Your test gives you three chlorine numbers, and only one of them is actually protecting the people in the water. I've been reading chlorine tests on South Florida pools since the late 1990s — my father's been a licensed Florida pool contractor since 1989, and checking chemicals was the first job he ever handed me.
The three chlorine numbers
Chlorine is the workhorse of pool sanitation. But “chlorine” isn't one number — it's three: free chlorine (FC), combined chlorine (CC), and total chlorine (TC). Understanding the difference is the whole game.
| Number | What it means | Target |
|---|---|---|
| Free chlorine (FC) | Available to sanitize. This is the one that protects you. | 1–3 ppm (residential) |
| Combined chlorine (CC) | Chlorine that's already reacted with contaminants — mostly worthless. | <0.4 ppm (MAHC ceiling); <0.2 ppm best practice |
| Total chlorine (TC) | FC + CC. The number on basic test strips. | FC + CC |
If you only measure total chlorine, you're flying blind.A pool can read 3 ppm total chlorine and still be unsafe because all of it is combined (chloramines) — dead chlorine that already did its job and quit. The strip says 3. The pool says otherwise.
How CYA changes everything
Here's the detail most homeowners miss: chlorine's killing power depends on the ratioof FC to CYA (cyanuric acid). The classic “1–3 ppm” rule assumes zero CYA. In a stabilized outdoor pool with 40 ppm CYA, you need more like 3–5 ppm FC to have the same sanitizing effect. This is known as the FC/CYA ratio and the rule of thumb is:
Keep minimum FC at roughly 7.5% of your CYA level. So 40 ppm CYA → 3 ppm FC minimum.
Do that math on your own pool before you blame the chlorine. If you don't know your CYA number — or you're on tablets and it's been climbing for two seasons — read our cyanuric acid guide next. The stabilizer that saves your chlorine from the sun is the same one that quietly shuts it off.
Breakpoint chlorination: why you shock
When combined chlorine climbs above 0.5 ppm, normal dosing won't burn it off. You have to hit breakpoint: add enough chlorine to reach roughly 10x the combined chlorine level, all at once. That's what “shocking” is — not “add one bag per week.” Half a dose doesn't half-work; it just feeds the problem. The full walkthrough is in our guide to shocking a pool.
Symptoms of low free chlorine
- Algae starting (green tint, slimy walls) — past “starting,” go straight to the green algae recovery guide
- Cloudy water that won't clear
- Strong “chlorine smell” (that's actually chloramines — ironic, right?)
- Irritated eyes and skin after swimming
Symptoms of too-high free chlorine
- Bleached swimsuits and hair
- Corroded equipment over time
- Burned plant life around the pool
Briefly hitting 10+ ppm during shocking is fine; living there long-term damages your equipment. I've replaced more than one pump seal that years of hot, over-chlorinated water chewed up.
How to test chlorine properly
Use a FAS-DPD test kit(Taylor K-2006 or similar). It separates free from combined chlorine to the nearest 0.2 ppm. Regular OTO/DPD test strips only measure total chlorine — useful for a quick gut-check, not for real management. For the full rundown of what's worth carrying, see our pool testing methods and tools guide.
Chlorine in South Florida: the summer math
Down here the sun is the biggest chlorine thief on the route. Summer UV can burn through an unstabilized pool's free chlorine in 4–6 hours — a morning dose is half gone by lunch, which is why we dose late in the day whenever the route allows. Then the afternoon storm rolls in, dumps rain and yard debris into the water, and that organic load converts more of your FC into CC.
Salt pools change the numbers too. Most salt systems run CYA at 60–80 ppm for cell efficiency, and the 7.5% rule doesn't care how the chlorine gets made: at 70 ppm CYA your minimum FC is north of 5 ppm, not the 1–3 printed on the strip bottle. That gap — a “fine” reading on a high-CYA pool — is how South Florida pools go green in July with chlorine still in the water.
When to call a pro
If your pool won't hold chlorine overnight, if combined chlorine creeps back within days of shocking, or if the FAS-DPD reads near zero no matter what you dose, stop pouring money into the deep end — something upstream is wrong. Usually it's CYA lock, a phosphate load feeding algae faster than you can sanitize, or circulation that isn't turning the water over. We test free, combined, and stabilizer on every visit and hand you the actual numbers. Get in touch and we'll get eyes on your water.
Pool chlorine calculator
Pick your product, enter your numbers — get the dose plus the side effects the label doesn't mention.
Add 30.6 fl oz
Raises free chlorine by 2 ppm in 15,000 gallons using liquid chlorine 12.5% (pool grade). Add in the evening (sunlight burns unstabilized chlorine), pump running, then retest.
Cheapest per ppm — what service routes use. Loses ~10% strength per month in Florida heat; buy fresh, store shaded.
Never mix chlorine products — trichlor + cal-hypo react violently. One product, one dose, water moving.
Frequently asked questions
1–3 ppm free chlorine is the baseline for a residential pool with little or no stabilizer. Once CYA is in the water, the minimum scales up: free chlorine should sit at roughly 7.5% of your CYA reading, so a pool stabilized at 40 ppm needs at least 3 ppm free chlorine to sanitize properly.
Want a pro to handle this?
Our CPO-certified techs run this exact playbook on every weekly service visit.