Free chlorine
Last reviewed August 11, 2026
Dosing
12 oz of 12.5% liquid chlorine raises free chlorine 2 ppm in 15,000 gallons. A jug is 128 oz. Scale it to the pool you're standing at: 30,000 gallons takes double, 10,000 gallons two thirds. What to buy and what to carry: starter chemicals.
Free chlorine is the chlorine that's still working. Total chlorine counts that plus the chlorine that's already been used up, so a total reading always looks better than the pool really is. Test free chlorine.
Why stabilizer changes the target
Cyanuric acid holds onto chlorine and shields it from sunlight. Chlorine that's being held isn't sanitizing. So the more stabilizer in the water, the more free chlorine you need to get the same job done.
The rule is a minimum of 7.5% of your CYA reading (CYA × 0.075). At CYA 70 that is 5.3 ppm, so a pool testing 4 ppm looks fine against the 3–6 range and is still short. Full chart in the FC–CYA ratio guide.
What the reading tells you
| Reading | What it means | What to do |
|---|---|---|
| 0 | Nothing is sanitizing the pool | Shock it. Check for a dead cell or an empty feeder. Retest before you leave. |
| Under the CYA floor | Short, even though it reads in range | Dose up. If it keeps dropping, the pool has demand. |
| 3–6 ppm at normal CYA | Where you want it | Log it and move on. |
| Over 10 ppm | Overshot, or somebody shocked it | Note it. It'll come down on its own. |
When chlorine won't hold
If it reads fine when you dose and near zero a week later, something is eating it. Check these in order:
- Algae, including a bloom you can't see yet
- CYA under 30, so the sun burns it off
- Phosphates feeding growth
- A salt cell that's quit, or a feeder that ran empty
- Heavy swimmer load, or rain
Logging chemistry on every stop? PoolSpartan keeps the readings, works out the LSI as you type them, and sends the customer a service report when you close the visit. Free to start. We only get paid when you do.
Sign up · free