Shock dose calculator
That chlorine smell is not chlorine. It is chloramine — chlorine already spent on sweat and urine — and the only thing that clears it is enough free chlorine, added at once, to break the molecule apart. Half a dose does nothing but cost money.
Shocking is not a scoop-and-hope. Combined chlorine is total minus free; breakpoint needs free chlorine at roughly ten times that number, all at once.
- Combined chlorine
- 1.4 ppm
- Breakpoint target
- 14 ppm
- Rise needed
- 12 ppm
Add in one dose
3lb Cal-hypo 73%
Do not swim
Keep the pool closed until free chlorine falls back under 3 ppm — the ceiling WHO sets for public and semi-public pools. Dose at dusk; sunlight destroys the reserve before it can finish the job.
What breakpoint actually is
Add chlorine to water carrying ammonia compounds and it first forms chloramines — the eye-stinging, pool-smelling stuff. Keep adding and, at roughly ten times the combined chlorine reading, the chemistry tips over and destroys them. Stop short of that point and you have made more chloramine, not less. That is why this tool asks for two readings and gives one number.
When to shock, and when not to
| Situation | Shock? | What to do instead |
|---|---|---|
| Combined chlorine over 0.4 ppm | Yes | — |
| Water green | No — SLAM | Hold shock level for days, not one dose |
| Water cloudy, CC 0.1 ppm | No | Filter and check pH first |
| After heavy bather load | Yes | — |
| After a faecal release | Yes | Then filter six turnover cycles (WHO §5.8) |
| Every week, on schedule | No | Test; shock the reading, not the calendar |
The 0.4 ppm line is the action level in CDC’s Model Aquatic Health Code (5th edition, 2024), which names superchlorination and water exchange among the fixes. WHO’s 2006 pool guidelines put the ideal under 0.2 mg/l.
Stabiliser changes the arithmetic
In a pool holding a lot of cyanuric acid, a breakpoint dose can take free chlorine to a level that bleaches a vinyl liner and eats a solar cover. Check CYA before a big shock, and dilute first if it is over the ceiling.
Dosage notes
Pre-dissolve cal-hypo in a bucket of pool water before it goes in — dropped dry on a vinyl floor it bleaches a white patch you will be explaining for years. Dose at dusk, leave the pump running overnight, and re-test in the morning before anyone swims. The pool reopens when free chlorine is back under 3 ppm.