Calculator.
- Combined Chlorine (CC)
- Chlorine that has already bound to contaminants and is no longer effective. It causes the harsh “chlorine smell” and eye irritation. When it climbs above about 0.5 ppm, it's time to shock the pool.
- Total Alkalinity (TA)
- A buffer that keeps pH stable, measured in ppm (ideal range roughly 100 to 150). When alkalinity is right, pH stops bouncing around. Adjust it before you adjust pH.
- Calcium Hardness (CH)
- How much dissolved calcium is in the water (ideal roughly 200 to 400 ppm). Too low pulls calcium from plaster and grout; too high leaves scale on surfaces and equipment.
- Cyanuric Acid / Stabilizer (CYA)
- Sunscreen for your chlorine. Cyanuric acid shields free chlorine from being burned off by UV, so outdoor pools hold chlorine far longer with it. Aim for about 30 to 50 ppm (higher for saltwater pools). Too much, and chlorine becomes sluggish. Size a dose with the Stabilizer (CYA) Calculator.
- LSI (Langelier Saturation Index) (LSI)
- A single number that combines pH, temperature, alkalinity, and calcium to tell you whether water is corrosive (will eat surfaces) or scaling (will leave deposits). Balanced water sits near zero.
Sanitizing & Chemicals
The products and processes that keep your water clean and how they fit together.
- Shocking
- Adding a large dose of chlorine to blast through contaminants that normal levels can't handle — usually raising free chlorine to about 10 to 12 ppm. Do it after heavy use, storms, or when the water turns cloudy or green.
- Breakpoint Chlorination
- The point where you've added enough chlorine to fully burn off combined chlorine and start fresh. The rule of thumb is a shock dose of about 10 times your combined chlorine reading.
- Liquid Chlorine
- Sodium hypochlorite, typically 10 to 12.5% strength. Dissolves instantly and adds nothing but chlorine, making it a favorite for routine dosing and shocking.
- Cal-Hypo
- Calcium hypochlorite, a strong granular shock (around 65 to 73% available chlorine). Long shelf life, but it adds calcium and should be pre-dissolved before adding.
- Saltwater Chlorine Generator (SWG)
- A cell that makes chlorine from dissolved salt, so you don't add chlorine by hand. It needs the salt level in a specific range — size it with the Salt Calculator.
Equipment & Maintenance
The gear and routines that circulate, filter, and clean your water.
- Turnover
- The time it takes your pump and filter to circulate the entire pool volume once. Most pools need one to two turnovers a day, which is why 8 to 12 hours of pump run time is typical.
- Backwashing
- Reversing water flow through a sand or D.E. filter to flush out trapped dirt, then sending it to waste. Do it when the pressure gauge reads about 8 to 10 psi above its clean baseline.
- Filter Pressure
- The reading on your filter's gauge. A rising number means the filter is getting dirty and restricting flow — your cue to clean or backwash it.
- Priming
- Getting water flowing through the pump so it can move water instead of spinning on air. A pump that won't hold prime usually has an air leak or a low water level.
Put it into practice
Ready to use what you've learned? Start with our free tools: check if your readings are healthy with the Water Balance Checker, size a chlorine dose with the Chlorine Calculator, or explore all of our pool tools and calculators. Working through a specific problem? Try the Troubleshooting Clinic.