Pool chlorine guides
Free chlorine (FC) is the front line of water safety, but getting it right is more nuanced than just adding liquid and hoping. FC disappears faster in hot weather, your CYA level sets the minimum you actually need, and combined chlorine — the gap between TC and FC — tells you when there's a problem chlorine alone won't fix. These guides cover the full picture: how to read your numbers, why FC vanishes overnight, which chlorine type to use, and the FC/CYA relationship that determines your real target.
Understanding pool chlorine
Free chlorine (FC) is the only form that sanitizes — it's the chlorine ready to react with bacteria, algae, and organic contaminants on contact. When FC does its job and gets used up, it becomes combined chlorine (CC): spent chlorine that still registers on a test kit but no longer protects your pool. Total chlorine (TC) = FC + CC. The gap between TC and FC is your early warning number — keep it below 0.5 ppm and your sanitizer is working efficiently. A gap above 0.5 ppm means chloramines are accumulating and a shock dose is needed.
Your FC target isn't a flat 2–4 ppm — it's a percentage of your CYA (cyanuric acid) level. CYA stabilizes chlorine against UV loss, but at higher concentrations it also reduces the active fraction of FC. The practical rule: FC minimum = CYA × 7.5% (or × 5% for saltwater pools). A pool with CYA 50 ppm needs FC above 3.75 ppm; a pool with CYA 80 ppm needs FC above 6 ppm. The FC/CYA chart translates this into a ready reference for any stabilizer level.
Common chlorine problems follow predictable patterns. FC dropping overnight: high demand from heat, bather load, organic debris, or CYA that's too low. CC above 0.5 ppm: chloramines accumulating — shock to breakpoint chlorination to clear them. Chlorine that won't hold after adding product: usually a CYA problem or invisible early-stage algae. The guides below cover each scenario with the specific diagnosis and fix.
FC/CYA Chart: How Much Chlorine You Need
Your chlorine target depends on your CYA. Use this chart to find the right FC minimum and avoid algae.
Pool Chlorine Levels Chart
Clear reference for FC, TC, and CC readings. What range to aim for, what's too high, and when to act.
Free Chlorine vs Total Chlorine
Your test kit gives two chlorine numbers. The gap between them is a warning sign most pool owners miss.
Pool Chlorine Too Low? Causes and How to Fix It
FC below target means algae, bacteria, and cloudy water. 6 common causes and the step-by-step fix.
Why Won't Chlorine Stay in My Pool?
Add chlorine and it's gone the next day? 7 causes — from low CYA to invisible algae — plus the overnight test that tells you exactly what's wrong.
What Causes High Combined Chlorine?
That strong chlorine smell isn't too much chlorine — it's chloramines. 6 causes of high CC and the breakpoint fix.
How to Raise Pool Chlorine Safely
Liquid, granular, or tablets — each works differently. The right method and dose based on your CYA level.
Does Liquid Chlorine Raise CYA?
No — and knowing which chlorine types do raise CYA is the key to keeping your pool chemistry balanced all season.
Liquid Chlorine vs Tablets: Pros and Cons
Both sanitize, but they affect your water differently. The CYA buildup from tablets is what most owners miss.
How to Shock a Pool: Step-by-Step Guide
Shock target = CYA × 40%. The full shock-and-hold process, when to start, and the three tests that tell you when you're done.
How Long After Shocking Can You Swim?
Don't count hours — test. Safe swimming FC depends on your CYA level. Here's exactly what to check before anyone gets in.
Why Does My Pool Smell Like Chlorine?
A strong chlorine smell means too little chlorine, not too much. Combined chlorine (chloramines) causes the odor — here's the fix.
Chlorine Lock: Myth or Real?
Chlorine lock isn't a real chemistry phenomenon. The actual causes of "chlorine not working" are diagnosable — and fixable.
Pool Chlorine Too High — What to Do
High FC after shocking is expected. Here's what "too high" actually means, how to bring it down, and when to just wait.
Track your FC/CYA ratio over time
PoolChem Tracker shows your FC trend against your CYA target — so you can see when FC is slipping before it becomes a problem.
